Examination of Mach wave coalescence in a Mach 3 jet flow

Prior acoustic measurements of the sound field produced by a laboratory-scale, Mach 3 jet flow [Baars et al., AIAA (2013); Fievet et al., AIAA (2016)] showed a discrepancy between the theoretical prediction that shocks should not form, based on effective Gol’dberg numbers, and the apparent observation of steepened waves. This suggests an additional mechanism relating to wave coalescence is responsible for increased wave steepening. To better understand this new mechanism, high framerate schlieren images of sound waves propagating from the post potential core of the same Mach 3 jet are studied. A shock detection algorithm is developed to isolate shock-like features in the images and along the propagation path, which are then used to determine occurrences of coalescence-induced steepening in the flow. Numerical models of coalescing waves using the Khokhlov–Zabolotskaya–Kuznetzov (KZK) equation are then leveraged to determine identifiable characteristics of coalescence events to improve detection. POD-based reduced-order representations of the isolated events are then studied in a Lagrangian frame. Ensemble averages of tracked events are used to identify common patterns for coalescence, which is believed to contribute to “crackle” noise in full-scale jet flows. [WAW is supported by the ARL:UT Chester M. McKinney Graduate Fellowship in Acoustics.]