A Simple Model for Mixing and Cooling in Cloud-Wind Interactions

We introduce a simple entropy-based formalism to characterize the role of mixing in pressure-balanced multiphase clouds and demonstrate example applications using enzo-e (magneto)hydrodynamic simulations. Under this formalism, the high-dimensional description of the system's state at a given time is simplified to the joint distribution of mass over pressure (P) and entropy (K = P rho (-gamma )). As a result, this approach provides a way to (empirically and analytically) quantify the impact of different initial conditions and sets of physics on the system evolution. We find that mixing predominantly alters the distribution along the K direction and illustrate how the formalism can be used to model mixing and cooling for fluid elements originating in the cloud. We further confirm and generalize a previously suggested criterion for cloud growth in the presence of radiative cooling and demonstrate that the shape of the cooling curve, particularly at the low-temperature end, can play an important role in controlling condensation. Moreover, we discuss the capacity of our approach to generalize such a criterion to apply to additional sets of physics and to build intuition for the impact of subtle higher-order effects not directly addressed by the criterion.

Automated summary

This study introduces a simple entropy-based formalism to characterize mixing in pressure-balanced multiphase clouds, using enzo-e (magneto)hydrodynamic simulations. The formalism simplifies the high-dimensional description of the system's state to the joint distribution of mass over pressure and entropy, providing a way to quantify the impact of different initial conditions and physics on system evolution. The approach can model mixing and cooling for fluid elements in the cloud and generalize the criteria for cloud growth in the presence of radiative cooling.