Unsteady Characteristics of Mode Oscillation for Screeching Jets

Jet screech is the result of a resonant aeroacoustic feedback loop sustained by the intrinsic instability waves of the jet flow. Experimental study has been carried out at the University of Bristol for jet Mach number ranging from M = 1.1 - 1.4 to characterize the non-linear instability wave development in the shear layer of a round supersonic cold screeching jet. Near-field pressure fluctuations using a linear array of microphones on a traverse was used to measure the instability wave development in the jet shear layers. Time-frequency analysis was carried out to determine resonant nonlinear interaction between multiple instability modes that occur in screeching jets. Wavelet analysis was carried out to track the time evolution of the various scales for the near-field pressure fluctuations. Furthermore, the coexistence of the multiple acoustic modes from jet and unsteady screech tone modulation are studied. Stochastic analysis of the wavelet coefficients of the near-field pressure fluctuations are used to statistically represent the various scales and the screech tone. Persistence spectrum of the fluctuating pressure signal is used to visualise the distribution and intensity of the scales and screech tones over time. This experimental study contributes new data set and a wide variety of analysis to provide an insight into the unsteady characteristics of single and multiple screech tones.