arXiv subjects

Wolf-Christian Müller

Publications and source records attributed to Wolf-Christian Müller.

21 records · Page 2Linked to original sources

arXiv2009

Nonlinear cascades in two-dimensional turbulent magnetoconvection

The dynamics of spectral transport in two-dimensional turbulent convection of electrically conducting fluids is studied by means of direct numerical simulations (DNS) in the frame of the magnetohydrodynamic (MHD) Boussinesq approximation. The system performs quasi-oscillations between two different regimes of small-scale turbulence: one dominated by nonlinear MHD interactions, the other governed by buoyancy forces. The self-excited change of turbulent states is reported here for the first time. The process is controlled by the ideal invariant cross-helicity, $H^\mathrm{C}=\int_S \mathrm{d}S \mathbf{v}\cdot\mathbf{b}$. The observations are explained by the interplay of convective driving with the nonlinear spectral transfer of total MHD energy and cross-helicity.

astro-ph.SR↗

arXiv2008

Probability distributions of turbulent energy

Probability density functions (PDFs) of scale-dependent energy fluctuations, $P[δE(\ell)]$, are studied in high-resolution direct numerical simulations of Navier-Stokes and incompressible magnetohydrodynamic (MHD) turbulence. MHD flows with and without a strong mean magnetic field are considered. For all three systems it is found that the PDFs of inertial range energy fluctuations exhibit self-similarity and monoscaling in agreement with recent solar-wind measurements [B. Hnat et al., Geophys. Res. Lett. 29(10), 86-1 (2002)]. Furthermore, the energy PDFs exhibit similarity over all scales of the turbulent system showing no substantial qualitative change of shape as the scale of the fluctuations varies. This is in contrast to the well-known behavior of PDFs of turbulent velocity fluctuations. In all three cases under consideration the $P[δE(\ell)]$ resemble Lévy-type gamma distributions $\sim Δ^{-1}\exp(-|δE|/Δ)|δE|^{-γ}$ The observed gamma distributions exhibit a scale-dependent width $Δ(\ell)$ and a system-dependent $γ$. The monoscaling property reflects the inertial-range scaling of the Elsässer-field fluctuations due to lacking Galilei invariance of $δE$. The appearance of Lévy distributions is made plausible by a simple model of energy transfer.

physics.flu-dyn↗

arXiv2007

Diffusion and dispersion of passive tracers: Navier-Stokes versus MHD turbulence

A comparison of turbulent diffusion and pair-dispersion in homogeneous, macroscopically isotropic Navier-Stokes (NS) and nonhelical magnetohydrodynamic (MHD) turbulence based on high-resolution direct numerical simulations is presented. Significant differences between MHD and NS systems are observed in the pair-dispersion properties, in particular a strong reduction of the separation velocity in MHD turbulence as compared to the NS case. It is shown that in MHD turbulence the average pair-dispersion is slowed down for $τ_\mathrm{d}\lesssim t\lesssim 10 τ_\mathrm{d}$, $τ_\mathrm{d}$ being the Kolmogorov time, due to the alignment of the relative Lagrangian tracer velocity with the local magnetic field. Significant differences in turbulent single-particle diffusion in NS and MHD turbulence are not detected. The fluid particle trajectories in the vicinity of the smallest dissipative structures are found to be characterisically different although these comparably rare events have a negligible influence on the statistics investigated in this work.

physics.plasm-ph↗