Structural excitation of SERNs during supersonic operation: A source of screech amplitude modulation

Abstract

This paper investigates the structural response of rectangular nozzles during their supersonic operation. The nozzles in this study are known as Single Expansion Ramp Nozzles (SERNs), with Throat Aspect Ratios (TARs) ranging from 6 to 9 and a designed Mach number of 1.5. Under and approaching the strongest off-design flow conditions, in both underexpanded and overexpanded flow regimes, periodic modulation of screech amplitude is observed. The nearfield pressure signature of the modulation phenomenon and its frequency characteristics show that its origin is the excitation of a structural mode of the nozzle wall and not a fluid–acoustic phenomenon, like screech with Strouhal number characteristics. The modulation behavior corresponds to the wide side of the converging–diverging channel, which forms the non-expanding wall of the SERN. The structures are stable when not under these flow conditions and thus lack this modulation.