Christopher David Tomkins

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 ...

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...

Vortex organization in a turbulent boundary layer overlying sparse roughness elements

Vortex organization in the outer layer of a turbulent boundary layer overlying sparse, hemispherical roughness elements is explored with two-component particle-image velocimetry (PIV) in multiple streamwise–wall-normal measurement planes downstream and between elements. The presence of sparse roughness elements causes a shortening of the streamwise length scale in the near-wall region. These measurements confirm that vortex packets exist in the outer layer of flow over rough walls, but that their organization is altered, and this is interpreted as the underlying cause of the length-scale reduction. In particular, the elements shed vortices which appear to align in the near-wall region, but are distinct from the packets. Further, it is observed that ejection events triggered in the element wakes are more intense compared to the ejection events in smooth wall. We speculate that this may initiate a self-sustaining mechanism leading to the formation of hairpin packets as a much more effective instability compared to those typical of smooth-wall turbulence.

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.

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.

HIGH RESOLUTION EXPERIMENTAL MEASUREMENTS OF RICHTMYER-MESHKOV TURBULENCE IN FLUID LAYERS AFTER RESHOCK USING SIMULTANEOUS PIV-PLIF

True ensemble‐averaged density self‐correlation and Reynolds stress turbulence statistics in a Richtmyer‐Meshkov unstable fluid layer after reshock are measured for the first time using simultaneous Particle‐Image Velocimetry (PIV)‐Planar Laser‐Induced Fluorescence (PLIF) diagnostic. These high‐quality experiments with advanced diagnostics provide important insights into the physics of RM turbulence that cannot be obtained using simple shadowgraphy or Schlieren diagnostics. The double peaked nature of the density self‐correlation and the asymmetric character of the Reynolds stress distributions are discussed. Error estimates and convergence rates for several turbulence quantities are also provided.

Planar velocity and scalar concentration measurements in shock-accelerated unstable fluid interfaces

We report applications of several high-speed photographic techniques to diagnose fluid instability and the onset of turbulence in an ongoing experimental study of the evolution of shock-accelerated, heavy-gas cylinders. Results are at Reynolds numbers well above that associated with the turbulent and mixing transitions. Recent developments in diagnostics enable high-resolution, planar (2D) measurements of velocity fields (using particle image velocimetry, or PIV) and scalar concentration (using planar laser-induced fluorescence, or PLIF). The purpose of this work is to understand the basic science of complex, shock-driven flows and to provide high-quality data for code validation and development. The combination of these high-speed optical methods, PIV and PLIF, is setting a new standard in validating large codes for fluid simulations. The PIV velocity measurements provide quantitative evidence of transition to turbulence. In the PIV technique, a frame transfer camera with a 1 ms separation is used to image flows illuminated by two 10 ns laser pulses. Individual particles in a seeded flow are tracked from frame to frame to produce a velocity field. Dynamic PLIF measurements of the concentration field are high-resolution, quantitative dynamic data that reveal finely detailed structure at several instances after shock passage. These structures include those associated with the incipient secondary instability and late-time transition. Multiple instances of the flow are captured using a single frame Apogee camera and laser pulses with 140 ?s spacing. We describe tradeoffs of diagnostic instrumentation to provide PLIF images.