Amplitude and phase modulation of cavity modes in a supersonic flow

An experimental study of supersonic flow over a cavity is carried out. The study includes flow visualization and unsteady pressure measurements. From the flow visualization it is observed that the flow field is highly unsteady and that four types of waves are present in the flow field. Spectra of the unsteady pressure data display high amplitude cavity tones. The pressure data is analysed by means of the continuous wavelet transform. Wavelet analysis using the Morlet wavelet function shows that the cavity acoustic modes are both amplitude and phase modulated simultaneously in time. However it is observed that the amplitude of the modulations differs from one cavity mode to the other. The power spectral density of individual time series signals obtained from the wavelet transform indicates that the cavity modes modulate with multiple frequencies rather than at a single frequency. The dominant frequency of amplitude modulation of the cavity modes is the frequency of the dominant cavity mode. The temporal behaviour of phase modulation indicates that there exist non-linear interactions between the cavity modes.