Ceci n'est pas une Couche de Mélange: The Meaning of Resolved Turbulent Radiative Mixing

Abstract: Turbulent Radiative Mixing Layers (TRMLs) are of fundamental importance to the transport of energy and momentum in multi-phase, astrophysical fluids. We use measurements of the "micro" and "macro" properties of these layers in high-resolution \texttt{AthenaK} simulations to investigate when their properties can be considered \textit{well}-resolved. In particular, we demonstrate that the previously noticed resolution independence of total cooling, $\dot{E}{\rm cool}$, in these simulations is due to a remarkable, and perhaps fortuitous, cancellation of the countervailing effects of numerical dissipation and numerical viscosity. This calls into question the degree to which we can trust the results of these experiments, as there is no physical picture that explains this cancellation. We also demonstrate that in order to correctly resolve the phase structure in these layers, important for accurate predictions of their observable properties, one must resolve the scale on which turbulent diffusion acts on time-scales comparable to the cooling time. This "turbulent Field length", $λ{\rm F,turb}$, is where the eddy turnover time is equal to the cooling time ($t_{\rm eddy}(λ{\rm F,turb}) = t{\rm cool}$). We demonstrate that resolving this scale results in converged phase-structure and spatially resolved transitions in the gas phases.