Two-phase azimuthal instability generated by a supersonic jet impinging on a granular bed

A persistent azimuthal pattern featuring alternate high and low concentration of ejecta emanating from the area where a supersonic jet (Mach 5.3) impinges on a bed of particles in a near-lunar vacuum condition is studied experimentally. Although this peculiar phenomenon has been documented in early studies motivated by extraterrestrial landing, the mechanism of this pattern is not clearly understood. Recently, a series of experiments were conducted at NASA Marshall Space Flight Center inside a 4.5 m vacuum chamber over a range of reduced ambient pressure. Experimental results show vibrant azimuthal patterns, which are clearest at low ambient pressure. The pattern is shown to be driven by the flow instability at low Reynolds number, despite the supersonic jet employed. The flow instability leads to azimuthal vortex lines, which expel ejecta into streaks between two neighboring vortex lines. A simple model is proposed to explain the observed phenomenon.