Updated: Oct 11, 2019
Chevalier & Clegg (1985) [henceforth CC85] proposed a model for a steady thermalized hot wind expanding radially outward from a central starburst. This can be taken as the simplest model for superbubble feedback due to coalescing supernovae (Sharma et al. 2014), which results in a galactic outflow. One of the crucial problems in galactic outflows is whether cold clouds can survive in them. Recent plane-parallel wind tunnel simulations (Gronke & Oh 2018) suggest that this may be possible if the cloud size is sufficiently large. It is important to answer this question in a more realistic spherical wind. In past, such a spherically expanding wind has been modeled locally as a coordinate expansion (Scannapieco 2017; Gronke & Oh 2019). Mathematically, this is analogous to the use of comoving coordinates to account for Hubble (cosmological) expansion of the Universe. However, these works do not present the evolution equations explicitly. Moreover, we highlight that, unlike isotropic cosmological expansion, the local coordinate expansion due to a radial wind is anisotropic occurring only in the angular directions but not radially. The aim of this Note is to present the governing equations explicitly.