Analysis of shock unsteadiness in a supersonic over-expanded planar nozzle

An experimental investigation has been conducted to study the unsteady nature of the shock structure in an over-expanded planar nozzle. The main emphasis was to investigate (i) the effect of resonance associated with laminar shock-wave/boundary-layer interactions on the unsteady characteristics of the wall pressure signals in the region of the interaction and, (ii) the change in these characteristics as the state of boundary layer undergoes transition to turbulence and a resonance free interaction. The tests were conducted in a Mach 2 planar nozzle with wall-half angle of 5.7∘. For almost all the NPR for which the nozzle is tested, the boundary layer state is seen to be in the laminar/transitional range and exhibits resonance for nozzle pressure ratios less than 2.3. The resonance tones associated with the laminar interactions result in a considerable increase in the length of the intermittent region. However, as the boundary layer state goes into the transitional regime and the resonance tones disappear, the length of intermittent region is seen to decrease significantly. A decrease in this length also causes the peak standard deviation values to decrease. Further a free interaction region is observed for separated nozzle flows irrespective of the NPR of operation for resonance free interactions.