Mode-switching and nonlinear effects in supersonic jet noise

The paper employs STFT (Short Time Fourier Transform) to extract and analyze the noise components of a supersonic jet through four rectangular nozzles with different AR (Aspect Ratio) since it can capture and represent the evolution of screech and its harmonics more directly. The results indicate that the screech of jet noise becomes pure and the fundamental frequency increases as the AR is reduced, and the more noticeable dominant interval moves towards lower pressure regions. Various oscillation modes of the shock cells results in the different fundamental frequencies of screech in free jet noise. The results also suggest more complicated components of screech and harmonics in the jet through an AR=2 nozzle. In addition, no multi-frequency relation is found between the fundamental frequency and the harmonics, and a relatively long switching duration of oscillation modes is observed that may cause more complicated interactions between Mach waves, screech, harmonics, and shock cells. The AR1 nozzle is recommended in engineering applications, however, small scale resonance may be initiated due to its high screech frequency and result in structural damage. STFT is the better method in applications for fast identification of the signal components of jet noise