Direct Measurements of Concentration and Velocity in the Richtmyer-Meshkov Instabilit

The development of the Richtmyer-Meshkov instability (RMI) is experimentally investigated in a vertical shock tube using a broadband initial condition imposed on an interface between a helium-acetone mixture and argon (Atwood number A ≈ 0.7). The shear layer used in the present work serves as a statistically repeatable, broadband initial condition to the RMI, and is accelerated by either an M = 1.6 or M = 2.2 planar shock wave. The development of the ensuing mixing layer is investigated using simultaneous planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV). PLIF images are processed to reveal the light-gas mole fraction, while PIV particle image pairs yield corresponding full-field velocity results. Field structure and distribution is explored through probability density functions (PDFs), and a decomposition is performed on concentration and velocity results to obtain a mean flow field and define fluctuations. Simultaneous concentration and velocity field measurements allow -- for the first time in this regime -- experimentally determined turbulence quantities such as Reynolds stresses, turbulent mass-flux velocities, and turbulent kinetic energy. We show that by the latest times the mixing layer has passed the turbulent threshold, and there is evidence of turbulent mixing occuring sooner for the higher Mach number case. Interface measurements show nonlinear growth with a power-law fit to the thickness data, and integral measurements of mixing layer thickness are proportional to threshold measurements. Spectral analysis demonstrates the emergence of an inertial range with a slope ∼ k--5/3 when considering both density and velocity effects in planar turbulent kinetic energy (TKE) measurements.