A wavelet transform analysis applied to unsteady aspects of supersonic jet screech resonance

The multiple acoustic modes and shear layer instability waves which characterize a supersonic underexpanded rectangular jet are investigated experimentally via the Morlet wavelet transform. Because of its quasi-locality in both physical space and Fourier space, the wavelet transformation allows one to track the time evolution of the various scales in both acoustic and velocity fluctuation signals. Using this technique, it is demonstrated that multiple acoustic modes produced by the jet coexist and are not due to a mode switching mechanism. Unsteady screech tone modulation is observed, and a mechanism for its occurrence is proposed. (Author)