Christopher K. W. Tam

TL;DR: In this paper, a modification to the κ-e model is proposed to mimic the effect of a large density gradient when used to compute the mean flow and noise of very hot jets.

Abstract: It is known that the mixing or spreading rate of free mixing layers decreases with an increase in the convective Mach number of the flow. At supersonic convective Mach number the natural rate of mixing of the shear layers is very small. It is believed that the decrease in mixing rate is directly related to the decrease in the rate of growth of the instabilities of these flows. In an earlier study (Tarn & Hu 1989) it was found that inside a rectangular channel supersonic free shear layers can support two families of instability waves and two families of acoustic wave modes. In this paper the possibility of driving these normal acoustic wave modes into resonant instability by using a periodic Mach wave system is investigated. The Mach waves can be generated by wavy walls. By properly choosing the wavelength of the periodic Mach wave system mutual secular excitation of two selected acoustic wave modes can be achieved. In undergoing resonant instability, the acoustic modes are locked into mutual simultaneous forcing. The periodic Mach waves serve as a catalyst without actually being involved in energy transfer. The resonant instability process is analysed by the method of multiple scales. Numerical results indicate that by using wavy walls with an amplitude-to-wavelength ratio of 1½% it is possible to obtain a total spatial growth of e9 folds over a distance of ten channel heights. This offers reasonable promise for mixing enhancement. The results of a parametric study of the effects of flow Mach numbers, temperature ratio, shear-layer thickness, modal numbers as well as three-dimensional effects on the spatial growth rate of the resonant instability are reported and examined so as to provide basic information needed for future feasibility analysis.

TL;DR: In this article, the relationship between broadband shock associated noise and screech tones is examined and it is shown that from the spectral characteristics point of view, the screech tone may be regarded as a special case of broadband wave associated noise.

TL;DR: The spectral shape of combustion noise is the same as the similarity spectrum of the noise from the large turbulence structures of high-speed jets as discussed by the authors, regardless of the turbulence level in the combustion process, the equivalence ratio of the fuel or whether the fuel is a gas or a liquid as long as it is hydrocarbon.

TL;DR: In this article, it was found by direct numerical simulation that sound waves at high intensity can induce vortex shedding at the mouths of the resonators of an acoustic liner, which is a dominant dissipation mechanism of resonant acoustic liners.