Dynamic Localised Turbulent Diffusion and its Impact on the Galactic
  Ecosystem
  Modelling the turbulent diffusion of thermal energy, momentum, and metals is
required in all galaxy evolution simulations due to the ubiquity of turbulence
in galactic environments. The most commonly employed diffusion model, the
Smagorinsky model, is known to be over-diffusive due to its strong dependence
on the fluid velocity shear. We present a method for dynamically calculating a
more accurate, locally appropriate, turbulent diffusivity: the dynamic
localised Smagorinsky model. We investigate a set of standard
astrophysically-relevant hydrodynamical tests, and demonstrate that the dynamic
model curbs over-diffusion in non-turbulent shear flows and improves the
density contrast in our driven turbulence experiments. In galactic discs, we
find that the dynamic model maintains the stability of the disc by preventing
excessive angular momentum transport, and increases the metal-mixing timescale
in the interstellar medium. In both our isolated Milky Way-like galaxies and
cosmological simulations, we find that the interstellar and circumgalactic
media are particularly sensitive to the treatment of turbulent diffusion. We
also examined the global gas enrichment fractions in our cosmological
simulations, to gauge the potential effect on the formation sites and
population statistics of Population III stars and supermassive black holes,
since they are theorised to be sensitive to the metallicity of the gas out of
which they form. The dynamic model is, however, not for galaxy evolution
studies only. It can be applied to all astrophysical hydrodynamics simulations,
including those modelling stellar interiors, planetary formation, and star
formation.
