Effect of Nozzle Geometry on Near-Field Modal Content of a Screeching Jet

The acoustic near-field of a supersonic underexpanded jet at Mj = 1.34 was studied exper- imentally. A near-field azimuthal microphone array was utilized to study the jet oscillation dynamics and infer the geometrical pattern of the upstream-traveling acoustic waves involved in the jet screech mechanism. Operation of the jet in isolation resulted in a lateral oscillation (mode B) with an acoustic field containing strong screech tones. Addition of a conical reflector of 60◦ cone half-angle changed the oscillation dynamics, with new mode E appearing promi- nently and coexisting with mode B. The cross-spectral densities at the distinct tone frequencies are utilized to discern the jet oscillation dynamics and screech frequency and wavelength pre- diction formulas are utilized to assess if mode E is an extension of previously observed screech modes.