We present an experimental study of planar shock interaction with an initially cylindrical, diffuse density interface, where the angle a between the plane of the shock and the axis of the cylinder can be zero (planar normal interaction) or non-zero (oblique interaction). The interface is formed by injecting a laminar jet of a heavy gas mixture (sulfur hexafluoride, acetone, nitrogen) into quiescent air. The jet is stabilized by an annular co-flow of air to minimize diffusion. Interaction between the pressure gradient (shock front) and density gradient leads to vorticity deposition, and during the subsequent evolution, the flow undergoes mixing (injected material - air) and eventually transitions to turbulence. Several parameters affect this evolution, including the angle a, the Atwood number (density ratio), and the Mach number of the shock. For quantitative and qualitative characterization of the influence of these parameters, we use flow visualization in two planes that relies on planar laser-induced fluorescence (PLIF) in acetone, which forms part of the injected material.

We consider two cases of interaction between a planar shock and a cylindrical density interface. In the first case (planar normal shock), the axis of the gas cylinder is parallel to the shock front, and baroclinic vorticity deposited by the shock is predominantly two-dimensional (directed along the axis of the cylinder). In the second case, the cylinder is tilted, resulting in an oblique shock interaction, and a fully three-dimensional shock-induced vorticity field. The statistical properties of the flow for both cases are analyzed based on images from two orthogonal visualization planes, using structure functions of the intensity maps of fluorescent tracer pre-mixed with the heavy gas. At later times, these structure functions exhibit power-law-like behavior over a considerable range of scales. Manifestation of this behavior is remarkably consistent in terms of dimensionless time defined based on Richtmyer's linear theory within the range of Mach numbers from 1.1 to 2.0 and the range of gas cylinder tilt angles with respect to the plane of the shock front (0 to 30 degrees).

An experimental study examines shock acceleration with an initially diffuse cylindrical column of sulfur hexafluoride surrounded by air and inclined with respect to the shock front. Three-dimensional vorticity deposition produces flow patterns whose evolution is captured with planar laser-induced fluorescence in two planes. Both planes are parallel to the direction of the shock propagation. The first plane is vertical and passes through the axis of the column. The second visualization plane is normal to the first plane and passes through the centerline of the shock tube. Vortex formation in the vertical and centerline planes is initially characterized by different rates and morphologies due to differences in initial vorticity deposition. In the vertical plane, the vortex structure manifests a periodicity that varies with Mach number. The dominant wavelength in the vertical plane can be related to the geometry and compressibility of the initial conditions. At later times, the vortex interaction produces a complex and irregular three-dimensional pattern suggesting transition to turbulence. Highly repeatable experimental data are presented for Mach numbers 1.13, 1.4, 1.7, and 2.0 at column incline angles of 0\(^{\circ }\), 20\(^{\circ }\), and 30\(^{\circ }\) for about 50 nominal cylinder diameters (30 cm) of downstream travel.

A cylindrical, initially diffuse density interface is formed by injecting a laminar jet of heavy gas into the test section of a shock tube. The injected gas is mixed with a fluorescent gaseous tracer, small liquid droplets, or smoke particles. The shock tube is tilted with respect to the horizontal. Thus the axis of the gravitystabilized heavy gas jet is at an oblique angle with the plane of the arriving shock front. The flow structure forming after the oblique shock wave interaction with the column of heavy gas is revealed by visualization in multiple planes. We observe the formation of the well-known counter-rotating vortex columns (same as caused by normal shock waves). However, along with them, periodic co-rotating vortices form in the vertical plane in the flow downstream of the oblique shock. The size of these vortices varies both with the Mach number and with the initial angle between the column and the shock front.

Both Rayleigh-Taylor instability (RTI [1, 2]) and Richtmyer-Meshkov instability (RMI [3, 4]) develop on a fluid (or gaseous) density interface undergoing sustained (RTI) or impulsive (RMI) acceleration. Misalignment between pressure and density gradients (baroclinicity) leads to vorticity production, and thus to interface perturbation, vortex formation, onset of secondary instabilities, and ultimately to turbulence

Richtmyer–Meshkov instability (RMI) has long been the subject of interest for analytical, numerical, and experimental studies. In comparing results of experiment with numerics, it is important to understand the limitations of experimental techniques inherent in the chosen method(s) of data acquisition. We discuss results of an experiment where a laminar, gravity-driven column of heavy gas is injected into surrounding light gas and accelerated by a planar shock. A popular and well-studied method of flow visualization (using glycol droplet tracers) does not produce a flow pattern that matches the numerical model of the same conditions, while revealing the primary feature of the flow developing after shock acceleration: the pair of counter-rotating vortex columns. However, visualization using fluorescent gaseous tracer confirms the presence of features suggested by the numerics; in particular, a central spike formed due to shock focusing in the heavy-gas column. Moreover, the streamwise growth rate of the spike appears to exhibit the same scaling with Mach number as that of the counter-rotating vortex pair (CRVP).

This work presents an experimental and numerical consideration of the flow developing after a planar shock wave accelerates a cylindrical column of heavy gas seeded with glycol droplets and surrounded by lighter unseeded gas. Special consideration is given to the formation of the gas column, and it is shown that careful modeling of the initial conditions and the tracer behavior is required to accurately match experimental results. Numerical analysis of the instability formation was performed using the Eulerian hydrodynamics code SHAMRC (second-order hydrodynamic automatic mesh refinement code), while the commercial CFD code FLUENT was used to model the formation of the initial conditions used in experiments. Experimental images and numerical results are presented for Mach numbers ranging from 1.2 to 2.1.

It was recently observed that vortices develop in multiphase media of non-uniform average density undergoing acceleration.This vortex formation is somewhat akin to vortex roll-up in gases of fluids (single-phase) due to Rayleigh-Taylor or Richtmyer-Meshkov instabilities.Differences in the underlying physics are negligible in the case of sustained modest acceleration (Rayleigh-Taylor instability), where conservation of momentum accounts for different velocities of volumes with different average densities.In the case of impulsive acceleration, the mechanism responsible for the multiphase analog of Richtmyer-Meshkov instability is peculiar to multiphase flow and is explained by post-acceleration interaction of the embedded phase (e.g., droplets) with the embedding phase (e.g., gas).Impulsive acceleration of a multiphase medium can also produce spatial rearrangement of the embedded particles or droplets in accordance with their size, noticeably altering the observed flow morphology.A careful numerical simulation explicitly accounting for the embedded phase behavior is required to faithfully reproduce the experimental results.

AbstractFlow past a circular cylinder executing sinusoidal rotary oscillations about its own axis is studied experimentally. The experiments are carried out at a Reynolds number of 185, oscillation amplitudes varying from $\mathrm{\pi} / 8$ to $\mathrm{\pi} $, and at non-dimensional forcing frequencies (ratio of the cylinder oscillation frequency to the vortex-shedding frequency from a stationary cylinder) varying from 0 to 5. The diagnostic is performed by extensive flow visualization using the hydrogen bubble technique, hot-wire anemometry and particle-image velocimetry. The wake structures are related to the velocity spectra at various forcing parameters and downstream distances. It is found that the phenomenon of lock-on occurs in a forcing frequency range which depends not only on the amplitude of oscillation but also the downstream location from the cylinder. The experimentally measured lock-on diagram in the forcing amplitude and frequency plane at various downstream locations ranging from 2 to 23 diameters is presented. The far-field wake decouples, after the lock-on at higher forcing frequencies and behaves more like a regular Bénard–von Kármán vortex street from a stationary cylinder with vortex-shedding frequency mostly lower than that from a stationary cylinder. The dependence of circulation values of the shed vortices on the forcing frequency reveals a decay character independent of forcing amplitude beyond forcing frequency of ${\sim }1. 0$ and a scaling behaviour with forcing amplitude at forcing frequencies ${\leq }1. 0$. The flow visualizations reveal that the far-field wake becomes two-dimensional (planar) near the forcing frequencies where the circulation of the shed vortices becomes maximum and strong three-dimensional flow is generated as mode shape changes in certain forcing parameter conditions. It is also found from flow visualizations that even at higher Reynolds number of 400, forcing the cylinder at forcing amplitudes of $\mathrm{\pi} / 4$ and $\mathrm{\pi} / 2$ can make the flow field two-dimensional at forcing frequencies greater than ${\sim }2. 5$.

The behavior of respirable particles being swept off a surface by the passage of a shock wave presents an interesting but little-studied problem. This problem has wide-ranging applications, from military to aerospace, and is being studied both numerically and experimentally. Here, we describe how a shock tube facility was modified to provide a dependable platform for such a study, with highly repeatable and well-characterized initial conditions. During the experiments, particle size distribution, surface chemical composition (that determines adhesion force between the particles and the surface), and the Mach number are closely controlled. Time-resolved visualization of the particle cloud forming after the shock passage provides insights into the physics of the flow, including the effect of the adhesion force on the growth of the cloud.

A Richtmyer-Meshkov Instability (RMI) [1, 2] is generated when an interface between two different fluids is impulsively accelerated. The instability develops due to misalignment of the density and pressure interfaces. This misalignment results in the deposition of vorticity, causing the formation of an instability that grows nonlinearly with time and eventually may transition to fully turbulent flow. It has been recently shown that a similar class of instability can evolve in a multi-phase flow [3], where the density gradient is caused by a second, non-fluid phase.

We present an experimental and numerical study of post-shock evolution of gas initially seeded with small droplets or particles. In two-phase media with gas being the embedding phase occupying most of the volume, shock acceleration can lead to vortex formation. The physical mechanism responsible for the vorticity deposition in this case is different from that of Richtmyer-Meshkov instability that would emerge on a gas-gas density interface. After the shock passage, the particles or droplets lag behind the surrounding gas. Momentum exchange between the embedded phase and the embedding phase leads to non-uniform local equilibrium velocity distribution, and thus to shear and vortex formation. Here we investigate shock interaction with a cylindrical particle-seeded column (with and without reshock).

Flow past two uniformly rotating cylinders with the same rotation rates in a side-by-side configuration is studied experimentally. The experiments are carried out at Reynolds numbers, Re, of 100, 200, 300, 400, and 500 and nondimensional rotation rates, α, varying from 0 to 5. The spacing ratios, T/D, are 1.8, 2.5, 4.0, and 7.5. Two possibilities of rotations are considered with the cylinder surfaces in between the two cylinders moving upstream in one case (inward rotation case) and downstream in the other (outward rotation case). The diagnostics is done by flow visualization using hydrogen bubble technique and quantitative measurements using particle image velocimetry (PIV). We present, using extensive flow visualization, the global view of the wake structure at Re of 200 for various rotation rates, and two senses. Vortex shedding suppression is studied through flow visualization and/or PIV at various Re’s, T/D’s, and two senses. Vortex shedding is found to be suppressed in the inward rotation cases at all Re and T/D’s. The value of α corresponding to vortex shedding suppression, αs, in the inward rotation case is ∼2.0 for Re of 200–500 at all T/D’s. The value of αs for Re of 100 in the case of inward rotation shows an increasing trend with T/D from T/D=1.8 to 4.0 with αs changing from 1.2 to 1.7; further increase of T/D does not change αs. For outward rotation cases, vortex shedding suppression is clearly observed for Re of 100 and for all values of T/D; however, for higher Re, vortex shedding suppression is observed for T/D of 4.0 and 7.5 only. The measurements of αs in this case showed a decreasing trend with increasing T/D. Symmetry breaking is reported for inward rotation case near α=1.35 for T/D=2.5 at Re of 200 where the wake pattern changes from in-phase to antiphase mode.