Characteristics of overexpanded nozzle flows in imposed oscillating condition

• Effect of imposed oscillation of pressure ratio is studied on nozzle flow structure.
• Flow structures are different in increasing and decreasing states during oscillation.
• The locations of shock structures show irreversible behaviors during oscillation.
• This behaviors are amplified at higher oscillation frequencies.

A computation study is performed to investigate the effect of imposed oscillation of nozzle pressure ratio (NPR) on the flow structure in a two-dimensional, non-axisymmetric supersonic converging–diverging nozzle. In this study, the overexpanded flow conditions are considered which are dominated by the shock-induced boundary-layer interaction and corresponding free shock separation. The computational results are well validated with the available experimental measurements. Results showed that the internal flow structure of the nozzle is dependent on the process of change of pressure ratio during the oscillation. Distinct flow structures are observed during increasing and decreasing processes of the change of pressure ratio even when the nozzle is at the same NPR. Irreversible behaviors in the locations of free shock separation, Mach stem, and the strength of Mach stem are observed at the same NPRs during this oscillation. However, the nozzle thrust performance does not show the same order of irreversibility as in the cases of shock structures. Further, the effect of oscillation frequency is explored on this irreversible behavior.