Secondary Instabilities In Shock-inducedTransition To Turbulence

Abstract

Richtmyer-Meshkovinstability(RMI)occurswhereveradensitygradientisimpulsively accelerated, e.g., by a shock wave. Misalignment between pressure and density gradients leads to baroclinic production of vorticity, the latter resulting in formation of vortical structures after the shock wave passage. The vortex-dominated evolutionof theflow eventuallyleads to turbulence.In the processof RMI-induced transition to turbulence, several secondary instabilities could develop in the flow, driven, e.g., by shear (Kelvin-Helmholtz) or by density-pressure gradient misalignment (secondary baroclinic instability). The exact nature of the secondary instabilities has been the subject of some discussion in the literature, with different authors observing shear-induced and baroclinic secondary instabilities. To resolve the issue, we have undertaken an experimental study of a Mach 1.2 shockaccelerated column of heavy gas (sulfur hexafluoride) immersed in a lighter gas (air). For visualization and quantitative analysis purposes, we use planar laserinducedfluorescence of acetone tracer pre-mixedwith the heavy gas, which makes it possible to resolve the small-scale (down to 12 microns) structure of the flow. Our observations of the RMI-driven flow around the gas column show the presence of two apparently distinct secondary instabilities: instability inside the vortex cores as well as the instability along the outer edge of the primary vortex spirals of the heavy gas. The former is consistent with the reports of baroclinic instability, while the latter is likely shear-induced.