Kathy Prestridge

An experimental investigation of mixing mechanisms in shock-accelerated flow

An experimental investigation of mixing mechanisms in a shock-induced instability flow is described. We obtain quantitative two-dimensional maps of the heavy-gas (SF6) concentration using planar laser-induced fluorescence for the case of a shock- accelerated cylinder of heavy gas in air. The instantaneous scalar dissipation rate, or mixing rate, χ, is estimated experimentally for the first time in this type of flow, and used to identify the regions of most intense post-shock mixing and examine the underlying mechanisms. We observe instability growth in certain regions of the flow beginning at intermediate times. The mixing rate results show that while these unstable regions play a significant role in the mixing process, a large amount of mixing also occurs by mechanisms directly associated with the primary instability, including gradient intensification via the large-scale strain field in a particular non-turbulent region of the flow.

Simultaneous particle-image velocimetry–planar laser-induced fluorescence measurements of Richtmyer–Meshkov instability growth in a gas curtain with and without reshock

The structure of the concentration and velocity fields in a light-heavy-light fluid layer subjected to an impulsive acceleration by a shock wave (Richtmyer–Meshkov instability) is studied using simultaneous particle-image velocimetry and planar laser-induced fluorescence (PLIF) measurements (performed in such flows for the first time). The initial condition prior to shock impact is accurately characterized using calibrated PLIF measurements to enable comparisons of the evolving structure to numerical simulations. Experiments performed on a SF6 curtain in air (Atwood number, At=0.67), after single shock by a Mach 1.2 shock wave and reshock by the reflected wave, show that the reshock wave has a dramatic impact on the evolution of the unstable structure. After first shock and in the absence of reshock(s), the structure widths agree well with an analytical extension to the nonlinear point vortex model [J. W. Jacobs et al., “Nonlinear growth of the shock-accelerated instability of a thin fluid layer,” J. Flui...

A Mach number study of the Richtmyer–Meshkov instability in a varicose, heavy-gas curtain

A varicose-perturbed, thin, heavy-gas curtain is impulsively accelerated by a planar shock wave of varying strength and investigated experimentally using concentration field visualization. Experiments were performed with Mach 1.2 and 1.5 incident shock waves, acquiring images of the initial conditions, and 18 different times after shock interaction in each case. Repeatability of the initial conditions allows for visualization of flow feature development over time for both Mach numbers despite capturing only one dynamic, postshock image per run of the experiment. Good agreement between integral width experimental data and a mixing width model is demonstrated for early to intermediate times in the flow. Integral width growth rates for Mach 1.2 and 1.5 are shown to collapse using a scaling based upon the convection velocity of the curtain. The diffusion driven instantaneous mixing rate, χ, is also estimated and compared between experiments. Results from this gradient based metric show differences in mixing t...

Stretching of material lines in shock-accelerated gaseous flows

A Mach 1.2 planar shock wave impulsively accelerates one of five different configurations of heavy-gas (SF6) cylinders surrounded by lighter gas (air), producing one or more pairs of interacting vortex columns. The interaction of the columns is investigated with planar laser-induced fluorescence in the plane normal to the axes of the cylinders. For the first time, we experimentally measure the early time stretching rate (in the first 220μs after shock interaction before the development of secondary instabilities) of material lines in shock-accelerated gaseous flows resulting from the Richtmyer-Meshkov instability at Reynolds number ∼25000 and Schmidt number ∼1. The early time specific stretching rate exponent associated with the stretching of material lines is measured in these five configurations and compared with the numerical computations of Yang et al. [AIAA J. 31, 854 (1993)] in some similar configurations and time range. The stretching rate is found to depend on the configuration and orientation of ...

Experimental study of initial condition dependence on Richtmyer-Meshkov instability in the presence of reshock

We present an experimental study on the dependence of initial condition parameters, namely, the amplitude δ and wavenumber κ (κ = 2π/λ, where λ is the wavelength) of perturbations, on turbulence and mixing in shock-accelerated Richtmyer-Meshkov (R-M) unstable fluid layers. A single mode, membrane-free varicose heavy gas curtain (air-SF6-air) at a shock Mach number M = 1.2 was used in our experiments. The density (concentration) and velocity fields for this initial configuration were measured using planar laser -induced fluorescence (PLIF) and particle image velocimetry (PIV). In order to understand the effects of multi-mode initial conditions on shock-accelerated mixing, the evolving fluid interface formed during the incident shock (M = 1.2) was shocked again by a reflected shock wave at various times using a movable wall, thus enabling us to change both δ and κ simultaneously. A dimensionless length-scale defined as η = κδ is proposed to parametrically link the initial condition dependence to late-time m...

A Survey of High Explosive‐Induced Damage and Spall in Selected Metals Using Proton Radiography

Multiple spall and damage layers can be created in metal when the free surface reflects a Taylor wave generated by high explosives. These phenomena have been explored in different thicknesses of several metals (tantalum, copper, 6061 T6‐aluminum, and tin) using high‐energy proton radiography. Multiple images (up to 21) can be produced of the dynamic evolution of damaged material on the microsecond time scale with a <50 ns “shutter” time. Movies and multiframe still images of areal and (Abel inverted) volume densities are presented. An example of material that is likely melted on release (tin) is also presented.

Incident shock Mach number effects on Richtmyer-Meshkov mixing in a heavy gas layer

Experiments were performed at the horizontal shock tube facility at Los Alamos National Laboratory to study the effect of incident shock Mach number (M) on the development of Richtmyer-Meshkov instability after a shock wave impulsively accelerates a varicose-perturbed, heavy-gas curtain. Three cases of incident shock strength were experimentally investigated: M = 1.21, 1.36, and 1.50. We discuss the state of the mixing and the mechanisms that drive the mixing at both large and small scales by examining the time evolution of 2D density fields derived from quantitative planar laser-induced fluorescence measurements. Several differences in qualitative flow features are identified as a result of Mach number variation, and differences in vortex interaction, observed using particle image velocimetry, play a critical role in the development of the flow field. Several quantities, including mixing layer width, mixing layer area, interface length, instantaneous mixing rate, the density self-correlation parameter, p...

Experimental study of initial condition dependence on turbulent mixing in shock-accelerated Richtmyer–Meshkov fluid layers

Experimental evidence is needed to verify the hypothesis that the memory of initial conditions is retained at late times in variable density flows. If true, this presents an opportunity to “design” and “control” late-time turbulence, with an improved understanding in the prediction of inertial confinement fusion and other general fluid mixing processes. In this communication, an experimental and theoretical study on the effects of initial condition parameters, namely, the amplitude δ0 and wavenumber κ0 , where λ0 is the initial wavelength) of perturbations, on late-time turbulence and mixing in shock-driven Richtmyer–Meshkov (R-M) unstable fluid layers in a 2D plane is presented. Single and multi-mode membrane-free initial conditions in the form of a gas curtain having a light-heavy-light configuration (air-SF6-air) with an Atwood number of A= 0.57 were used in our experiments. A planar shock wave with a shock Mach number M= 1.21 drives the R-M instability, and the evolution of this instability after incident shock is captured using high resolution simultaneous planar laser induced fluorescence (PLIF) and particle image velocimetry (PIV) diagnostics. Time evolution of statistics such as amplitude of the mixing layer, 2D turbulent kinetic energy, Reynolds number, rms of velocity fluctuations, probability density functions, and density-specific volume correlation were observed to quantify the amount of mixing and understand the nature of turbulence in this flow. Based on these results, it was found that the R-M mixing layer is asymmetric and non-Boussinesq. There is a correlation between initial condition parameters and large-scale, and small-scale mixing at late times, indicating an initial condition dependence on R-M mixing.

Dependence of growth patterns and mixing width on initial conditions in Richtmyer?Meshkov unstable fluid layers

A preliminary investigation of the impact of initial modal composition on the mixing of a shocked, membraneless fluid layer is performed. The growth patterns that emerge upon the impulsive acceleration of three different initial conditions (varicose, sinuous and large-wavelength sinuous) by a Mach 1.2 shock wave are investigated using planar laser induced fluorescence (PLIF) in an air‐SF6‐air fluid layer. Time-series images of the flow evolution in each of these cases indicate the presence of concentrated regions of vorticity, with the intensity and stability of the resulting vortex configurations dictating the post-shock evolution. In the sinuous case, self advection of the nonuniformly spaced vortices generates a pattern of two streamwise separated regions of material concentration after first shock. However, upon reshock, substantial mixing occurs and results in a structure where the separated regions merge to create a density distribution with a single, broad plateau. This profile contrasts with the varicose case, in which the streamwise density profile is characterized by a narrow peak.

Mixing transition in a shocked variable-density flow

We measure two-dimensional velocity and density fluctuations in a shock-driven heavy gas curtain for three different incident Mach numbers (M = 1.21, 1.36, and 1.50) and a fixed initial perturbation. We study the time evolution of the velocity and density fields and observe two different mixing transitions in this unsteady flow. The first transition is caused by small-scale mixing in vortex cores, while the second transition is related to increased homogenization across the mixing layer and a drive towards isotropy. By measuring the anisotropy of the velocity fluctuations and the evolution of the turbulent kinetic energy, we are able to assess the anisotropy of the flow. For the first time in Richtmyer-Meshkov (RM) flows, we measure and compare turbulent length scales derived from both the density and velocity field measurements, and we find ratios of Liepmann-Taylor to inner-viscous scales (λL/λν) that are inconsistent with those found using Reynolds number scaling based on circulation, ReΓ, or based on ...