Stretching of material lines in shock-accelerated gaseous flows (original) (raw)

Flow morphologies of two shock-accelerated unstable gas cylinders

Journal of Visualization, 2002

Our highly reproducible shock-tube experiments examine the interaction of two unstable, compressible gas cylind ers 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 permit s 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.

Effects of inclination angle on a shock-accelerated heavy gas column

WIT transactions on engineering sciences, 2015

When a shock encounters a multiphase interface at an oblique angle, threedimensional (3D) flow effects are produced. Experiments using advanced optical diagnostics seek to elucidate the 3D nature of the flow as it transitions to turbulence. Planar laser-induced fluorescence (PLIF) images capture the development of flow instabilities in a shock-accelerated heavy gas column. Early time images show the counter-rotating vortex pair (CRVP) associated with the Richtmyer-Meshkov instability (RMI) both for normal planar and for oblique shocks, with the cores of the vortex pair parallel to the axis of the original gas column. For the oblique case, a shear-driven Kelvin-Helmholtz instability (KHI) also develops along the axis of the column due to 3D vorticity deposition. The influence of inclination angle of the column with respect to the shock direction on this secondary instability and thus upon the fully 3D flow, is assessed. The 3D data collected in these experiments is essential to the validation of numerical codes predicting a range of problems from scramjets to supernovae.

Simultaneous density-field visualization and PIV of a shock-accelerated gas curtain

Experiments in Fluids, 2000

We describe a highly-detailed experimental characterization of the Richtmyer-Meshkov instability (the impulsively driven Rayleigh-Taylor instability) . In our experiment, a vertical curtain of heavy gas (SF 6 )¯ows into the test section of an air-®lled, horizontal shock tube. The instability evolves after a Mach 1.2 shock passes through the curtain. For visualization, we pre-mix the SF 6 with a small ($10 )5 ) volume fraction of sub-micron-sized glycol/water droplets. A horizontal section of the¯ow is illuminated by a light sheet produced by a combination of a customized, burst-mode Nd:YAG laser and a commercial pulsed laser. Three CCD cameras are employed in visualization. The``dynamic imaging camera'' images the entire test section, but does not detect the individual droplets. It produces a sequence of instantaneous images of local droplet concentration, which in the post-shock¯ow is proportional to density. The gas curtain is convected out of the test section about 1 ms after the shock passes through the curtain. A second camera images the initial conditions with high resolution, since the initial conditions vary from test to test. The third camera,``PIV camera,'' has a spatial resolution suf®cient to detect the individual droplets in the light sheet. Images from this camera are interrogated using Particle Image Velocimetry (PIV) to recover instantaneous snapshots of the velocity ®eld in a small (19´14 mm) ®eld of view. The ®delity of the¯ow-seeding technique for density-®eld acquisition and the reliability of the PIV technique are both quanti®ed in this paper. In combination with wide-®eld density data, PIV measurements give us additional physical insight into the evolution of the Richtmyer-Meshkov instability in a problem which serves as an excellent test case for general transition-to-turbulence studies.

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

Physics of Fluids, 2003

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 (SF 6 ) cylinders have an initial spanwise separation of S/D ͑where D is the cylinder diameter͒ 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/DϷ1.2 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/Dϭ1.5. A correlation-based ensemble-averaging procedure extracts the persistent character of the unstable flow structures, and permits decomposition of the concentration fields into mean ͑deterministic͒ and fluctuating ͑stochastic͒ components.

Shock bowing and vorticity dynamics during propagation into different transverse density profiles

Physica D: Nonlinear Phenomena, 2002

A 2D numerical investigation is presented of shock wave propagation into a gas whose density is modulated in the transverse direction across the width of a shock tube. These density modulations represent temperature distributions in which low density corresponds to high temperature gas and high density corresponds to low temperature gas. This work is motivated by recent shock-plasma experiments, and mechanisms to explain the experimentally observed shock "splitting" signatures are investigated. It is found that the shock splitting signatures are more pronounced when the shock wave is more strongly curved or bowed. This occurs as the depth of the initial density profile is increased. The gross features of the shock splitting signatures are relatively insensitive to variations in the shape of the initial density profile (into which the shock propagates). Several interesting features of vorticity production and evolution are also indicated.

Oblique shock interaction with a cylindrical density interface

WIT transactions on engineering sciences, 2015

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.

Numerical Investigation of a Shock Accelerated Heavy Gas Cylinder in the Self-Similar Regime

A detailed numerical simulation of a shock accelerated heavy gas (SF6) cylinder surrounded by air gas is presented. It is a simplified configuration of the more general shock-accelerated inhomogeneous flows which occur in a wide variety of astrophysical systems. From the snapshots of the time evolution of the gas cylinder, we find that the evolution of the shock accelerated gas cylinder is in some ways similar to the roll-ups of a vortex sheet for both roll up into a spiral and fall into a self-similar behavior. The systemic and meaningful analyses of the negative circulation, the center of vorticity and the vortex spacing are in a good agreement with results obtained from the prediction of vorticity dynamics. Unlike the mixing zone width in single-mode or multi-mode Richtmyer-Meshkov instability which doesn’t exist, a single power law of time owing to the bubble and spike fronts follow a power law of tθ with different power exponents, the normalized length of the shock accelerated gas cylinder follows a single power law with θ = 0.43 in its self-similar regime obtained from the numerical results.

Oblique shock interaction with a laminar cylindrical jet

2017

We present an experimental study of planar shock interaction with an initially cylindrical, diffuse density interface, where the angle α 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 α, 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.

Vortex Formation in a Shock-Accelerated Gas Induced by Particle Seeding

Physical Review Letters, 2011

An instability forms in gas of constant density (air) with an initial nonuniform seeding of small particles or droplets as a planar shock wave passes through the two-phase medium. The seeding nonuniformity is produced by vertical injection of a slow-moving jet of air premixed with glycol droplets or smoke particles into the test section of a shock tube, with the plane of the shock parallel to the axis of the jet. After the shock passage, two counterrotating vortices form in the plane normal to that axis. 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 initial seeding, resulting in vortex formation in the flow.

Shock layer instability near the Newtonian limit of hypervelocity flows

Physics of Fluids, 2001

The curved bow shock in hypersonic flow over a blunt body generates a shear layer with smoothly distributed vorticity. The vorticity magnitude is approximately proportional to the density ratio across the shock, which may be very large in hypervelocity flow, making the shear layer unstable. A computational study of the instability reveals that two distinct nonlinear growth mechanisms occur in such flows: First, the vortical structures formed in the layer move supersonically with respect to the flow beneath them and form shock waves that reflect from the body and reinforce the structures. Second, the structures deform the bow shock, forming triple points from which shear layers issue that feed the main shear layer. Significant differences exist between plane and axisymmetric flow. Particularly rapid growth is observed for free-stream disturbances with the wavelength approximately equal to the nose radius. The computational study indicates that the critical normal shock density ratio for which disturbances grow to large amplitudes within a few nose radii is approximately 14. This served as a guide to the design of a physical experiment in which a spherical projectile moves at high speed through propane or carbon dioxide gas. The experiment confirms the approximate value of the critical density ratio, as well as the features of the computed flows. Comparisons of calculations of perfect gas flows over a sphere with shadowgraphs of the projectile show very good agreement. The Newtonian theory of hypersonic flow, which applies at high density ratio, makes the assumption that the flow remains smooth. The results show that high density ratio also causes this assumption to fail.