Generation of Solenoidal Modes and Magnetic Fields in Turbulence Driven by Compressive Driving
Abstract
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 (B0) and the sonic Mach number (Ms). 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 B0, the level of solenoidal velocity component after saturation increases as Ms increases. A similar trend is observed for generation of magnetic field when B0 is small. Second, when we fix the value of Ms, HD and MHD simulations result in similar level of the solenoidal component when B0 0.2 (or Alfven Mach number of 5). However, the level increases when B0 0.2. Roughly speaking, the magnetic energy density after saturation is a linearly increasing function of B0 irrespective of Ms. 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 Ms and B0 yield larger quantity of the solenoidal velocity component.