Abstract
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.
Highlights
Supersonic jets generate different types of noise including turbulent mixing noise, broadband shock-associated noise, and single wave discrete frequency screech tones.[1]
The Fast Fourier Transform (FFT) ignores the time-varying of the signal which may result in the missing of some characteristic frequencies, so as to incompletely obtain the frequency features of the signal during different pressure drop intervals
The results suggest that: 1) With the decrease of aspect ratio (AR), the screech components of jet noise become pure, the fundamental frequency increases, and the screech dominant interval moves towards the low pressure region
Summary
Supersonic jets generate different types of noise including turbulent mixing noise, broadband shock-associated noise, and single wave discrete frequency screech tones.[1]. Morlet wavelet transform is used by researchers in investigations on multiple acoustic modes and shear layer instability waves in supersonic under-expanded jets through rectangular nozzles. This approach revealed the co-existence of multiple acoustic modes, as well as indicated it is not the switching mechanism that results in this phenomenon.[37]. The FFT ignores the time-varying of the signal which may result in the missing of some characteristic frequencies, so as to incompletely obtain the frequency features of the signal during different pressure drop intervals It is not a judicious choice for analysis of free jets by FFT as they are non-stationary and non-linear.
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