The variable pressure can be replaced by the sound speed c(r,t) since pressure can be obtained from the formula c^2=\gamma p/\rho. The following non-dimensional self-similar variables are introduced,Landau, L. D., & Lifshitz, E. M. (1987). Fluid mechanics. Translated from the Russian by JB Sykes and WH Reid. Course of Theoretical Physics, 6. Section 106, pp. 403-407.Sedov, L. I. (1993). Similarity and dimensional methods in mechanics. CRC press.
