P. Vorobieff

Experimental observations of the mixing transition in a shock-accelerated gas curtain

Richtmyer–Meshkov instability of a thin curtain of heavy gas (SF6) embedded in air and accelerated by a planar shock wave (Mach 1.2) leads to the growth of interfacial perturbations in the curtain and to mixing. Our experiments produce a phenomenological description of the mixing transition and incipient turbulence during the first millisecond after the shock interaction. Growth of scales both larger and smaller than that of initial perturbations is visually observed and quantified by applying a wavelet transform to laser-sheet images of the evolving gas curtain. Histogram and wavelet analyses show an abrupt mixing transition for a multimode initial perturbation that is not apparent for single-mode perturbations.

Evolution of a shock-accelerated thin fluid layer

Multi-exposure flow visualization experiments with laser-sheet illumination provide growth-rate measurement of Richtmyer–Meshkov instability of a thin, perturbed heavy-gas layer embedded in a lower-density gas and accelerated by a planar shock wave. The measurements clearly show the three-stage transition to turbulence of this gas-curtain instability and the single-event coexistence of the three primary flow morphologies discovered previously. The shock-induced circulation for each event is estimated using a simple model based on Richtmyer–Meshkov instability and an infinite linear array of vortex points. These estimates are consistent with simplified flow simulations using a finite-core vortex-blob model.

A quantitative study of the interaction of two Richtmyer-Meshkov-unstable gas cylinders

We experimentally investigate the evolution and interaction of two Richtmyer–Meshkov-unstable gas cylinders using concentration field visualization and particle image velocimetry. The heavy-gas (SF6) cylinders have an initial spanwise separation of S/D (where D is the cylinder diameter) and are simultaneously impacted by a planar, Mach 1.2 shock. The resulting flow morphologies are highly reproducible and highly sensitive to the initial separation, which is varied from S/D≈1.2 to 2.0. The effects of the cylinder–cylinder interaction are quantified using both visualization and high-resolution velocimetry. Vorticity fields reveal that a principal interaction effect is the weakening of the inner vortices of the system. We observe a nonlinear, threshold-type behavior of inner vortex formation around S/D=1.5. A correlation-based ensemble-averaging procedure extracts the persistent character of the unstable flow structures, and permits decomposition of the concentration fields into mean (deterministic) and fluc...

Flow Morphologies of Two Shock-accelerated Unstable Gas Cylinders

Our highly reproducible shock-tube experiments examine the interaction of two unstable, compressible gas cylinders accelerated by a planar shock wave. Flow visualization shows that the evolution of the double-cylinder flow morphologies is dominated by two counter-rotating vortex pairs, the strength and behavior of which are observed to be highly sensitive to the initial cylinder separation. Simulations of the flow based on idealized vortex dynamics predict grossly different morphologies than those observed experimentally, suggesting that interactions at early time weaken the inner vortices. A correlation-based ensemble averaging procedure permits decomposition of the concentration field into mean and fluctuating components, providing evidence that energy is transferred from the intermediate to the small scales at late time.

Two-dimensional simulation of a shock-accelerated gas cylinder

The Richtmyer-Meshkov Instability (RMI) arises when a density gradient in a fluid (gas) is subjected to an impulsive acceleration (e.g., due to a shock wave passage). The evolution of RMI is non-linear and hydrodynamically complex and hence is a very good test problem to validate numerical codes. In this paper, we present a two-dimensional numerical simulation of RMI-driven evolution of the flow produced by shock acceleration of a diffuse heavy gaseous cylinder embedded in lighter gas. The initial conditions employed in the simulation are a very close match to the initial conditions realised in a well-characterised experiment, facilitating a detailed quantitative comparison with experimental measurements, as well as with other simulations of the same experiment. Comparison of the late-time flow statistics between experiment and numerics elucidates the limitations inherently present in a two-dimensional simulation of a spatially three-dimensional flow, even if the large-scale flow structure is nominally two-dimensional.