Staging behaviour in screeching elliptical jets

The staging behaviour of jet screech in an elliptical nozzle with with an aspect ratio AR = 2.0is examined across a range of pressure ratios. Acoustic measurements and high resolution imaging are used to assess the staging behaviour. Based on the frequency of the dominant screech tone, the jet staging behaviour can be separated into five distinct modes over the range of pressures studied. A statistical analysis of the schlieren images suggests that the first mode is varicose, the next three modes are flapping instabilities, and the final mode is helical in nature. A comparison of characteristic length scales in the jet is undertaken to explain the mode switches. A shift from the classical “weakest link” feedback model to an acoustic waveguide model, associated with a shift in characteristic length scale of the jet, is used to explain the discontinuous jumps in screech tone frequency. The results indicate that the different acoustic feedback mechanisms need not necessarily be associated with a particular instability mode shape.