Effect of rotational nonequilibrium on shock standoff distances in intermediate hypersonic range

The shock standoff distances for sphere in intermediate hypersonic flows (2.5-4.5 km/s) are calculated by CFD codes that account for (i) vibrational nonequilibrium assigning three distinct vibrational temperatures for each molecular species and (ii) rotational nonequilibrium. In this study, the effect of vibrational-translational relaxation time model over the shock standoff distance is first examined Next, the rotational collision number of N₂ is deduced from the available shock tube experimental data. At the same time, attempts are made to reproduce experimental data of both rotational and vibrational temperatures simultaneously by scaling the vibrational-translational relaxation time. Using these values, the shock standoff distances are calculated for the conditions which the conventional two-temperature model fails to reproduce the experimental data measured in a ballistic range. It is found that the use of vibrational-translational relaxation time model derived from the SSH theory slightly improves the agreement of the shock standoff distance. Inclusion of rotational nonequilibrium alone, however, gives lMe improvement If we assumed a larger collision number and employ the scaled vibrational-translational relaxation time besides the rotational nonequilibrium, then agreement is significantly improved This suggests the importance of vibrational- translational relaxation process over the shock standoff distance in this particular flow regime.