Resolving high Reynolds numbers in SPH simulations of subsonic
  turbulence
  Accounting for the Reynolds number is critical in numerical simulations of
turbulence, particularly for subsonic flow. For Smoothed Particle Hydrodynamics
(SPH) with constant artificial viscosity coefficient alpha, it is shown that
the effective Reynolds number in the absence of explicit physical viscosity
terms scales linearly with the Mach number - compared to mesh schemes, where
the effective Reynolds number is largely independent of the flow velocity. As a
result, SPH simulations with alpha=1 will have low Reynolds numbers in the
subsonic regime compared to mesh codes, which may be insufficient to resolve
turbulent flow. This explains the failure of Bauer and Springel (2011,
arXiv:1109.4413v1) to find agreement between the moving-mesh code AREPO and the
GADGET SPH code on simulations of driven, subsonic (v ~ 0.3 c_s) turbulence
appropriate to the intergalactic/intracluster medium, where it was alleged that
SPH is somehow fundamentally incapable of producing a Kolmogorov-like turbulent
cascade. We show that turbulent flow with a Kolmogorov spectrum can be easily
recovered by employing standard methods for reducing alpha away from shocks.
