Mixing matters
  All hydrodynamical simulations of turbulent astrophysical phenomena require
sub-grid scale models to properly treat energy dissipation and metal mixing. We
present the first implementation and application of an anisotropic eddy
viscosity and metal mixing model in Lagrangian astrophysical simulations,
including a dynamic procedure for the model parameter. We compare these two
models directly to the common Smagorinsky and dynamic variant. Using the
mesh-free finite mass method as an example, we show that the anisotropic model
is best able to reproduce the proper Kolmogorov inertial range scaling in
homogeneous, isotropic turbulence. Additionally, we provide a method to
calibrate the metal mixing rate that ensures numerical convergence. In our
first application to cosmological simulations, we find that all models strongly
impact the early evolution of galaxies leading to differences in enrichment and
thermodynamic histories. The anisotropic model has the strongest impact, with
little difference between the dynamic and the constant-coefficient variant. We
also find that the metal distribution functions in the circumgalactic gas are
significantly tighter at all redshifts, with the anisotropic model providing
the tightest distributions. This is contrary to a recent study that found metal
mixing to be relatively unimportant on cosmological scales. In all of our
experiments the constant-coefficient Smagorinsky and anisotropic models rivaled
their dynamic counterparts, suggesting that the computationally inexpensive
constant-coefficient models are viable alternatives in cosmological contexts.
