Flowfield Establishment in Hypervelocity Shock-Wave/Boundary-Layer Interactions

Shock/boundary-layer interactions generated over double-wedge and double-cone models in high-enthalpy, hypersonic flows are known to be sensitive to the thermochemical state of the gas. In this study, the transient evolution of shock interactions and separated flow is examined in nitrogen and air freestream conditions with stagnation enthalpies ranging from 2 to 8 MJ/kg and Mach numbers from 4 to 7. The time-dependent flowfield and associated time scales required to reach mean values for both viscous and inviscid processes are investigated using fast-response thermocouples and high-speed schlieren and chemiluminescence imaging. In all cases, the oblique/bow-shock triple point is observed to propagate upstream to a mean location as the bow-shock standoff distance increases with time. For all freestream conditions, the triple point reaches a mean position in less time for the conical than the wedge flow. Distinct differences between nitrogen and air both in the evolution and mean flow features are observed in the highest-enthalpy test case. The triple-point establishment time is greater in nitrogen than air, corresponding to an increased bow-shock standoff distance, whereas the viscous establishment times in the region of peak heating are comparable. Although the observed dependence on freestream composition and enthalpy can be used to quantify the effect of thermochemistry, we note the time scales for viscous and inviscid processes are of the same order of magnitude for all conditions studied. Normalized establishment times of 2-8 are measured, in reasonable agreement with existing experimental data from surface gauges (6-11).