Improvement of organic solar cell morphology and device operation due to controlled polymer aggregation in solution. - In: Joint Meeting of the DPG and EPS Condensed Matter Divisions together with the Statistical and Nonlinear Physics Division of the EPS and the Working Groups: Equal Opportunities, Industry and Business, Young DPG, Philosophy of Physics, (all DPG) EPS Young Minds, EPS History of Physics Group, (2018), CPP 46.35
Reversals in infinite-Prandtl-number Rayleigh-Bénard convection. - In: Physical review, ISSN 2470-0053, Bd. 98 (2018), 2, 023109, insges. 11 S.
Using direct numerical simulations, we study the statistical properties of reversals in two-dimensional Rayleigh-Bénard convection for infinite Prandtl number. We find that the large-scale circulation reverses irregularly, with the waiting time between two consecutive genuine reversals exhibiting a Poisson distribution on long timescales, while the interval between successive crossings on short timescales shows a power-law distribution. We observe that the vertical velocities near the sidewall and at the center show different statistical properties. The velocity near the sidewall shows a longer autocorrelation and 1/f2 power spectrum for a wide range of frequencies, compared to shorter autocorrelation and a narrower scaling range for the velocity at the center. The probability distribution of the velocity near the sidewall is bimodal, indicating a reversing velocity field. We also find that the dominant Fourier modes capture the dynamics at the sidewall and at the center very well. Moreover, we show a signature of weak intermittency in the fluctuations of velocity near the sidewall by computing temporal structure functions.
https://doi.org/10.1103/PhysRevE.98.023109
Turbulent magnetohydrodynamic flow in a square duct: comparison of zero and finite magnetic Reynolds number cases. - In: Physical review fluids, ISSN 2469-990X, Bd. 3 (2018), 8, 083701, insges. 23 S.
Three-dimensional turbulent magnetohydrodynamic flow in a duct with a square cross section and insulating walls is investigated by direct numerical simulations. The flow evolves in the presence of a uniform vertical magnetic field and is driven by an applied mean pressure gradient. A boundary element technique is applied to treat the magnetic field boundary conditions at the walls consistently. Our primary focus is on the large- and small-scale characteristics of turbulence in the regime of moderate magnetic Reynolds numbers up to Rm ˜ 10^2 and a comparison of the simulations with the quasistatic limit at Rm = 0. The present simulations demonstrate that differences to the quasistatic case arise for the accessible magnetic Prandtl number Pm ˜ 10^-2 and different Hartmann numbers up to Ha = 43.5. Hartmann and Shercliff layers at the duct walls are affected differently when a dynamical coupling to secondary magnetic fields is present. This becomes manifest by the comparison of the mean streamwise velocity profiles as well as the skin friction coefficients. While large-scale properties change only moderately, the impact on small-scale statistics is much stronger as quantified by an analysis of local anisotropy based on velocity derivatives. The small-scale anisotropy is found to increase at moderate Rm. These differences can be attributed to the additional physical phenomena which are present when secondary magnetic fields evolve, such as the expulsion of magnetic flux in the bulk of the duct or the presence of turbulent electromotive forces.
https://doi.org/10.1103/PhysRevFluids.3.083701
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), R2, S. R2-1-R2-12
Three-dimensional turbulent magnetoconvection at a Rayleigh number of Ra 10^7 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