Computational analysis of characteristics of Mach wave sources in supersonic free-jets

For the prediction of acoustic waves from a rocket plume, source characteristics of Mach waves are investigated with using high resolution schemes. Four ideally-expanded supersonic jets, M=2.0 cold jet, M=3.0 cold jet, M=2.0 hot jet and M=3.0 hot jet, are computed and analyzed. With regard to computation, the seventh order weighted compact non-linear scheme and the tenth order compact scheme are used for the fluid analysis and near fields acoustics propagation, respectively. Source positions are investigated with the focused array method and the visualization of sources. The results of the focused array methods show that the high frequency sources are located at the upstream region, while the low frequency sources are located at the downstream region. In addition, Mach waves emitted from the high frequency sources are propagated with the large angle to the axis, while that from the low frequency sources are propagated with the small angle. Mach wave sources are located in the ambient supersonic region, in which fluid-velocity is higher than the sound speed of the ambient. The almost same features are obtained with using the visualization of sources. Then the normalization of the source frequency is investigated. Using the shear layer thickness and the velocity at the axis, we can normalize the source frequency of various supersonic jets well without the temperature effects. Finally our computational results show that the lowest frequency of Mach waves is determined by the ambient supersonic region length and the potential core length.