Models of coalescing Mach waves based on experiment and simulation

Turbulent structures in supersonic jet flows generate acoustic Mach waves, which have the potential to intersect and coalesce. The coalescence process has been proposed as a significant contributor to acoustic waveform steepening in the near field of a jet and consequently to the noise referred to as crackle [Baars et al., AIAA (2013); Fiévet et al., AIAA (2016)]. Simulations of intersecting waveforms using the Khokhlov-Zabolotskaya-Kuznetzov nonlinear parabolic wave equation demonstrate that coalescence does increase steepening, dependent on intersection angle, waveform duration, and geometrical spreading [Willis et al., AIAA (2022)]. To observe this experimentally, a spark source is used to generate high-amplitude waveforms inside an enclosure with side-by-side holes, thus producing intersecting waveforms. Schlieren images of the coalescing waves are analyzed using Proper Orthogonal Decomposition (POD) in translating coordinates to identify modal behavior. By isolating coalescence events in schlieren images of Mach waves emitted from a lab-scale Mach 3 jet flow, modal behavior can be compared between the simulation, simplified experiment, and actual turbulence-generated waveforms.