Hypersonic problem
 

What I am trying to do is to find the solution where pressure, temperature and Mach number are 8.1757 Pa, 227 K and 22.41 (freestream conditions). I started with Mach 2 and increased the Mach number using the same pressure and temperature. At Mach 8 the absolute pressure was limited to the minimum 1 in a number of cells and the Mach number at some point in the domain became 90. Why would this happen? I am running a 2D inviscid implicit density based model.

Thanks, Subramani