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Center for Nonlinear Studies |
There are two well-known hydrodynamic instabilities that develop on an interface between two fluids (or gases): Rayleigh-Taylor instability (RTI) driven by sustained acceleration (e.g., gravity) andimpulsively-driven (e.g., shock-accelrerated) Richtmyer-Meshkov instability (RMI). For both RTI and RMI, instability develops due to misalignment between the gradients of pressure and density. Recently it was discovered that a similar phenomenon can also develop in multiphase flow, where there is no macroscopic density interface. In our experiments, we observe an instability forming in gas of constant density (air) with an initially nonuniform seeding of small particles or droplets, so that the spatially averaged density of the medium is non-constant. We produce such non-uniform seeding with a laminar jet of the two-phase mixture (air+droplets) vertically injected into the test section of a shock tube (unseeded air) prior to shock arrival. After a planar shock wave passes through the test section, two counter-rotating vortices form in the plane normal to the axis of the jet. The physical mechanism of the instability we observe is peculiar to multiphase flow, where the shock acceleration causes the second (embedded) phase to move with respect to the embedding medium. With sufficient seeding concentration, this leads to entrainment of the embedding phase that acquires a relative velocity dependent on the properties of the initial seeding, resulting in vortex formation in the flow. This is an interesting new addition to the variety of other shock-driven phenomena we observe, including flow features forming due to shock focusing and "instant chaos" transition to turbulence.
Host: Bob Ecke