Next projects B review : October 15 2018. Proposal must be submitted before September 30 2018.
Publications and communications which have benefited of mésocentre resources should contain : « This work was granted access to the HPC resources of Aix-Marseille Université financed by the project Equip@Meso (ANR-10-EQPX-29-01) of the program « Investissements d’Avenir » supervised by the Agence Nationale de la Recherche.»
Recent publications :
2017 |
A Pivano; V O Dolocan Systematic motion of magnetic domain walls in notched nanowires under ultrashort current pulses In: Phys. Rev. B, 96 , pp. 224431, 2017. @article{PhysRevB.96.224431, title = {Systematic motion of magnetic domain walls in notched nanowires under ultrashort current pulses}, author = {A Pivano and V O Dolocan}, url = {https://link.aps.org/doi/10.1103/PhysRevB.96.224431}, doi = {10.1103/PhysRevB.96.224431}, year = {2017}, date = {2017-12-01}, journal = {Phys. Rev. B}, volume = {96}, pages = {224431}, publisher = {American Physical Society}, abstract = {The precise manipulation of transverse magnetic domain walls in finite/infinite nanowires with artificial defects under the influence of very short spin-polarized current pulses is investigated. We show that for a classical 3d ferromagnet material like nickel, the exact positioning of the domain walls at room temperature is possible only for pulses with very short rise and fall time that move the domain wall reliably to nearest neighboring pinning position. The influence of the shape of the current pulse and of the transient effects on the phase diagram current-pulse length are discussed. We show that large transient effects appear even when α=β, below a critical value, due to the domain wall distortion caused by the current pulse shape and the presence of the notches. The transient effects can oppose or amplify the spin-transfer torque (STT), depending on the ratio β/α. This enlarges the physical comprehension of the domain wall (DW) motion under STT and opens the route to the DW displacement in both directions with unipolar currents.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The precise manipulation of transverse magnetic domain walls in finite/infinite nanowires with artificial defects under the influence of very short spin-polarized current pulses is investigated. We show that for a classical 3d ferromagnet material like nickel, the exact positioning of the domain walls at room temperature is possible only for pulses with very short rise and fall time that move the domain wall reliably to nearest neighboring pinning position. The influence of the shape of the current pulse and of the transient effects on the phase diagram current-pulse length are discussed. We show that large transient effects appear even when α=β, below a critical value, due to the domain wall distortion caused by the current pulse shape and the presence of the notches. The transient effects can oppose or amplify the spin-transfer torque (STT), depending on the ratio β/α. This enlarges the physical comprehension of the domain wall (DW) motion under STT and opens the route to the DW displacement in both directions with unipolar currents. |
L -A Couston; D Lecoanet; B Favier; M Le Bars Dynamics of mixed convectivechar21stably-stratified fluids In: Phys. Rev. Fluids, 2 , pp. 094804, 2017. @article{PhysRevFluids.2.094804, title = {Dynamics of mixed convectivechar21stably-stratified fluids}, author = {L -A Couston and D Lecoanet and B Favier and M Le Bars}, url = {https://link.aps.org/doi/10.1103/PhysRevFluids.2.094804}, doi = {10.1103/PhysRevFluids.2.094804}, year = {2017}, date = {2017-09-01}, journal = {Phys. Rev. Fluids}, volume = {2}, pages = {094804}, publisher = {American Physical Society}, abstract = {We study the dynamical regimes of a density-stratified fluid confined between isothermal no-slip top and bottom boundaries (at temperatures Tt and Tb) via direct numerical simulation. The thermal expansion coefficient of the fluid is temperature dependent and chosen such that the fluid density is maximum at the inversion temperature Tb>Ti>Tt. Thus, the lower layer of the fluid is convectively unstable while the upper layer is stably stratified. We show that the characteristics of the convection change significantly depending on the degree of stratification of the stable layer. For strong stable stratification, the convection zone coincides with the fraction of the fluid that is convectively unstable (i.e., where T>Ti), and convective motions consist of rising and sinking plumes of large density anomaly, as is the case in canonical Rayleigh-Bénard convection; internal gravity waves are generated by turbulent fluctuations in the convective layer and propagate in the upper layer. For weak stable stratification, we demonstrate that a large fraction of the stable fluid (i.e., with temperature T pubstate = {published}, tppubtype = {article} } We study the dynamical regimes of a density-stratified fluid confined between isothermal no-slip top and bottom boundaries (at temperatures Tt and Tb) via direct numerical simulation. The thermal expansion coefficient of the fluid is temperature dependent and chosen such that the fluid density is maximum at the inversion temperature Tb>Ti>Tt. Thus, the lower layer of the fluid is convectively unstable while the upper layer is stably stratified. We show that the characteristics of the convection change significantly depending on the degree of stratification of the stable layer. For strong stable stratification, the convection zone coincides with the fraction of the fluid that is convectively unstable (i.e., where T>Ti), and convective motions consist of rising and sinking plumes of large density anomaly, as is the case in canonical Rayleigh-Bénard convection; internal gravity waves are generated by turbulent fluctuations in the convective layer and propagate in the upper layer. For weak stable stratification, we demonstrate that a large fraction of the stable fluid (i.e., with temperature T<Ti) is instead destabilized and entrained by buoyant plumes emitted from the bottom boundary. The convection thus mixes cold patches of low density-anomaly fluid with hot upward plumes and the end result is that the Ti isotherm sinks within the bottom boundary layer and that the convection is entrainment dominated. We provide a phenomenological description of the transition between the regimes of plume-dominated and entrainment-dominated convection through analysis of the differences in the heat transfer mechanisms, kinetic energy density spectra, and probability density functions for different stratification strengths. Importantly, we find that the effect of the stable layer on the convection decreases only weakly with increasing stratification strength, meaning that the dynamics of the stable layer and convection should be studied self-consistently in a wide range of applications. |
Thomas Le Reun; Benjamin Favier; Adrian J Barker; Michael Le Bars Inertial Wave Turbulence Driven by Elliptical Instability In: Phys. Rev. Lett., 119 , pp. 034502, 2017. @article{PhysRevLett.119.034502, title = {Inertial Wave Turbulence Driven by Elliptical Instability}, author = {Thomas Le Reun and Benjamin Favier and Adrian J Barker and Michael Le Bars}, url = {https://link.aps.org/doi/10.1103/PhysRevLett.119.034502}, doi = {10.1103/PhysRevLett.119.034502}, year = {2017}, date = {2017-07-01}, journal = {Phys. Rev. Lett.}, volume = {119}, pages = {034502}, publisher = {American Physical Society}, abstract = {The combination of elliptical deformation of streamlines and vorticity can lead to the destabilization of any rotating flow via the elliptical instability. Such a mechanism has been invoked as a possible source of turbulence in planetary cores subject to tidal deformations. The saturation of the elliptical instability has been shown to generate turbulence composed of nonlinearly interacting waves and strong columnar vortices with varying respective amplitudes, depending on the control parameters and geometry. In this Letter, we present a suite of numerical simulations to investigate the saturation and the transition from vortex-dominated to wave-dominated regimes. This is achieved by simulating the growth and saturation of the elliptical instability in an idealized triply periodic domain, adding a frictional damping to the geostrophic component only, to mimic its interaction with boundaries. We reproduce several experimental observations within one idealized local model and complement them by reaching more extreme flow parameters. In particular, a wave-dominated regime that exhibits many signatures of inertial wave turbulence is characterized for the first time. This regime is expected in planetary interiors.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The combination of elliptical deformation of streamlines and vorticity can lead to the destabilization of any rotating flow via the elliptical instability. Such a mechanism has been invoked as a possible source of turbulence in planetary cores subject to tidal deformations. The saturation of the elliptical instability has been shown to generate turbulence composed of nonlinearly interacting waves and strong columnar vortices with varying respective amplitudes, depending on the control parameters and geometry. In this Letter, we present a suite of numerical simulations to investigate the saturation and the transition from vortex-dominated to wave-dominated regimes. This is achieved by simulating the growth and saturation of the elliptical instability in an idealized triply periodic domain, adding a frictional damping to the geostrophic component only, to mimic its interaction with boundaries. We reproduce several experimental observations within one idealized local model and complement them by reaching more extreme flow parameters. In particular, a wave-dominated regime that exhibits many signatures of inertial wave turbulence is characterized for the first time. This regime is expected in planetary interiors. |
Simon Cabanes, Jonathan Aurnou, Benjamin Favier & Michael Le Bars A laboratory model for deep-seated jets on the gas giants In: Nature Physics, 2017. @article{Cabanes2017, title = {A laboratory model for deep-seated jets on the gas giants}, author = {Simon Cabanes, Jonathan Aurnou, Benjamin Favier & Michael Le Bars}, editor = {Nature Physics}, url = {http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys4001.html}, doi = {10.1038/nphys4001}, year = {2017}, date = {2017-01-23}, journal = {Nature Physics}, abstract = {The strong east–west jet flows on the gas giants, Jupiter1 and Saturn2, have persisted for hundreds of years. Yet, experimental studies cannot reach the planetary regime and similarly strong and quasi-steady jets have been reproduced in numerical models only under simplifying assumptions and limitations. Two models have been proposed: a shallow model where jets are confined to the weather layer and a deep model where the jets extend into the planetary molecular envelope. Here we show that turbulent laboratory flows naturally generate multiple, alternating jets in a rapidly rotating cylindrical container. The observed properties of gas giants’ jets are only now reproduced in a laboratory experiment emulating the deep model. Our findings demonstrate that long-lived jets can persist at high latitudes even under conditions including viscous dissipation and friction and bear relevance to the shallow versus deep models debate in the context of the ongoing Juno mission.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The strong east–west jet flows on the gas giants, Jupiter1 and Saturn2, have persisted for hundreds of years. Yet, experimental studies cannot reach the planetary regime and similarly strong and quasi-steady jets have been reproduced in numerical models only under simplifying assumptions and limitations. Two models have been proposed: a shallow model where jets are confined to the weather layer and a deep model where the jets extend into the planetary molecular envelope. Here we show that turbulent laboratory flows naturally generate multiple, alternating jets in a rapidly rotating cylindrical container. The observed properties of gas giants’ jets are only now reproduced in a laboratory experiment emulating the deep model. Our findings demonstrate that long-lived jets can persist at high latitudes even under conditions including viscous dissipation and friction and bear relevance to the shallow versus deep models debate in the context of the ongoing Juno mission. |
P. Tamain; C. Colin; L. Colas; C. Baudoin; G. Ciraolo; R. Futtersack; D. Galassi; Ph Ghendrih; N. Nace; F. Schwander; E. Serre In: Nuclear Materials and Energy, pp. -, 2017, ISSN: 2352-1791. @article{Tamain2017, title = {Numerical analysis of the impact of an RF sheath on the Scrape-Off Layer in 2D and 3D turbulence simulations}, author = {P. Tamain and C. Colin and L. Colas and C. Baudoin and G. Ciraolo and R. Futtersack and D. Galassi and Ph Ghendrih and N. Nace and F. Schwander and E. Serre}, url = {http://www.sciencedirect.com/science/article/pii/S2352179116302757}, doi = {http://dx.doi.org/10.1016/j.nme.2016.12.022}, issn = {2352-1791}, year = {2017}, date = {2017-01-01}, journal = {Nuclear Materials and Energy}, pages = {-}, abstract = {Abstract Motivated by Radio Frequency (RF) heating studies, the response of the plasma of tokamaks to the presence of a locally polarized limiter is studied. In a first part, we use the TOKAM3X 3D global edge turbulence code to analyse the impact of such biasing in a realistic geometry. Key features of experimental observations are qualitatively recovered, especially the extension of a potential and density perturbation on long, but finite, distances along connected field lines. The perturbation is also found to extend in the transverse direction. Both observations demonstrate the influence of perpendicular current loops on the plasma confirming the need for an accurate description in reduced models. In a second part, we use the TOKAM2D slab turbulence code to determine the validity of using a transverse Ohm's law for this purpose. Results indicate that a local Ohm's law with a constant and uniform perpendicular resistivity appears at least as an oversimplified description of perpendicular charge transport in a turbulent Scrape-Off Layer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Abstract Motivated by Radio Frequency (RF) heating studies, the response of the plasma of tokamaks to the presence of a locally polarized limiter is studied. In a first part, we use the TOKAM3X 3D global edge turbulence code to analyse the impact of such biasing in a realistic geometry. Key features of experimental observations are qualitatively recovered, especially the extension of a potential and density perturbation on long, but finite, distances along connected field lines. The perturbation is also found to extend in the transverse direction. Both observations demonstrate the influence of perpendicular current loops on the plasma confirming the need for an accurate description in reduced models. In a second part, we use the TOKAM2D slab turbulence code to determine the validity of using a transverse Ohm's law for this purpose. Results indicate that a local Ohm's law with a constant and uniform perpendicular resistivity appears at least as an oversimplified description of perpendicular charge transport in a turbulent Scrape-Off Layer. |
H. Bufferand; C. Baudoin; J. Bucalossi; G. Ciraolo; J. Denis; N. Fedorczak; D. Galassi; Ph. Ghendrih; R. Leybros; Y. Marandet; N. Mellet; J. Morales; N. Nace; E. Serre; P. Tamain; M. Valentinuzzi Implementation of drift velocities and currents in SOLEDGE2D–EIRENE In: Nuclear Materials and Energy, pp. -, 2017, ISSN: 2352-1791. @article{Bufferand2017, title = {Implementation of drift velocities and currents in SOLEDGE2D–EIRENE}, author = {H. Bufferand and C. Baudoin and J. Bucalossi and G. Ciraolo and J. Denis and N. Fedorczak and D. Galassi and Ph. Ghendrih and R. Leybros and Y. Marandet and N. Mellet and J. Morales and N. Nace and E. Serre and P. Tamain and M. Valentinuzzi}, url = {http://www.sciencedirect.com/science/article/pii/S2352179116301946}, doi = {http://dx.doi.org/10.1016/j.nme.2016.11.031}, issn = {2352-1791}, year = {2017}, date = {2017-01-01}, journal = {Nuclear Materials and Energy}, pages = {-}, abstract = {Abstract In order to improve cross-field transport description, drifts and currents have been implemented in SOLEDGE2D–EIRENE. The derivation of an equation for the electric potential is recalled. The resolution of current equation is tested in a simple slab case. WEST divertor simulations in forward-B and reverse-B fields are also discussed. A significant increase of ExB shear is observed in the forward-B configuration that could explain a favorable L-H transition in this case.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Abstract In order to improve cross-field transport description, drifts and currents have been implemented in SOLEDGE2D–EIRENE. The derivation of an equation for the electric potential is recalled. The resolution of current equation is tested in a simple slab case. WEST divertor simulations in forward-B and reverse-B fields are also discussed. A significant increase of ExB shear is observed in the forward-B configuration that could explain a favorable L-H transition in this case. |
G. Ciraolo; H. Bufferand; J. Bucalossi; Ph. Ghendrih; P. Tamain; Y. Marandet; M. Valentinuzzi; J. Denis; N. Fedorczak; E. Hodille; N. Mellet; B. Pegourie; C. Grisolia; C. Bourdelle; E. Tsitrone; D. Galassi; R. Leybros; G. Giorgiani; E. Serre In: Nuclear Materials and Energy, pp. -, 2017, ISSN: 2352-1791. @article{Ciraolo2017, title = {H-mode WEST tungsten divertor operation: deuterium and nitrogen seeded simulations with SOLEDGE2D-EIRENE}, author = {G. Ciraolo and H. Bufferand and J. Bucalossi and Ph. Ghendrih and P. Tamain and Y. Marandet and M. Valentinuzzi and J. Denis and N. Fedorczak and E. Hodille and N. Mellet and B. Pegourie and C. Grisolia and C. Bourdelle and E. Tsitrone and D. Galassi and R. Leybros and G. Giorgiani and E. Serre}, url = {http://www.sciencedirect.com/science/article/pii/S235217911630271X}, doi = {http://dx.doi.org/10.1016/j.nme.2016.12.025}, issn = {2352-1791}, year = {2017}, date = {2017-01-01}, journal = {Nuclear Materials and Energy}, pages = {-}, abstract = {Abstract Simulations of WEST H-mode divertor scenarios have been performed with SOLEDGE2D-EIRENE edge plasma transport code, both for pure deuterium and nitrogen seeded discharge. In the pure deuterium case, a target heat flux of 8 MW/m2 is reached, but misalignment between heat and the particle outflux yields 50 eV plasma temperature at the target plates. With nitrogen seeding, the heat and particle outflux are observed to be aligned so that lower plasma temperatures at the target plates are achieved together with the required high heat fluxes. This change in heat and particle outflux alignment is analysed with respect to the role of divertor geometry and the impact of vertical vs horizontal target plates on neutrals spreading.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Abstract Simulations of WEST H-mode divertor scenarios have been performed with SOLEDGE2D-EIRENE edge plasma transport code, both for pure deuterium and nitrogen seeded discharge. In the pure deuterium case, a target heat flux of 8 MW/m2 is reached, but misalignment between heat and the particle outflux yields 50 eV plasma temperature at the target plates. With nitrogen seeding, the heat and particle outflux are observed to be aligned so that lower plasma temperatures at the target plates are achieved together with the required high heat fluxes. This change in heat and particle outflux alignment is analysed with respect to the role of divertor geometry and the impact of vertical vs horizontal target plates on neutrals spreading. |
Athanassoula, E.; Rodionov, S. A.; Prantzos, N. Metallicity-dependent kinematics and morphology of the Milky Way bulge In: Monthly Notices of the Royal Astronomical Society: Letters, 467 (1), pp. L46, 2017. @article{doi:10.1093/mnrasl/slw255, title = {Metallicity-dependent kinematics and morphology of the Milky Way bulge}, author = {Athanassoula, E. and Rodionov, S. A. and Prantzos, N.}, url = {+ http://dx.doi.org/10.1093/mnrasl/slw255}, doi = {10.1093/mnrasl/slw255}, year = {2017}, date = {2017-01-01}, journal = {Monthly Notices of the Royal Astronomical Society: Letters}, volume = {467}, number = {1}, pages = {L46}, abstract = {We use N-body chemo-dynamic simulations to study the coupling between morphology, kinematics and metallicity of the bar/bulge region of our Galaxy. We make qualitative comparisons of our results with available observations and find very good agreement. We conclude that this region is complex, since it comprises several stellar components with different properties -- i.e. a boxy/peanut bulge, thin and thick disc components, and, to lesser extents, a disky pseudobulge, a stellar halo and a small classical bulge -- all cohabiting in dynamical equilibrium. Our models show strong links between kinematics and metallicity, or morphology and metallicity, as already suggested by a number of recent observations. We discuss and explain these links. }, keywords = {}, pubstate = {published}, tppubtype = {article} } We use N-body chemo-dynamic simulations to study the coupling between morphology, kinematics and metallicity of the bar/bulge region of our Galaxy. We make qualitative comparisons of our results with available observations and find very good agreement. We conclude that this region is complex, since it comprises several stellar components with different properties -- i.e. a boxy/peanut bulge, thin and thick disc components, and, to lesser extents, a disky pseudobulge, a stellar halo and a small classical bulge -- all cohabiting in dynamical equilibrium. Our models show strong links between kinematics and metallicity, or morphology and metallicity, as already suggested by a number of recent observations. We discuss and explain these links. |
Gao, Xing; Bai, Shuming; Fazzi, Daniele; Niehaus, Thomas; Barbatti, Mario; Thiel, Walter Evaluation of Spin-Orbit Couplings with Linear-Response Time-Dependent Density Functional Methods In: Journal of Chemical Theory and Computation, 13 (2), pp. 515-524, 2017, (PMID: 27959528). @article{doi:10.1021/acs.jctc.6b00915, title = {Evaluation of Spin-Orbit Couplings with Linear-Response Time-Dependent Density Functional Methods}, author = {Gao, Xing and Bai, Shuming and Fazzi, Daniele and Niehaus, Thomas and Barbatti, Mario and Thiel, Walter}, url = {http://dx.doi.org/10.1021/acs.jctc.6b00915}, doi = {10.1021/acs.jctc.6b00915}, year = {2017}, date = {2017-01-01}, journal = {Journal of Chemical Theory and Computation}, volume = {13}, number = {2}, pages = {515-524}, abstract = {A new versatile code based on Python scripts was developed to calculate spin–orbit coupling (SOC) elements between singlet and triplet states. The code, named PySOC, is interfaced to third-party quantum chemistry packages, such as Gaussian 09 and DFTB+. SOCs are evaluated using linear-response (LR) methods based on time-dependent density functional theory (TDDFT), the Tamm-Dancoff approximation (TDA), and time-dependent density functional tight binding (TD-DFTB). The evaluation employs Casida-type wave functions and the Breit-Pauli (BP) spin–orbit Hamiltonian with an effective charge approximation. For validation purposes, SOCs calculated with PySOC are benchmarked for several organic molecules, with SOC values spanning several orders of magnitude. The computed SOCs show little variation with the basis set, but are sensitive to the chosen density functional. The benchmark results are in good agreement with reference data obtained using higher-level spin–orbit Hamiltonians and electronic structure methods, such as CASPT2 and DFT/MRCI. PySOC can be easily interfaced to other third-party codes and other methods yielding CI-type wave functions.}, note = {PMID: 27959528}, keywords = {}, pubstate = {published}, tppubtype = {article} } A new versatile code based on Python scripts was developed to calculate spin–orbit coupling (SOC) elements between singlet and triplet states. The code, named PySOC, is interfaced to third-party quantum chemistry packages, such as Gaussian 09 and DFTB+. SOCs are evaluated using linear-response (LR) methods based on time-dependent density functional theory (TDDFT), the Tamm-Dancoff approximation (TDA), and time-dependent density functional tight binding (TD-DFTB). The evaluation employs Casida-type wave functions and the Breit-Pauli (BP) spin–orbit Hamiltonian with an effective charge approximation. For validation purposes, SOCs calculated with PySOC are benchmarked for several organic molecules, with SOC values spanning several orders of magnitude. The computed SOCs show little variation with the basis set, but are sensitive to the chosen density functional. The benchmark results are in good agreement with reference data obtained using higher-level spin–orbit Hamiltonians and electronic structure methods, such as CASPT2 and DFT/MRCI. PySOC can be easily interfaced to other third-party codes and other methods yielding CI-type wave functions. |
Dmytro Kandaskalov; Philippe Maugis In: Computational Materials Science, 128 , pp. 278 - 286, 2017, ISSN: 0927-0256. @article{Kandaskalov2017278, title = {A first-principle study of the structural, elastic, lattice dynamical and thermodynamic properties of and phases}, author = {Dmytro Kandaskalov and Philippe Maugis}, url = {http://www.sciencedirect.com/science/article/pii/S0927025616305730}, doi = {http://dx.doi.org/10.1016/j.commatsci.2016.11.022}, issn = {0927-0256}, year = {2017}, date = {2017-01-01}, journal = {Computational Materials Science}, volume = {128}, pages = {278 - 286}, abstract = {Abstract Low temperature decomposition of ferrous martensite leads to the formation of alternate C-rich and Fe-rich domains. Although some contradictions exist in the experimental results, the C-rich domains are often supposed to be the ordered α ″ - Fe 16 C 2 phase. The aim of our theoretical study is to investigate the fundamental properties of the α ″ - Fe 16 C 2 phase and to compare it to the isomorphic known phase α ″ - Fe 16 N 2 . In this paper, we present the structural, electronic, magnetic, vibrational and elastic properties of α ″ - Fe 16 C 2 and Fe 16 N 2 phases, calculated within the formalism of the DFT theory. This study is used to discuss the relative properties and stabilities of these two phases.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Abstract Low temperature decomposition of ferrous martensite leads to the formation of alternate C-rich and Fe-rich domains. Although some contradictions exist in the experimental results, the C-rich domains are often supposed to be the ordered α ″ - Fe 16 C 2 phase. The aim of our theoretical study is to investigate the fundamental properties of the α ″ - Fe 16 C 2 phase and to compare it to the isomorphic known phase α ″ - Fe 16 N 2 . In this paper, we present the structural, electronic, magnetic, vibrational and elastic properties of α ″ - Fe 16 C 2 and Fe 16 N 2 phases, calculated within the formalism of the DFT theory. This study is used to discuss the relative properties and stabilities of these two phases. |
Bilel Sanhaji Testing for Nonlinearity in Conditional Covariances In: 2017, ISSN: 21946507. @article{Sanhaji2017, title = {Testing for Nonlinearity in Conditional Covariances}, author = {Bilel Sanhaji}, editor = {Journal of Time Series Econometrics}, url = {https://doi.org/10.1515/jtse-2016-0010}, doi = {jtse-2016-0010}, issn = {21946507}, year = {2017}, date = {2017-01-01}, abstract = {We propose two Lagrange multiplier tests for nonlinearity in conditional covariances in multivariate GARCH models. The null hypothesis is the scalar BEKK model in which covolatilities of time series are driven by a linear function of their own lags and lagged squared innovations. The alternative hypothesis is an extension of the model in which covolatilities are modeled by a nonlinear function of the lagged squared innovations, represented by an exponential or a logistic transition function. Moreover, on the same basis we develop two other tests that are robust to leverage effects. We investigate the size and power of these tests through Monte Carlo experiments, and we provide empirical illustrations in many of which cases these tests encourage the use of nonlinearity in conditional covariances.}, keywords = {}, pubstate = {published}, tppubtype = {article} } We propose two Lagrange multiplier tests for nonlinearity in conditional covariances in multivariate GARCH models. The null hypothesis is the scalar BEKK model in which covolatilities of time series are driven by a linear function of their own lags and lagged squared innovations. The alternative hypothesis is an extension of the model in which covolatilities are modeled by a nonlinear function of the lagged squared innovations, represented by an exponential or a logistic transition function. Moreover, on the same basis we develop two other tests that are robust to leverage effects. We investigate the size and power of these tests through Monte Carlo experiments, and we provide empirical illustrations in many of which cases these tests encourage the use of nonlinearity in conditional covariances. |
E Constant; J Favier; M Meldi; P Meliga; E Serre An immersed boundary method in OpenFOAM : Verification and validation In: Computers & Fluids, 157 , pp. 55 - 72, 2017, ISSN: 0045-7930. @article{CONSTANT201755, title = {An immersed boundary method in OpenFOAM : Verification and validation}, author = {E Constant and J Favier and M Meldi and P Meliga and E Serre}, url = {http://www.sciencedirect.com/science/article/pii/S004579301730275X}, doi = {https://doi.org/10.1016/j.compfluid.2017.08.001}, issn = {0045-7930}, year = {2017}, date = {2017-01-01}, journal = {Computers & Fluids}, volume = {157}, pages = {55 - 72}, abstract = {The present work proposes a modified Pressure-Implicit Split-Operator (PISO) solver integrating the recent Immersed Boundary Method (IBM) proposed by [1] in order to perform reliable simulations of incompressible flows around bluff bodies using the open source toolbox OpenFOAM version 2.2 ([2]). The (IBM) allows for a precise representation of fixed and moving solid obstacles embedded in the physical domain, using uniform or stretched Cartesian meshes. Owing to this feature, the maximum level of accuracy and scalability of the numerical solvers can be systematically achieved. An iterative scheme based on sub-iterations between (IBM) and pressure correction has been implemented in the native (PISO) solver of OpenFOAM. This allows one to use fast optimized Poisson solvers while satisfying simultaneously the divergence-free flow state and the no-slip condition at the body surface. To compute the divergence of the momentum equation (in the PISO loop) and the interpolation of the fluxes, we propose an hybrid calculation with an analytical resolution (using the kernel function equation) of the quantities involving the force term (singular quantities). A careful and original verification study has been carried out which allows to estimate three different errors related to the discretization and to the (IBM). Various 2D and 3D well-documented test cases of academic flows around fixed or moving solid bodies (cylinder and sphere) have been simulated and carefully validated against existing data from the literature in a large range of Reynolds number}, keywords = {}, pubstate = {published}, tppubtype = {article} } The present work proposes a modified Pressure-Implicit Split-Operator (PISO) solver integrating the recent Immersed Boundary Method (IBM) proposed by [1] in order to perform reliable simulations of incompressible flows around bluff bodies using the open source toolbox OpenFOAM version 2.2 ([2]). The (IBM) allows for a precise representation of fixed and moving solid obstacles embedded in the physical domain, using uniform or stretched Cartesian meshes. Owing to this feature, the maximum level of accuracy and scalability of the numerical solvers can be systematically achieved. An iterative scheme based on sub-iterations between (IBM) and pressure correction has been implemented in the native (PISO) solver of OpenFOAM. This allows one to use fast optimized Poisson solvers while satisfying simultaneously the divergence-free flow state and the no-slip condition at the body surface. To compute the divergence of the momentum equation (in the PISO loop) and the interpolation of the fluxes, we propose an hybrid calculation with an analytical resolution (using the kernel function equation) of the quantities involving the force term (singular quantities). A careful and original verification study has been carried out which allows to estimate three different errors related to the discretization and to the (IBM). Various 2D and 3D well-documented test cases of academic flows around fixed or moving solid bodies (cylinder and sphere) have been simulated and carefully validated against existing data from the literature in a large range of Reynolds number |
P Manas; Y Camenen; S Benkadda; H Weisen; C Angioni; F J Casson; C Giroud; M Gelfusa; M Maslov Gyrokinetic modeling of impurity peaking in JET H-mode plasmas In: Physics Of Plasmas, 24 (6), pp. 12. 062511, 2017. @article{Manas:230792, title = {Gyrokinetic modeling of impurity peaking in JET H-mode plasmas}, author = {P Manas and Y Camenen and S Benkadda and H Weisen and C Angioni and F J Casson and C Giroud and M Gelfusa and M Maslov}, url = {https://aip.scitation.org/doi/abs/10.1063/1.4985330?journalCode=php}, year = {2017}, date = {2017-01-01}, journal = {Physics Of Plasmas}, volume = {24}, number = {6}, pages = {12. 062511}, publisher = {Amer Inst Physics}, address = {Melville}, abstract = {Quantitative comparisons are presented between gyrokinetic simulations and experimental values of the carbon impurity peaking factor in a database of JET H-modes during the carbon wall era. These plasmas feature strong NBI heating and hence high values of toroidal rotation and corresponding gradient. Furthermore, the carbon profiles present particularly interesting shapes for fusion devices, i.e., hollow in the core and peaked near the edge. Dependencies of the experimental carbon peaking factor (R/L-nC) on plasma parameters are investigated via multilinear regressions. A marked correlation between R/L-nC and the normalised toroidal rotation gradient is observed in the core, which suggests an important role of the rotation in establishing hollow carbon profiles. The carbon peaking factor is then computed with the gyrokinetic code GKW, using a quasi-linear approach, supported by a few non-linear simulations. The comparison of the quasi-linear predictions to the experimental values at mid-radius reveals two main regimes. At low normalised collisionality, nu*, and T-e/T-i < 1, the gyrokinetic simulations quantitatively recover experimental carbon density profiles, provided that rotodiffusion is taken into account. In contrast, at higher nu* and T-e/T-i > 1, the very hollow experimental carbon density profiles are never predicted by the simulations and the carbon density peaking is systematically over estimated. This points to a possible missing ingredient in this regime.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Quantitative comparisons are presented between gyrokinetic simulations and experimental values of the carbon impurity peaking factor in a database of JET H-modes during the carbon wall era. These plasmas feature strong NBI heating and hence high values of toroidal rotation and corresponding gradient. Furthermore, the carbon profiles present particularly interesting shapes for fusion devices, i.e., hollow in the core and peaked near the edge. Dependencies of the experimental carbon peaking factor (R/L-nC) on plasma parameters are investigated via multilinear regressions. A marked correlation between R/L-nC and the normalised toroidal rotation gradient is observed in the core, which suggests an important role of the rotation in establishing hollow carbon profiles. The carbon peaking factor is then computed with the gyrokinetic code GKW, using a quasi-linear approach, supported by a few non-linear simulations. The comparison of the quasi-linear predictions to the experimental values at mid-radius reveals two main regimes. At low normalised collisionality, nu*, and T-e/T-i < 1, the gyrokinetic simulations quantitatively recover experimental carbon density profiles, provided that rotodiffusion is taken into account. In contrast, at higher nu* and T-e/T-i > 1, the very hollow experimental carbon density profiles are never predicted by the simulations and the carbon density peaking is systematically over estimated. This points to a possible missing ingredient in this regime. |
P Migliano; D Zarzoso; F J Artola; Y Camenen; X Garbet In: Plasma Physics and Controlled Fusion, 59 (9), pp. 095004, 2017. @article{0741-3335-59-9-095004, title = {An improved approximation for the analytical treatment of the local linear gyro-kinetic plasma dispersion relation in toroidal geometry}, author = {P Migliano and D Zarzoso and F J Artola and Y Camenen and X Garbet}, url = {http://stacks.iop.org/0741-3335/59/i=9/a=095004}, year = {2017}, date = {2017-01-01}, journal = {Plasma Physics and Controlled Fusion}, volume = {59}, number = {9}, pages = {095004}, abstract = {The analytical treatment of plasma kinetic linear instabilities in toroidal geometry is commonly tackled employing a power series expansion of the resonant part of the dispersion relation. This expansion is valid under the assumption that the modulus of the mode frequency is smaller than the magnitude of the frequencies characterising the system (the drift, bounce and transit frequencies for example). We will refer to this approximation as high frequency approximation (HFA). In this paper the linear plasma dispersion relation is derived in the framework of the gyro-kinetic model, for the electrostatic case, in the local limit, in the absence of collisions, for a non rotating plasma, considering adiabatic electrons, in toroidal circular geometry, neglecting the parallel dynamics effect. A systematic analysis of the meaning and limitations of the HFA is performed. As already known, the HFA is not valid for tokamak relevant parameters. A new way to approximate the resonant part of the dispersion relation, called here Improved high frequency approximation (IHFA), is therefore proposed. A quantitative analysis of the ion temperature gradient (ITG) instability is presented. The IHFA is shown to be applicable to the treatment of the ITG instability in tokamaks.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The analytical treatment of plasma kinetic linear instabilities in toroidal geometry is commonly tackled employing a power series expansion of the resonant part of the dispersion relation. This expansion is valid under the assumption that the modulus of the mode frequency is smaller than the magnitude of the frequencies characterising the system (the drift, bounce and transit frequencies for example). We will refer to this approximation as high frequency approximation (HFA). In this paper the linear plasma dispersion relation is derived in the framework of the gyro-kinetic model, for the electrostatic case, in the local limit, in the absence of collisions, for a non rotating plasma, considering adiabatic electrons, in toroidal circular geometry, neglecting the parallel dynamics effect. A systematic analysis of the meaning and limitations of the HFA is performed. As already known, the HFA is not valid for tokamak relevant parameters. A new way to approximate the resonant part of the dispersion relation, called here Improved high frequency approximation (IHFA), is therefore proposed. A quantitative analysis of the ion temperature gradient (ITG) instability is presented. The IHFA is shown to be applicable to the treatment of the ITG instability in tokamaks. |
M Muraglia; O Agullo; A Poyé; S Benkadda; N Dubuit; X Garbet; A Sen Amplification of a turbulence driven seed magnetic island by bootstrap current In: Nuclear Fusion, 57 (7), pp. 072010, 2017. @article{0029-5515-57-7-072010, title = {Amplification of a turbulence driven seed magnetic island by bootstrap current}, author = {M Muraglia and O Agullo and A Poyé and S Benkadda and N Dubuit and X Garbet and A Sen}, url = {http://stacks.iop.org/0029-5515/57/i=7/a=072010}, year = {2017}, date = {2017-01-01}, journal = {Nuclear Fusion}, volume = {57}, number = {7}, pages = {072010}, abstract = {The amplification of a turbulence driven seed magnetic island by the bootstrap current is investigated numerically in a reduced magnetohydrodynamic model as a possible additional channel for the excitation and development of a neoclassical tearing mode (NTM) in a tokamak. The novel mechanism for such an excitation involves a two-step process, namely, the formation of a seed island due to a nonlinear beating of interchange turbulence modes followed by an amplification of the seed island by the bootstrap current leading to a large saturated island. Our study indicates that the level of turbulence controls both the critical seed island size and the saturated island size.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The amplification of a turbulence driven seed magnetic island by the bootstrap current is investigated numerically in a reduced magnetohydrodynamic model as a possible additional channel for the excitation and development of a neoclassical tearing mode (NTM) in a tokamak. The novel mechanism for such an excitation involves a two-step process, namely, the formation of a seed island due to a nonlinear beating of interchange turbulence modes followed by an amplification of the seed island by the bootstrap current leading to a large saturated island. Our study indicates that the level of turbulence controls both the critical seed island size and the saturated island size. |
O Agullo; M Muraglia; S Benkadda; A Poyé; N Dubuit; X Garbet; A Sen Nonlinear dynamics of turbulence driven magnetic islands. II. Numerical simulations In: Physics of Plasmas, 24 (4), pp. 042309, 2017. @article{doi:10.1063/1.4981230, title = {Nonlinear dynamics of turbulence driven magnetic islands. II. Numerical simulations}, author = {O Agullo and M Muraglia and S Benkadda and A Poyé and N Dubuit and X Garbet and A Sen}, url = {https://doi.org/10.1063/1.4981230}, doi = {10.1063/1.4981230}, year = {2017}, date = {2017-01-01}, journal = {Physics of Plasmas}, volume = {24}, number = {4}, pages = {042309}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
O Agullo; M Muraglia; S Benkadda; A Poyé; N Dubuit; X Garbet; A Sen Nonlinear dynamics of turbulence driven magnetic islands. I. Theoretical aspects In: Physics of Plasmas, 24 (4), pp. 042308, 2017. @article{doi:10.1063/1.4981229, title = {Nonlinear dynamics of turbulence driven magnetic islands. I. Theoretical aspects}, author = {O Agullo and M Muraglia and S Benkadda and A Poyé and N Dubuit and X Garbet and A Sen}, url = {https://doi.org/10.1063/1.4981229}, doi = {10.1063/1.4981229}, year = {2017}, date = {2017-01-01}, journal = {Physics of Plasmas}, volume = {24}, number = {4}, pages = {042308}, abstract = {The nonlinear dynamics of a turbulence driven magnetic island (TDMI) is investigated numerically in a reduced magnetohydrodynamic fluid model. The significance of identifying a characteristic signature of a TDMI for its experimental observation is discussed. The principal focus of our simulations is on the nature of the pressure profile flattening inside a TDMI, and we show that, in agreement with analytical predictions, a partial flattening occurs when the island size exceeds a critical value that is a function of the small scale interchange dynamics. We also present a model and test it numerically, which links explicitly the interchange turbulence and the island pressure flattening.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The nonlinear dynamics of a turbulence driven magnetic island (TDMI) is investigated numerically in a reduced magnetohydrodynamic fluid model. The significance of identifying a characteristic signature of a TDMI for its experimental observation is discussed. The principal focus of our simulations is on the nature of the pressure profile flattening inside a TDMI, and we show that, in agreement with analytical predictions, a partial flattening occurs when the island size exceeds a critical value that is a function of the small scale interchange dynamics. We also present a model and test it numerically, which links explicitly the interchange turbulence and the island pressure flattening. |
Michael Stenrup; Elisa Pieri; Vincent Ledentu; Nicolas Ferre In: Phys. Chem. Chem. Phys., 19 , pp. 14073-14084, 2017. @article{C7CP00991G, title = {pH-Dependent absorption spectrum of a protein: a minimal electrostatic model of Anabaena sensory rhodopsin}, author = {Michael Stenrup and Elisa Pieri and Vincent Ledentu and Nicolas Ferre}, url = {http://dx.doi.org/10.1039/C7CP00991G}, doi = {10.1039/C7CP00991G}, year = {2017}, date = {2017-01-01}, journal = {Phys. Chem. Chem. Phys.}, volume = {19}, pages = {14073-14084}, publisher = {The Royal Society of Chemistry}, abstract = {A minimal electrostatic model is introduced which aims at reproducing and analyzing the visible-light absorption energy shift of a protein with pH. It relies on the existence of a protein structure, the prediction of titratable amino-acid pKa values and a very limited set of parameters. Applied to the case of the photochromic Anabaena sensory rhodopsin protein, the model succeeds in reproducing qualitatively the reported experimental data, confirming the importance of aspartic acid 217 in the observed blue shift in the [small lambda]max of ASR at neutral pH. It also suggests for the first time the role of two other amino acids, glutamic acid 36 at basic pH and aspartic acid 120 at acidic pH.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A minimal electrostatic model is introduced which aims at reproducing and analyzing the visible-light absorption energy shift of a protein with pH. It relies on the existence of a protein structure, the prediction of titratable amino-acid pKa values and a very limited set of parameters. Applied to the case of the photochromic Anabaena sensory rhodopsin protein, the model succeeds in reproducing qualitatively the reported experimental data, confirming the importance of aspartic acid 217 in the observed blue shift in the [small lambda]max of ASR at neutral pH. It also suggests for the first time the role of two other amino acids, glutamic acid 36 at basic pH and aspartic acid 120 at acidic pH. |
A M Grannan; B Favier; M Le Bars; J M Aurnou Tidally forced turbulence in planetary interiors In: Geophysical Journal International, 208 (3), pp. 1690-1703, 2017. @article{doi:10.1093/gji/ggw479, title = {Tidally forced turbulence in planetary interiors}, author = {A M Grannan and B Favier and M Le Bars and J M Aurnou}, doi = {10.1093/gji/ggw479}, year = {2017}, date = {2017-01-01}, journal = {Geophysical Journal International}, volume = {208}, number = {3}, pages = {1690-1703}, abstract = {The turbulence generated in the liquid metal cores and subsurface oceans of planetary bodies may be due to the role of mechanical forcing through precession/nutation, libration, tidal forcing and collisions. Here, we model the response of an enclosed constant density fluid to tidal forcing by combining laboratory equatorial velocity measurements with selected high-resolution numerical simulations to show, for the first time, the generation of bulk filling turbulence. The transition to saturated turbulence is characterized by an elliptical instability that first excites primary inertial modes of the system, then secondary inertial modes forced by the primary inertial modes, and then bulk filling turbulence. The amplitude of this saturated turbulence scales with the body's elliptical distortion, U ∼ β, while a time- and radially averaged azimuthal zonal flow scales with β2. The results of the current tidal experiments are compared with recent studies of the libration-driven turbulent flows studied by Grannan et al. and Favier et al. Tides and libration correspond to two end-member types of geophysical mechanical forcings. For satellites dominated by tidal forcing, the ellipsoidal boundary enclosing the internal fluid layers is elastically deformed while, for librational forcing, the core-mantle boundary possesses an inherently rigid, frozen-in ellipsoidal shape. We find striking similarities between tidally and librationally driven flow transitions to bulk turbulence and zonal flows. This suggests a generic fluid response independent of the style of mechanical forcing. Since β ≲ 10−4 in planetary bodies, it is often argued that mechanically driven zonal velocities will be small. In contrast, our linear scaling for mechanically driven bulk turbulence, U ∼ β, suggests geophysically significant velocities that can play a significant role in planetary processes including tidal dissipation and magnetic field generation.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The turbulence generated in the liquid metal cores and subsurface oceans of planetary bodies may be due to the role of mechanical forcing through precession/nutation, libration, tidal forcing and collisions. Here, we model the response of an enclosed constant density fluid to tidal forcing by combining laboratory equatorial velocity measurements with selected high-resolution numerical simulations to show, for the first time, the generation of bulk filling turbulence. The transition to saturated turbulence is characterized by an elliptical instability that first excites primary inertial modes of the system, then secondary inertial modes forced by the primary inertial modes, and then bulk filling turbulence. The amplitude of this saturated turbulence scales with the body's elliptical distortion, U ∼ β, while a time- and radially averaged azimuthal zonal flow scales with β2. The results of the current tidal experiments are compared with recent studies of the libration-driven turbulent flows studied by Grannan et al. and Favier et al. Tides and libration correspond to two end-member types of geophysical mechanical forcings. For satellites dominated by tidal forcing, the ellipsoidal boundary enclosing the internal fluid layers is elastically deformed while, for librational forcing, the core-mantle boundary possesses an inherently rigid, frozen-in ellipsoidal shape. We find striking similarities between tidally and librationally driven flow transitions to bulk turbulence and zonal flows. This suggests a generic fluid response independent of the style of mechanical forcing. Since β ≲ 10−4 in planetary bodies, it is often argued that mechanically driven zonal velocities will be small. In contrast, our linear scaling for mechanically driven bulk turbulence, U ∼ β, suggests geophysically significant velocities that can play a significant role in planetary processes including tidal dissipation and magnetic field generation. |
Simon Cabanes; Jonathan M Aurnou; Benjamin Favier; Michael Le Bars A laboratory model for deep-seated jets on the gas giants In: Nature Physics, 2017. @article{cabanes:hal-01478957, title = {A laboratory model for deep-seated jets on the gas giants}, author = {Simon Cabanes and Jonathan M Aurnou and Benjamin Favier and Michael Le Bars}, url = {https://hal.archives-ouvertes.fr/hal-01478957}, doi = {10.1038/nphys4001}, year = {2017}, date = {2017-01-01}, journal = {Nature Physics}, publisher = {Nature Publishing Group}, keywords = {}, pubstate = {published}, tppubtype = {article} } |
O Février; P Maget; H Lütjens; P Beyer Comparison of magnetic island stabilization strategies from magneto-hydrodynamic simulations In: Plasma Physics and Controlled Fusion, 59 (4), pp. 044002, 2017. @article{0741-3335-59-4-044002, title = {Comparison of magnetic island stabilization strategies from magneto-hydrodynamic simulations}, author = {O Février and P Maget and H Lütjens and P Beyer}, url = {http://stacks.iop.org/0741-3335/59/i=4/a=044002}, year = {2017}, date = {2017-01-01}, journal = {Plasma Physics and Controlled Fusion}, volume = {59}, number = {4}, pages = {044002}, abstract = {The degradation of plasma confinement in tokamaks caused by magnetic islands motivates to better understand their possible suppression using electron cyclotron current drive (ECCD) and to investigate the various strategies relevant for this purpose. In this work, we evaluate the efficiency of several control methods through nonlinear simulations of this process with the toroidal magneto-hydro-dynamic (MHD) code XTOR-2F (Lütjens and Luciani 2010 J. Comput. Phys. 229 [http://https://doi.org/10.1016/j.jcp.2010.07.013] 8130–43 ), which has been extended to incorporate in Ohm’s law a source term modeling the driven current resulting from the interaction of the EC waves with the plasma. A basic control system has been implemented in the code, allowing testing of advanced strategies that require feedback on island position or phase. We focus in particular on the robustness of the control strategies towards uncertainties that apply to the control and ECCD systems, such as the risk of misalignment of the current deposition or the possible inability to generate narrow current deposition.}, keywords = {}, pubstate = {published}, tppubtype = {article} } The degradation of plasma confinement in tokamaks caused by magnetic islands motivates to better understand their possible suppression using electron cyclotron current drive (ECCD) and to investigate the various strategies relevant for this purpose. In this work, we evaluate the efficiency of several control methods through nonlinear simulations of this process with the toroidal magneto-hydro-dynamic (MHD) code XTOR-2F (Lütjens and Luciani 2010 J. Comput. Phys. 229 [http://https://doi.org/10.1016/j.jcp.2010.07.013] 8130–43 ), which has been extended to incorporate in Ohm’s law a source term modeling the driven current resulting from the interaction of the EC waves with the plasma. A basic control system has been implemented in the code, allowing testing of advanced strategies that require feedback on island position or phase. We focus in particular on the robustness of the control strategies towards uncertainties that apply to the control and ECCD systems, such as the risk of misalignment of the current deposition or the possible inability to generate narrow current deposition. |
O A. Bessonov Analysis of mixed convection in the Czochralski model in a wide range of Prandtl numbers In: Fluid Dynamics, 52 , pp. 375-387, 2017. @article{articleb, title = {Analysis of mixed convection in the Czochralski model in a wide range of Prandtl numbers}, author = {O A. Bessonov}, url = {https://www.researchgate.net/publication/317912710_Analysis_of_mixed_convection_in_the_Czochralski_model_in_a_wide_range_of_Prandtl_numbers}, doi = {10.1134/S001546281703005X}, year = {2017}, date = {2017-01-01}, booktitle = {Fluid Dynamics}, journal = {Fluid Dynamics}, volume = {52}, pages = {375-387}, abstract = {The results of the calculations and analysis of the effect of separate and joint rotation of the crystal and the crucible on the flow stability are presented for a wide range of Prandtl numbers (from 0.01 to 10). The regimes with a high stability threshold are determined for different combinations of the rotation velocities. It is shown that for high Prandtl numbers, simultaneous rotation of the crystal and the crucible makes it possible to increase the critical Grashof number in 9–12 times. A resultant diagram (map) of the limiting regimes of natural and mixed convection is constructed. Themethodology of control and analysis of 2D and 3D instability modes is discussed. Analysis of mixed convection in the Czochralski model in a.... Available from: https://www.researchgate.net/publication/317912710_Analysis_of_mixed_convection_in_the_Czochralski_model_in_a_wide_range_of_Prandtl_numbers [accessed Apr 03 2018].}, keywords = {}, pubstate = {published}, tppubtype = {article} } The results of the calculations and analysis of the effect of separate and joint rotation of the crystal and the crucible on the flow stability are presented for a wide range of Prandtl numbers (from 0.01 to 10). The regimes with a high stability threshold are determined for different combinations of the rotation velocities. It is shown that for high Prandtl numbers, simultaneous rotation of the crystal and the crucible makes it possible to increase the critical Grashof number in 9–12 times. A resultant diagram (map) of the limiting regimes of natural and mixed convection is constructed. Themethodology of control and analysis of 2D and 3D instability modes is discussed. Analysis of mixed convection in the Czochralski model in a.... Available from: https://www.researchgate.net/publication/317912710_Analysis_of_mixed_convection_in_the_Czochralski_model_in_a_wide_range_of_Prandtl_numbers [accessed Apr 03 2018]. |
Oleg Bessonov Technological Aspects of the Hybrid Parallelization with OpenMP and MPI In: Malyshkin, Victor (Ed.): Parallel Computing Technologies, pp. 101–113, Springer International Publishing, Cham, 2017, ISBN: 978-3-319-62932-2. @inproceedings{10.1007/978-3-319-62932-2_9, title = {Technological Aspects of the Hybrid Parallelization with OpenMP and MPI}, author = {Oleg Bessonov}, editor = {Victor Malyshkin}, doi = {10.1007/978-3-319-62932-2_9}, isbn = {978-3-319-62932-2}, year = {2017}, date = {2017-01-01}, booktitle = {Parallel Computing Technologies}, pages = {101--113}, publisher = {Springer International Publishing}, address = {Cham}, abstract = {In this paper we present practical parallelization techniques for different explicit and implicit numerical algorithms. These algorithms are considered on the base of the analysis of characteristics of modern computer systems and the nature of modeled physical processes. Limits of applicability of methods and parallelization techniques are determined in terms of practical implementation. Finally, the unified parallelization approach for OpenMP and MPI for solving a CFD problem in a regular domain is presented and discussed.}, keywords = {}, pubstate = {published}, tppubtype = {inproceedings} } In this paper we present practical parallelization techniques for different explicit and implicit numerical algorithms. These algorithms are considered on the base of the analysis of characteristics of modern computer systems and the nature of modeled physical processes. Limits of applicability of methods and parallelization techniques are determined in terms of practical implementation. Finally, the unified parallelization approach for OpenMP and MPI for solving a CFD problem in a regular domain is presented and discussed. |
Patrick Maget; Fabien Widmer; Olivier Février; Xavier Garbet; Hinrich Lütjens Stabilization of a magnetic island by localized heating in a tokamak with stiff temperature profile 2017, (working paper or preprint). @unpublished{maget:hal-01614656, title = {Stabilization of a magnetic island by localized heating in a tokamak with stiff temperature profile}, author = {Patrick Maget and Fabien Widmer and Olivier Février and Xavier Garbet and Hinrich Lütjens}, url = {https://hal.archives-ouvertes.fr/hal-01614656}, year = {2017}, date = {2017-01-01}, abstract = {In tokamaks plasmas, turbulent transport is triggered above a threshold in the temperature gradient and leads to stiff profiles. This particularity, neglected so far in the problem of magnetic island stabilization by a localized heat source, is investigated analytically in the present paper. We show that the efficiency of the stabilization is larger than previously estimated due to the reduced turbulence level inside the island. }, note = {working paper or preprint}, keywords = {}, pubstate = {published}, tppubtype = {unpublished} } In tokamaks plasmas, turbulent transport is triggered above a threshold in the temperature gradient and leads to stiff profiles. This particularity, neglected so far in the problem of magnetic island stabilization by a localized heat source, is investigated analytically in the present paper. We show that the efficiency of the stabilization is larger than previously estimated due to the reduced turbulence level inside the island. |
Benjamin Kadoch; Wouter Bos; Kai Schneider Directional change of fluid particles in two-dimensional turbulence and of football players In: Physical Review Fluids, 2 (6), pp. 064604, 2017. @article{kadoch:hal-01705321, title = {Directional change of fluid particles in two-dimensional turbulence and of football players}, author = {Benjamin Kadoch and Wouter Bos and Kai Schneider}, url = {https://hal.archives-ouvertes.fr/hal-01705321}, doi = {10.1103/PhysRevFluids.2.064604}, year = {2017}, date = {2017-01-01}, journal = {Physical Review Fluids}, volume = {2}, number = {6}, pages = {064604}, publisher = {American Physical Society}, abstract = {Multiscale directional statistics are investigated in two-dimensional incompressible turbulence. It is shown that the short-time behavior of the mean angle of directional change of fluid particles is linearly dependent on the time lag and that no inertial range behavior is observed in the directional change associated with the enstrophy-cascade range. In simulations of the inverse-cascade range, the directional change shows a power law behavior at inertial range time scales. By comparing the directional change in space-periodic and wall-bounded flow, it is shown that the probability density function of the directional change at long times carries the signature of the confinement. The geometrical origin of this effect is validated by Monte Carlo simulations. The same effect is also observed in the directional statistics computed from the trajectories of football players (soccer players in American English).}, keywords = {}, pubstate = {published}, tppubtype = {article} } Multiscale directional statistics are investigated in two-dimensional incompressible turbulence. It is shown that the short-time behavior of the mean angle of directional change of fluid particles is linearly dependent on the time lag and that no inertial range behavior is observed in the directional change associated with the enstrophy-cascade range. In simulations of the inverse-cascade range, the directional change shows a power law behavior at inertial range time scales. By comparing the directional change in space-periodic and wall-bounded flow, it is shown that the probability density function of the directional change at long times carries the signature of the confinement. The geometrical origin of this effect is validated by Monte Carlo simulations. The same effect is also observed in the directional statistics computed from the trajectories of football players (soccer players in American English). |
D Galassi; P Tamain; C Baudoin; H Bufferand; G Ciraolo; N Fedorczak; Ph. Ghendrih; N Nace; E Serre Flux expansion effect on turbulent transport in 3D global simulations In: Nuclear Materials and Energy, 12 , pp. 953 - 958, 2017, ISSN: 2352-1791, (Proceedings of the 22nd International Conference on Plasma Surface Interactions 2016, 22nd PSI). @article{GALASSI2017953, title = {Flux expansion effect on turbulent transport in 3D global simulations}, author = {D Galassi and P Tamain and C Baudoin and H Bufferand and G Ciraolo and N Fedorczak and Ph. Ghendrih and N Nace and E Serre}, url = {http://www.sciencedirect.com/science/article/pii/S2352179116302216}, doi = {https://doi.org/10.1016/j.nme.2017.01.008}, issn = {2352-1791}, year = {2017}, date = {2017-01-01}, journal = {Nuclear Materials and Energy}, volume = {12}, pages = {953 - 958}, abstract = {The flux expansion effect on the Scrape-Off Layer equilibrium is inspected through TOKAM3X 3D turbulence simulations. Three magnetic equilibria with analytically controlled flux expansion are built, representing respectively a positive, a null and a negative Shafranov shift. Turbulent E × B fluxes across flux surfaces show similar amplitudes and poloidal distributions in all cases. The ballooning nature of the interchange instability is recovered, with an enhancement of turbulence in the vicinity of the limiter, probably due to a Kelvin–Helmoltz instability. Interestingly, the poloidally averaged density decay length is found to be shorter almost by a factor 2 in the case of flux surfaces compressed at the low-field side midplane, with respect to the opposite case, indicating the presence of unfavorable conditions for the turbulent transport. The difference in the magnetic field line shape is pointed out as a mechanism which affects the turbulent transport across the flux surfaces. Indeed the unstable region has a larger parallel extension when flux expansion in the low-field side is larger. Moreover, the configuration with a lower magnetic shear at the low-field side midplane shows a more unstable behavior. The role of this parameter in turbulence stabilization is qualitatively evaluated. The difference in the distribution of transport along the parallel direction is shown to affect also the parallel flows, which are analyzed for the three proposed cases.}, note = {Proceedings of the 22nd International Conference on Plasma Surface Interactions 2016, 22nd PSI}, keywords = {}, pubstate = {published}, tppubtype = {article} } The flux expansion effect on the Scrape-Off Layer equilibrium is inspected through TOKAM3X 3D turbulence simulations. Three magnetic equilibria with analytically controlled flux expansion are built, representing respectively a positive, a null and a negative Shafranov shift. Turbulent E × B fluxes across flux surfaces show similar amplitudes and poloidal distributions in all cases. The ballooning nature of the interchange instability is recovered, with an enhancement of turbulence in the vicinity of the limiter, probably due to a Kelvin–Helmoltz instability. Interestingly, the poloidally averaged density decay length is found to be shorter almost by a factor 2 in the case of flux surfaces compressed at the low-field side midplane, with respect to the opposite case, indicating the presence of unfavorable conditions for the turbulent transport. The difference in the magnetic field line shape is pointed out as a mechanism which affects the turbulent transport across the flux surfaces. Indeed the unstable region has a larger parallel extension when flux expansion in the low-field side is larger. Moreover, the configuration with a lower magnetic shear at the low-field side midplane shows a more unstable behavior. The role of this parameter in turbulence stabilization is qualitatively evaluated. The difference in the distribution of transport along the parallel direction is shown to affect also the parallel flows, which are analyzed for the three proposed cases. |
Umberto D'Ortona; Nathalie Thomas; Richard M Lueptow Axial segregation in spherical and cylindrical rotating tumblers In: EPJ Web of Conferences, 140 , pp. 03011, 2017. @article{dortona:hal-01671207, title = {Axial segregation in spherical and cylindrical rotating tumblers}, author = {Umberto D'Ortona and Nathalie Thomas and Richard M Lueptow}, url = {https://hal.archives-ouvertes.fr/hal-01671207}, doi = {10.1051/epjconf/201714003011}, year = {2017}, date = {2017-01-01}, journal = {EPJ Web of Conferences}, volume = {140}, pages = {03011}, publisher = {EDP Sciences}, series = {Powders and Grains 2017 -- 8th International Conference on Micromechanics on Granular Media}, abstract = {Monodisperse and bidisperse granular flows are studied in rotating tumblers using DEM. In spherical tumblers, flowing particles’ trajectories do not follow straight lines but are curved. At the same time particles near the surface drift toward the pole, inducing two global recirculation cells. Combined with radial segregation, drift and curvature compete to impose the axial segregation pattern: Small-Large-Small (SLS) or Large-Small-Large (LSL). Fill level, rotation speed and wall roughness influence drift and curvature, and modify the resulting segregation pattern. In cylindrical tumblers, equivalent recirculation cells occur next to the end walls. A second pair of recirculation cells with a weak drift in the opposite direction appears at the center for long enough tumblers. Unlike the sphere case, curvature and drift in the primary cells combine to push large particles toward the end walls, explaining why large particle bands appear at the end walls for axial segregation in cylinder.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Monodisperse and bidisperse granular flows are studied in rotating tumblers using DEM. In spherical tumblers, flowing particles’ trajectories do not follow straight lines but are curved. At the same time particles near the surface drift toward the pole, inducing two global recirculation cells. Combined with radial segregation, drift and curvature compete to impose the axial segregation pattern: Small-Large-Small (SLS) or Large-Small-Large (LSL). Fill level, rotation speed and wall roughness influence drift and curvature, and modify the resulting segregation pattern. In cylindrical tumblers, equivalent recirculation cells occur next to the end walls. A second pair of recirculation cells with a weak drift in the opposite direction appears at the center for long enough tumblers. Unlike the sphere case, curvature and drift in the primary cells combine to push large particles toward the end walls, explaining why large particle bands appear at the end walls for axial segregation in cylinder. |
Ljiljana Stojanović; Saadullah G Aziz; Rifaat H Hilal; Felix Plasser; Thomas A Niehaus; Mario Barbatti Nonadiabatic Dynamics of Cycloparaphenylenes with TD-DFTB Surface Hopping In: Journal of Chemical Theory and Computation, 13 (12), pp. 5846-5860, 2017, (PMID: 29140693). @article{doi:10.1021/acs.jctc.7b01000, title = {Nonadiabatic Dynamics of Cycloparaphenylenes with TD-DFTB Surface Hopping}, author = {Ljiljana Stojanović and Saadullah G Aziz and Rifaat H Hilal and Felix Plasser and Thomas A Niehaus and Mario Barbatti}, url = {https://doi.org/10.1021/acs.jctc.7b01000}, doi = {10.1021/acs.jctc.7b01000}, year = {2017}, date = {2017-01-01}, journal = {Journal of Chemical Theory and Computation}, volume = {13}, number = {12}, pages = {5846-5860}, abstract = {We implemented a version of the decoherence-corrected fewest switches surface hopping based on linear-response time-dependent density functional tight binding (TD-DFTB), enhanced by transition density analysis. The method has been tested for the gas-phase relaxation dynamics of two cycloparaphenylene molecules, [8]CPP and [10]CPP, explaining some important features of their nonadiabatic dynamics, such as the origin of their long fluorescence lifetimes (related to the slow radiative emission from the S1 state) and the trend of increasing the fluorescence rate with the molecular size (related to an increase in the S1–S0 energy gaps and oscillator strengths in the larger molecule). The quality of the TD-DFTB electronic structure information was assessed through four quantities: excitation energies; charge-transfer (CT) numbers, which estimate the charge transfer character of states; participation ratio (PR), which describes delocalization of electronic density; and participation ratio of natural transition orbitals (PRNTO), which describes the multiconfigurational character of states. These quantities were computed during dynamics and recomputed for the same geometries with the higher-level long-range-corrected TD-LC-DFTB and a lower-level single-determinant approximation for the excited states, SD-(LC)-DFTB. Taking TD-LC-DFTB as the standard, TD-DFTB underestimates the excitation energies by ∼0.5 eV and overestimates CT and PR. SD-DFTB underestimates excitation energies and overestimates CT to the same extent that TD-DFTB does, but it predicts reasonable PR distributions. SD-LC-DFTB leads to an extreme overestimation of the excitation energies by ∼3 eV, overestimates the charge transfer character of the state, but predicts the PR values very close to those obtained with TD-LC-DFTB.}, note = {PMID: 29140693}, keywords = {}, pubstate = {published}, tppubtype = {article} } We implemented a version of the decoherence-corrected fewest switches surface hopping based on linear-response time-dependent density functional tight binding (TD-DFTB), enhanced by transition density analysis. The method has been tested for the gas-phase relaxation dynamics of two cycloparaphenylene molecules, [8]CPP and [10]CPP, explaining some important features of their nonadiabatic dynamics, such as the origin of their long fluorescence lifetimes (related to the slow radiative emission from the S1 state) and the trend of increasing the fluorescence rate with the molecular size (related to an increase in the S1–S0 energy gaps and oscillator strengths in the larger molecule). The quality of the TD-DFTB electronic structure information was assessed through four quantities: excitation energies; charge-transfer (CT) numbers, which estimate the charge transfer character of states; participation ratio (PR), which describes delocalization of electronic density; and participation ratio of natural transition orbitals (PRNTO), which describes the multiconfigurational character of states. These quantities were computed during dynamics and recomputed for the same geometries with the higher-level long-range-corrected TD-LC-DFTB and a lower-level single-determinant approximation for the excited states, SD-(LC)-DFTB. Taking TD-LC-DFTB as the standard, TD-DFTB underestimates the excitation energies by ∼0.5 eV and overestimates CT and PR. SD-DFTB underestimates excitation energies and overestimates CT to the same extent that TD-DFTB does, but it predicts reasonable PR distributions. SD-LC-DFTB leads to an extreme overestimation of the excitation energies by ∼3 eV, overestimates the charge transfer character of the state, but predicts the PR values very close to those obtained with TD-LC-DFTB. |
Shuming Bai; Mario Barbatti Spatial Factors for Triplet Fusion Reaction of Singlet Oxygen Photosensitization In: The Journal of Physical Chemistry Letters, 8 (21), pp. 5456-5460, 2017, (PMID: 29058918). @article{doi:10.1021/acs.jpclett.7b02574, title = {Spatial Factors for Triplet Fusion Reaction of Singlet Oxygen Photosensitization}, author = {Shuming Bai and Mario Barbatti}, url = {https://doi.org/10.1021/acs.jpclett.7b02574}, doi = {10.1021/acs.jpclett.7b02574}, year = {2017}, date = {2017-01-01}, journal = {The Journal of Physical Chemistry Letters}, volume = {8}, number = {21}, pages = {5456-5460}, abstract = {First-principles quantum-chemical description of photosensitized singlet oxygen generation kinetics is challenging because of the intrinsic complexity of the underlying triplet fusion process in a floppy molecular complex with open-shell character. With a quantum-chemical kinetic model specifically tailored to deal with this problem, the reaction rates are investigated as a function of intermolecular incidence direction, orientation, and distance between O2 and the photosensitizer. The adopted photosensitizer, 6-azo-2-thiothymine, combines practical interest and prototypical variability. The study quantitatively determined maximum singlet oxygen generation rates for 15 incidence/orientation directions, showing that they span 5 orders of magnitude between the largest and the smallest rate. Such information may provide a hands-on guideline for the experimental molecular design of new photosensitizers as well as further higher-level theoretical research.}, note = {PMID: 29058918}, keywords = {}, pubstate = {published}, tppubtype = {article} } First-principles quantum-chemical description of photosensitized singlet oxygen generation kinetics is challenging because of the intrinsic complexity of the underlying triplet fusion process in a floppy molecular complex with open-shell character. With a quantum-chemical kinetic model specifically tailored to deal with this problem, the reaction rates are investigated as a function of intermolecular incidence direction, orientation, and distance between O2 and the photosensitizer. The adopted photosensitizer, 6-azo-2-thiothymine, combines practical interest and prototypical variability. The study quantitatively determined maximum singlet oxygen generation rates for 15 incidence/orientation directions, showing that they span 5 orders of magnitude between the largest and the smallest rate. Such information may provide a hands-on guideline for the experimental molecular design of new photosensitizers as well as further higher-level theoretical research. |
Shuming Bai; Mario Barbatti Divide-to-Conquer: A Kinetic Model for Singlet Oxygen Photosensitization In: Journal of Chemical Theory and Computation, 13 (11), pp. 5528-5538, 2017, (PMID: 28953379). @article{doi:10.1021/acs.jctc.7b00619, title = {Divide-to-Conquer: A Kinetic Model for Singlet Oxygen Photosensitization}, author = {Shuming Bai and Mario Barbatti}, url = {https://doi.org/10.1021/acs.jctc.7b00619}, doi = {10.1021/acs.jctc.7b00619}, year = {2017}, date = {2017-01-01}, journal = {Journal of Chemical Theory and Computation}, volume = {13}, number = {11}, pages = {5528-5538}, abstract = {Photosensitized singlet oxygen generation occurring in a PS–O2 complex, where PS is a photosensitizer chromophore, is a weakly coupled intermolecular energy-transfer process, a still challenging problem for theoretical chemistry. To investigate the reaction rate directly from quantum-chemical calculations, we built a semiclassical kinetic model that minimizes the computational effort for the calculation of diabatic couplings, activation energies, and reorganization energies, which are the components of the rate. The model splits the system into sets of orthogonal coordinates, which are then explored to compute the reaction rate. This model offers an effective way to evaluate the reaction probability of singlet oxygen generation along different directions and intramolecular distances of the PS–O2 complex. The model can also be applied to other similar intermolecular energy-transfer problems, to connect the reaction kinetics and quantum-chemical calculations.}, note = {PMID: 28953379}, keywords = {}, pubstate = {published}, tppubtype = {article} } Photosensitized singlet oxygen generation occurring in a PS–O2 complex, where PS is a photosensitizer chromophore, is a weakly coupled intermolecular energy-transfer process, a still challenging problem for theoretical chemistry. To investigate the reaction rate directly from quantum-chemical calculations, we built a semiclassical kinetic model that minimizes the computational effort for the calculation of diabatic couplings, activation energies, and reorganization energies, which are the components of the rate. The model splits the system into sets of orthogonal coordinates, which are then explored to compute the reaction rate. This model offers an effective way to evaluate the reaction probability of singlet oxygen generation along different directions and intramolecular distances of the PS–O2 complex. The model can also be applied to other similar intermolecular energy-transfer problems, to connect the reaction kinetics and quantum-chemical calculations. |
Daniele Fazzi; Mario Barbatti; Walter Thiel In: The Journal of Physical Chemistry Letters, 8 (19), pp. 4727-4734, 2017, (PMID: 28903560). @article{doi:10.1021/acs.jpclett.7b02144, title = {Hot and Cold Charge-Transfer Mechanisms in Organic Photovoltaics: Insights into the Excited States of Donor/Acceptor Interfaces}, author = {Daniele Fazzi and Mario Barbatti and Walter Thiel}, url = {https://doi.org/10.1021/acs.jpclett.7b02144}, doi = {10.1021/acs.jpclett.7b02144}, year = {2017}, date = {2017-01-01}, journal = {The Journal of Physical Chemistry Letters}, volume = {8}, number = {19}, pages = {4727-4734}, abstract = {The evolution of the excited-state manifold in organic D/A aggregates (e.g., the prototypical P3HT/PCBM) is investigated through a bottom-up approach via first-principles calculations. We show how the excited-state energies, the charge transfer (CT) states, and the electron–hole density distributions are strongly influenced by the size, the orientation, and the position (i.e., on-top versus on-edge phases) of P3HT/PCBM domains. We discuss how the structural order influences the excited-state electronic structure, providing an atomistic interpretation of the photophysics of organic blends. We show how the simultaneous presence of on-top and on-edge phases does not alter the optical absorption spectrum of the blend but does affect the photophysics. Photovoltaic processes such as (i) the simultaneous charge generation obtained from hot and cold excitations, (ii) the instantaneous and delayed charge separation, and (iii) the pump–push–probe charge generation can be interpreted based on our study.}, note = {PMID: 28903560}, keywords = {}, pubstate = {published}, tppubtype = {article} } The evolution of the excited-state manifold in organic D/A aggregates (e.g., the prototypical P3HT/PCBM) is investigated through a bottom-up approach via first-principles calculations. We show how the excited-state energies, the charge transfer (CT) states, and the electron–hole density distributions are strongly influenced by the size, the orientation, and the position (i.e., on-top versus on-edge phases) of P3HT/PCBM domains. We discuss how the structural order influences the excited-state electronic structure, providing an atomistic interpretation of the photophysics of organic blends. We show how the simultaneous presence of on-top and on-edge phases does not alter the optical absorption spectrum of the blend but does affect the photophysics. Photovoltaic processes such as (i) the simultaneous charge generation obtained from hot and cold excitations, (ii) the instantaneous and delayed charge separation, and (iii) the pump–push–probe charge generation can be interpreted based on our study. |
Josene M Toldo; Mario Barbatti; Paulo F B Goncalves A three-state model for the photo-Fries rearrangement In: Phys. Chem. Chem. Phys., 19 , pp. 19103-19108, 2017. @article{C7CP03777E, title = {A three-state model for the photo-Fries rearrangement}, author = {Josene M Toldo and Mario Barbatti and Paulo F B Goncalves}, url = {http://dx.doi.org/10.1039/C7CP03777E}, doi = {10.1039/C7CP03777E}, year = {2017}, date = {2017-01-01}, journal = {Phys. Chem. Chem. Phys.}, volume = {19}, pages = {19103-19108}, publisher = {The Royal Society of Chemistry}, abstract = {A three-state model for the photo-Fries rearrangement (PFR) is proposed based on multiconfigurational calculations. It provides a comprehensive mechanistic picture of all steps of the reaction, from the photoabsorption to the final tautomerization. The three states participating in the PFR are an aromatic 1[small pi][small pi]*, which absorbs the radiation; a pre-dissociative 1n[small pi]*, which transfers the energy to the dissociative region; and a 1[small pi][sigma]*, along which dissociation occurs. The transfer from 1[small pi][small pi]* to 1n[small pi]* involves pyramidalization of the carbonyl carbon, while transfer from 1n[small pi]* to 1[small pi][sigma]* takes place through CO stretching. Different products are available after a conical intersection with the ground state. Among them is a recombined radical intermediate, which can yield ortho-PFR products after an intramolecular 1,3-H tunneling. The three-state model is developed for phenyl acetate, the basic prototype for the PFR, and it reconciles the theory with a series of observations from time-resolved spectroscopy. It also delivers a rational way to optimize PFR yields, since, as shown for four different systems, diverse substituents can change the energetic order of the 1[small pi][small pi]* and 1n[small pi]* states, preventing or enhancing the PFR.}, keywords = {}, pubstate = {published}, tppubtype = {article} } A three-state model for the photo-Fries rearrangement (PFR) is proposed based on multiconfigurational calculations. It provides a comprehensive mechanistic picture of all steps of the reaction, from the photoabsorption to the final tautomerization. The three states participating in the PFR are an aromatic 1[small pi][small pi]*, which absorbs the radiation; a pre-dissociative 1n[small pi]*, which transfers the energy to the dissociative region; and a 1[small pi][sigma]*, along which dissociation occurs. The transfer from 1[small pi][small pi]* to 1n[small pi]* involves pyramidalization of the carbonyl carbon, while transfer from 1n[small pi]* to 1[small pi][sigma]* takes place through CO stretching. Different products are available after a conical intersection with the ground state. Among them is a recombined radical intermediate, which can yield ortho-PFR products after an intramolecular 1,3-H tunneling. The three-state model is developed for phenyl acetate, the basic prototype for the PFR, and it reconciles the theory with a series of observations from time-resolved spectroscopy. It also delivers a rational way to optimize PFR yields, since, as shown for four different systems, diverse substituents can change the energetic order of the 1[small pi][small pi]* and 1n[small pi]* states, preventing or enhancing the PFR. |
Claudia Climent; Mario Barbatti; Michael O Wolf; Christopher J Bardeen; David Casanova The photophysics of naphthalene dimers controlled by sulfur bridge oxidation In: Chem. Sci., 8 , pp. 4941-4950, 2017. @article{C7SC01285C, title = {The photophysics of naphthalene dimers controlled by sulfur bridge oxidation}, author = {Claudia Climent and Mario Barbatti and Michael O Wolf and Christopher J Bardeen and David Casanova}, url = {http://dx.doi.org/10.1039/C7SC01285C}, doi = {10.1039/C7SC01285C}, year = {2017}, date = {2017-01-01}, journal = {Chem. Sci.}, volume = {8}, pages = {4941-4950}, publisher = {The Royal Society of Chemistry}, abstract = {In this study we investigate in detail the photophysics of naphthalene dimers covalently linked by a sulfur atom. We explore and rationalize how the oxidation state of the sulfur-bridging atom directly influences the photoluminescence of the dimer by enhancing or depriving its radiative and non-radiative relaxation pathways. In particular, we discuss how oxidation controls the amount of electronic transfer between the naphthalene moieties and the participation of the SOn bridge in the low-lying electronic transitions. We identify the sulfur electron lone-pairs as crucial actors in the non-radiative decay of the excited sulfide and sulfoxide dimers, which are predicted to proceed via a conical intersection (CI). Concretely, two types of CI have been identified for these dimers, which are associated with the photo-induced pyramidal inversion and reverse fragmentation mechanisms found in aryl sulfoxide dimers. The obtained results and conclusions are general enough to be extrapolated to other sulfur-bridged conjugated dimers, therefore proportionating novel strategies in the design of strongly photoluminescent organic molecules with controlled charge transfer.}, keywords = {}, pubstate = {published}, tppubtype = {article} } In this study we investigate in detail the photophysics of naphthalene dimers covalently linked by a sulfur atom. We explore and rationalize how the oxidation state of the sulfur-bridging atom directly influences the photoluminescence of the dimer by enhancing or depriving its radiative and non-radiative relaxation pathways. In particular, we discuss how oxidation controls the amount of electronic transfer between the naphthalene moieties and the participation of the SOn bridge in the low-lying electronic transitions. We identify the sulfur electron lone-pairs as crucial actors in the non-radiative decay of the excited sulfide and sulfoxide dimers, which are predicted to proceed via a conical intersection (CI). Concretely, two types of CI have been identified for these dimers, which are associated with the photo-induced pyramidal inversion and reverse fragmentation mechanisms found in aryl sulfoxide dimers. The obtained results and conclusions are general enough to be extrapolated to other sulfur-bridged conjugated dimers, therefore proportionating novel strategies in the design of strongly photoluminescent organic molecules with controlled charge transfer. |
Shuming Bai; Mario Barbatti On the decay of the triplet state of thionucleobases In: Phys. Chem. Chem. Phys., 19 , pp. 12674-12682, 2017. @article{C7CP02050C, title = {On the decay of the triplet state of thionucleobases}, author = {Shuming Bai and Mario Barbatti}, url = {http://dx.doi.org/10.1039/C7CP02050C}, doi = {10.1039/C7CP02050C}, year = {2017}, date = {2017-01-01}, journal = {Phys. Chem. Chem. Phys.}, volume = {19}, pages = {12674-12682}, publisher = {The Royal Society of Chemistry}, abstract = {Singlet oxygen production upon photosensitization plays a critical role in drugs based on thionucleobases. While for immunosuppressants its yield must be near zero, for phototherapeutic drugs it should be near the unity. In this work, we apply high-level quantum chemical modelling to investigate the decay of the triplet state of thionucleobases, a main determinant of the singlet oxygen yield. Working on CASPT2 optimizations of two prototypical thiothymines (2-thiothymine and 6-aza-2-thiothymine), we showed that the T1 state is characterized by two [small pi][small pi]* minima and by the intersection of T1 with the singlet ground state. On the basis of this topography, we propose a two-step mechanistic model, which, depending on the energetic balance between the two minima, may have as a determining step either a slow transition between minima or a faster intersystem crossing to S0. Chemical kinetics modelling, as well as simulations of the transient absorption spectra, confirmed that the two-step model can explain the experimental results available for both molecules. Moreover, through additional investigations of 2-thiocytosine and 6-thioguanine, we show that such a T1 topography is a common theme for nucleobases. We also discuss how the triplet-state topography may be used to control the singlet oxygen yield, aiming at different medical applications.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Singlet oxygen production upon photosensitization plays a critical role in drugs based on thionucleobases. While for immunosuppressants its yield must be near zero, for phototherapeutic drugs it should be near the unity. In this work, we apply high-level quantum chemical modelling to investigate the decay of the triplet state of thionucleobases, a main determinant of the singlet oxygen yield. Working on CASPT2 optimizations of two prototypical thiothymines (2-thiothymine and 6-aza-2-thiothymine), we showed that the T1 state is characterized by two [small pi][small pi]* minima and by the intersection of T1 with the singlet ground state. On the basis of this topography, we propose a two-step mechanistic model, which, depending on the energetic balance between the two minima, may have as a determining step either a slow transition between minima or a faster intersystem crossing to S0. Chemical kinetics modelling, as well as simulations of the transient absorption spectra, confirmed that the two-step model can explain the experimental results available for both molecules. Moreover, through additional investigations of 2-thiocytosine and 6-thioguanine, we show that such a T1 topography is a common theme for nucleobases. We also discuss how the triplet-state topography may be used to control the singlet oxygen yield, aiming at different medical applications. |
Joanna Jankowska; Mario Barbatti; Joanna Sadlej; Andrzej L Sobolewski In: Phys. Chem. Chem. Phys., 19 , pp. 5318-5325, 2017. @article{C6CP08545H, title = {Tailoring the Schiff base photoswitching - a non-adiabatic molecular dynamics study of substituent effect on excited state proton transfer}, author = {Joanna Jankowska and Mario Barbatti and Joanna Sadlej and Andrzej L Sobolewski}, url = {http://dx.doi.org/10.1039/C6CP08545H}, doi = {10.1039/C6CP08545H}, year = {2017}, date = {2017-01-01}, journal = {Phys. Chem. Chem. Phys.}, volume = {19}, pages = {5318-5325}, publisher = {The Royal Society of Chemistry}, abstract = {Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract considerable attention due to their possible role as ultrafast, efficient, and photostable molecular photoswitches. Here, by means of static potential energy profile scan and on-the-fly non-adiabatic dynamics simulations we study the photodeactivation process of a minimal-chromophore aromatic Schiff base, salicylidene methylamine (SMA), and its two derivatives 6-cyano-salicylidene methylamine (6-CN-SMA) and 3-hydroxy-salicylidene methylamine (3-OH-SMA). We show that the dominant character of the lowest excited singlet state - [small pi][small pi]* vs. n[small pi]* - plays a crucial role in the system's photophysics and controls the ESIPT efficiency. We also show that the relative alignment of the [small pi][small pi]* and n[small pi]* states may be controlled through chemical substitutions made to the aromatic ring of the Schiff-base molecule. We believe that our findings will improve the rational-design strategies employed for the ESIPT systems, especially in the context of their possible photoswitching.}, keywords = {}, pubstate = {published}, tppubtype = {article} } Small molecular systems exhibiting Excited State Intramolecular Proton Transfer (ESIPT) attract considerable attention due to their possible role as ultrafast, efficient, and photostable molecular photoswitches. Here, by means of static potential energy profile scan and on-the-fly non-adiabatic dynamics simulations we study the photodeactivation process of a minimal-chromophore aromatic Schiff base, salicylidene methylamine (SMA), and its two derivatives 6-cyano-salicylidene methylamine (6-CN-SMA) and 3-hydroxy-salicylidene methylamine (3-OH-SMA). We show that the dominant character of the lowest excited singlet state - [small pi][small pi]* vs. n[small pi]* - plays a crucial role in the system's photophysics and controls the ESIPT efficiency. We also show that the relative alignment of the [small pi][small pi]* and n[small pi]* states may be controlled through chemical substitutions made to the aromatic ring of the Schiff-base molecule. We believe that our findings will improve the rational-design strategies employed for the ESIPT systems, especially in the context of their possible photoswitching. |
