Effects of Numerical Resolution on Simulated Cloud-Wind Interactions

TL;DR

The study addresses how numerical resolution affects cloud-wind interactions, a key uncertainty for modeling multiphase galactic outflows. It uses a suite of cloud-crushing simulations across adiabatic and radiative physics and across subsonic and supersonic winds to quantify how cloud survival and acceleration depend on resolution, revealing that no universal convergence threshold applies to all regimes. In radiative subsonic flows, mass growth and acceleration converge at roughly 4 cells per cloud radius, whereas in supersonic flows the dependence on resolution remains non-converged even at 48 cells per cloud radius, indicating that accurately capturing destruction may require far higher resolution than growth. The authors also present a simple ram-pressure–driven acceleration model that dominates early cloud velocities before mixing-driven acceleration becomes important, underscoring implications for subgrid models in cosmological simulations.

Abstract

Mixing by hydrodynamical instabilities plays a key role in cloud-wind interactions, causing cloud destruction in the adiabatic limit and facilitating cloud survival with efficient radiative cooling. However, the rate of mixing in numerical simulations is sensitive to the smallest resolved scale, and the relationship between resolution and cloud evolution is under-explored. Using a set of cloud-crushing simulations, we investigate the effects of numerical resolution on cloud survival and acceleration. Modeling both adiabatic and radiative cases, in a subsonic and supersonic wind, we find that cloud survival and velocity does depend on the numerical resolution, however, no single resolution requirement can be applied to all scenarios. In the radiative subsonic case, we find that mass growth and acceleration appear converged at only 4 cells per cloud radius. Conversely, in the supersonic regime, we see a clear dependence of cloud destruction and velocity on resolution that is not converged even at 48 cells per cloud radius, implying that accurately capturing cloud destruction may require higher resolution than capturing growth. We also present a simple model illustrating how ram pressure accelerates cool clouds at early times before mixing kicks in as an acceleration mechanism.