A comparison of shock-cloud and wind-cloud interactions: effect of increased cloud density contrast on cloud evolution

Abstract

The similarities, or otherwise, of a shock or wind interacting with a cloud of density contras t$ χ= 10$ were explored in a previous paper. Here, we investigate such interactions with clouds of higher density contrast. We compare the adiabatic hydrodynamic interaction of a Mach 10 shock with a spherical cloud of $χ= 10^{3}$ with that of a cloud embedded in a wind with identical parameters to the post-shock flow. We find that initially there are only minor morphological differences between the shock-cloud and wind-cloud interactions, compared to when $χ= 10$. However, once the transmitted shock exits the cloud, the development of a turbulent wake and fragmentation of the cloud differs between the two simulations. On increasing the wind Mach number, we note the development of a thin, smooth tail of cloud material, which is then disrupted by the fragmentation of the cloud core and subsequent `mass-loading' of the flow. We find that the normalized cloud mixing time ($t_{mix}$) is shorter at higher $χ$. However, a strong Mach number dependence on tmix and the normalized cloud drag time, $t'_{drag}$, is not observed. Mach-number-dependent values of $t_{mix}$ and $t'_{drag}$ from comparable shock-cloud interactions converge towards the Mach-number-independent time-scales of the wind-cloud simulations. We find that high $χ$ clouds can be accelerated up to 80-90 per cent of the wind velocity and travel large distances before being significantly mixed. However, complete mixing is not achieved in our simulations and at late times the flow remains perturbed.