High-temperature stable single carrier hole only device based on conjugated polymers. - In: Journal of materials research, ISSN 2044-5326, Bd. 33 (2018), 13, S. 1860-1867
https://doi.org/10.1557/jmr.2018.203
Wall modes in magnetoconvection at high Hartmann numbers. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 849 (2018), S. R2-1-R2-12
Three-dimensional turbulent magnetoconvection at a Rayleigh number of Ra 107 in liquid gallium at a Prandtl number Pr 0.025 is studied in a closed square cell for very strong external vertical magnetic fields B0 in direct numerical simulations which apply the quasistatic approximation. As B0, or equivalently the Hartmann number Ha, are increased, the convection flow, which is highly turbulent in the absence of magnetic fields, crosses the Chandrasekhar linear stability limit for which thermal convection ceases in an infinitely extended layer and which can be assigned a critical Hartmann number Hac. Similar to rotating Rayleigh-Bénard convection, our simulations reveal subcritical sidewall modes that maintain a small but finite convective heat transfer for Ha > Hac. We report a detailed analysis of the complex two-layer structure of these wall modes, their extension into the cell interior, and a resulting sidewall boundary layer composition that is found to scale with the Shercliff layer thickness.
https://doi.org/10.1017/jfm.2018.479
Koopman analysis of the long-term evolution in a turbulent convection cell. - In: Journal of fluid mechanics, ISSN 1469-7645, Bd. 847 (2018), S. 735-767
We analyse the long-time evolution of the three-dimensional flow in a closed cubic turbulent Rayleigh-Bénard convection cell via a Koopman eigenfunction analysis. A data-driven basis derived from diffusion kernels known in machine learning is employed here to represent a regularized generator of the unitary Koopman group in the sense of a Galerkin approximation. The resulting Koopman eigenfunctions can be grouped into subsets in accordance with the discrete symmetries in a cubic box. In particular, a projection of the velocity field onto the first group of eigenfunctions reveals the four stable large-scale circulation (LSC) states in the convection cell. We recapture the preferential circulation rolls in diagonal corners and the short-term switching through roll states parallel to the side faces which have also been seen in other simulations and experiments. The diagonal macroscopic flow states can last as long as 1000 convective free-fall time units. In addition, we find that specific pairs of Koopman eigenfunctions in the secondary subset obey enhanced oscillatory fluctuations for particular stable diagonal states of the LSC. The corresponding velocity-field structures, such as corner vortices and swirls in the midplane, are also discussed via spatiotemporal reconstructions.
https://doi.org/10.1017/jfm.2018.297
Large eddy simulation of hydrodynamic and magnetohydrodynamic channel flows with a collocated finite-volume scheme and improved subgrid-scale modeling. - In: European journal of mechanics, ISSN 1873-7390, Bd. 72 (2018), S. 189-198
We study hydrodynamic and magnetohydrodynamic channel flows by means of large eddy simulations (LES) on the basis of a second-order finite-volume scheme in collocated variable arrangement with a focus on the impact of numerical diffusion on the subgrid-scale (SGS) modeling. It is found that a mixed SGS model, which is based on the velocity increment tensor and an eddy viscosity model, performs best and is able to capture near-wall regions of energy backscatter from small to larger scales. Thereby, it improves the accuracy of the LES computations significantly. Our studies suggest that the mixed SGS model is thus applicable for a wide class for shear flow problems in liquid metal flows where finite-volume methods with collocated grids are applied.
https://doi.org/10.1016/j.euromechflu.2018.05.008
Improvement of polymer:fullerene bulk heterojunction morphology via temperature and anti-solvent effect. - In: Synthetic metals, Bd. 243 (2018), S. 8-16
https://doi.org/10.1016/j.synthmet.2018.05.011
Turbulent superstructures in Rayleigh-Bénard convection. - In: Nature Communications, ISSN 2041-1723, Bd. 9 (2018), 2118, S. 1-11
https://doi.org/10.1038/s41467-018-04478-0
Complexity of viscous dissipation in turbulent thermal convection. - In: Physics of fluids, ISSN 1089-7666, Bd. 30 (2018), 3, 031702, insges. 5 S.
https://doi.org/10.1063/1.5022316
Eulerian and Lagrangian perspectives on turbulent superstructures in Rayleigh-Bénard convection. - In: NIC Symposium 2018, (2018), S. 421-428
Large-scale computations in combination with new mathematical analysis tools make studies of the large-scale patterns, which are termed turbulent superstructures, in extended turbulent convection flows now accessible. Here, we report recent analyses in the Eulerian and Lagrangian frames of reference that reveal the characteristic spatial and temporal scales of the patterns as a function of Prandtl number, the dimensionless number which relates momentum to temperature diffusion in the working fluid.
Instabilities and turbulence in magnetohydrodynamic duct flows. - In: NIC Symposium 2018, (2018), S. 389-396
Liquid metal flows in the presence of a uniform magnetic field experience electromagnetic induction. Eddy currents and associated Lorentz force density modify the flow and give rise to thin electromagnetic boundary Jayerson the walls of the channel or duct. We describe features of transition to turbulence in the sidewall jets that occur in a duct flow when the Hartmann walls perpendicular to the magnetic field are electrically conducting. We also examine the modification of the magnetic field and the turbulence in a duct with insulatingwalls in the case when the magnetic diffusion time is not small compared to the flow time scale.
Temperature-tuning of optical properties and molecular aggregation in AnE-PVstat copolymer solution. - In: The journal of physical chemistry, ISSN 1932-7455, Bd. 122 (2018), 7, S. 3965-3969
https://doi.org/10.1021/acs.jpcc.7b10709