Wall Pressure and External Velocity Field Relation in Overexpanded Supersonic Jets

Abstract

Truncated ideal contour nozzles operating at offdesign conditions encounter offaxis loads for which the exact origin remains unclear. Although these loads are usually attributed to the nonaxisymmetric motion of the adaptation shock wave, the unsteady pressure field in the downstream separated region may also be of importance: as the distance from the nozzle’s structural attachment increases, small wall pressure fluctuations can generate undesired torque. Experiments are conducted on a truncated ideal contour nozzle operated at overexpanded conditions. The study investigates the unsteady characteristics of the pressure fluctuations in the downstream separation region and their links with the downstream developing jet flow dynamics. The analysis of the pressure fluctuations inside the nozzle reveals the existence of highly organized structures, both in time and in the azimuthal direction. It is shown that these organized fluctuations dominate the pressure signals in the separated region and that they correspond to an azimuthal mode responsible for the side-load generation. Moreover, it is shown that these specific pressure modes have a clear signature in the exiting jet flowfield. A discussion is given in the paper on the possible origins of these organized structures.