Upstream Sound Generation Mechanisms in Supersonic Impinging Jets*

A supersonic jet impinging on a flat plate is a fundamental problem with applications ranging from launch vehicles to cold spray manufacturing. The flow field is characterized by a self-sustaining aeroacoustic resonance between the downstream convecting shear layer structures and upstream travelling acoustic waves generated by their impingement. Despite extensive investigations into the feedback process, the precise mechanisms by which upstream travelling acoustic waves are generated remain unclear. This investigation aims to enhance our understanding of the generation of upstream acoustic waves upon jet impingement by employing two well-resolved numerical databases of Mach 1.27 and Mach 3.7 jets at different impingement heights. The databases are subsequently decomposed into their exact hydrodynamic, acoustic, and thermal components using Doak’s Momentum Potential Theory, allowing a straightforward examination of the downstream convecting and upstream radiating components of the feedback loop. By visualizing the hydrodynamic and acoustic components simultaneously, we gain insights into the sound generation mechanisms at the impingement zone. At both Mach numbers, upstream acoustic waves are generated due to the bifurcation and subsequent compression of a hydrodynamic structure between the plate and the preceding shear layer structure.