Investigation of Passive Flow Control Mechanisms on a Supersonic Shock-Shear Layer Interaction in a Multi-Stream Rectangular Nozzle

Within an experimental rectangular multi-aperture supersonic nozzle an undesirable vortex shedding instability forms at the trailing edge of a plate separating the core flow (Mach = 1.6) and bypass flow (M = 1.0). This instability generates large coherent vortical structures that strongly interact with the downstream recompression shock, causing it to oscillate and emit high frequency acoustic radiation that dominates both the near- and far-fields. Far-field acoustics and time-resolved shadowgraph photography are used to investigate a passive control parametric study aimed at diminishing the shedding phenomena, and thus attenuating the resulting acoustic emission. Interestingly, this study was motivated by an unexpected observation: a modification to the nozzle, made to accommodate a separate experiment, was found to attenuate the far-field acoustic peak associated with the vortex shedding. This reduction was further confirmed through time-resolved shadowgraph imaging, suggesting a potential passive control mechanism had emerged unintentionally. An extensive parametric study was conducted to examine the effects of acoustic resonance, passive actuator hole shape, and boundary layer bleeding as potential causes of the passive actuation effectiveness. It was found that the chief effect is the boundary layer bleeding, which disrupts the formation of the coherent Von Karman vortex street at the trailing edge of the dividing plate, eliminating the acoustic tone produced.