http://arxiv.org/abs/2001.05154
We perform numerical simulations of hydrodynamic (HD) and magnetohydrodynamic (MHD) turbulence driven by compressive driving to study generation of solenoidal velocity component and small-scale magnetic field. We mainly focus on the effects of mean magnetic field ($B_0$) and the sonic Mach number ($M_s$). We also consider two different driving schemes in terms of correlation timescale of forcing vectors: a finite-correlated driving and a delta-correlated driving. The former has a longer correlation timescale of forcing vectors, which is comparable to large-eddy turnover time, than the latter. Our findings are as follows. First, when we fix the value of $B_0$, the level of solenoidal velocity component after saturation increases as $M_s$ increases. A similar trend is observed for generation of magnetic field when $B_0$ is small. Second, when we fix the value of $M_s$, HD and MHD simulations result in similar level of the solenoidal component when $B_0$ $\lesssim$ 0.2 (or Alfven Mach number of $\sim$ 5). However, the level increases when $B_0$ $\gtrsim$ 0.2. Roughly speaking, the magnetic energy density after saturation is a linearly increasing function of $B_0$ irrespective of $M_s$. Third, generation of solenoidal velocity component is not sensitive to numerical resolution, but that of magnetic energy density is mildly sensitive. Lastly, when initial conditions are same, the finite-correlated driving always produces more solenoidal velocity and small-scale magnetic field components than the delta-correlated driving. We additionally analyze the vorticity equation to understand why higher $M_s$ and $B_0$ yield larger quantity of the solenoidal velocity component.
J. Lim, J. Cho and H. Yoon
Thu, 16 Jan 20
32/46
Comments: 20 pages, 23 figures, 1 table
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