Chris Weber

Experimental validation of a Richtmyer-Meshkov scaling law over large density ratio and shock strength ranges

A universal scaling law for the Richtmyer–Meshkov instability is validated with experimental results covering a wide range of density ratios and shock strengths. These results include the first membraneless, gas-phase, interface experiments for A>0.5 and M>1.5. The shock-accelerated, sinusoidal interface experiments are conducted in a vertical shock tube with a large square cross section and cover the experimental parameter space: 0.29<A<0.95, 1.1<M<3, and 3.1×104<Re<1.4×107. Results provide growth-rate data for comparison with computational fluid dynamics simulation codes and verify the nondimensional time and amplitude parameters chosen for scaling are the correct ones. Correct scaling is obtained by including a growth-reduction factor that accounts for diffusion at the interface. Planar imaging techniques are used to diagnose the instability development for a nearly single-mode interface, and results are reported for eight scenarios (including three distinct gas pairs) that span the linear and nonlinea...

Experimental investigation of a twice-shocked spherical density inhomogeneity

Results are presented from a series of experiments investigating the behavior of a twice-shocked spherical density inhomogeneity. The experiments are performed at the Wisconsin Shock Tube Laboratorys (WiSTL) 9?m long vertical shock tube with a square internal cross section, 0.254?m on a side, equipped with a pneumatically retracting bubble injector. Baroclinic vorticity deposition occurs during the interaction of the shock wave with the spherical density inhomogeneity, leading to the formation of a vortex ring. The incident shock wave reflects from the tube end wall and interacts with the translating vortex ring (reshock), resulting in additional vorticity deposition. After reshock, the ambient particle velocity is zero and the subsequent translational motion of the vortex ring is due entirely to the net circulation present. Circulation models are investigated for the reshock scenario and a new model is developed and applied to both once-shocked and reshocked scenarios. Two Atwood numbers (A=0.17,?0.68) and three initial Mach numbers (M=1.35,?2.00,?2.33) are investigated. High-speed cameras at 10?000?fps are used to observe the development of the vortex ring after reshock. An understanding of the shock-induced compression and vortex generation is vital to the future study of turbulent combustion initiated by the shock focusing phenomenon.

Richtmyer-Meshkov Parameter Study

The growth of an interfacial perturbation after acceleration by a shock wave, known as the Richtmyer-Meshkov instability (RMI), plays an important role in the compression of an ICF target. Experiments studying the RMI are performed in a vertical shock tube by observing the growth of the interface between a pair of gases after acceleration by a planar shock wave. A near 2D, sinusoidal, membraneless interface is created in a shock tube by oscillating rectangular pistons at the stagnation plane between the two gases. The interface is visualized by seeding one of the gases with acetone, smoke, or atomized oil and observing the fluorescence or Mie scattering from a planar laser sheet. The results presented here span a range of Atwood numbers, 0.30<A<0.95, and shock wave strengths, 1.1<M<3. Numerical simulations of the experimental conditions are performed and compared with the experiments using the 2D hydrodynamics code Raptor (LLNL).

Diffusion of degenerate minority carriers in a p-type semiconductor

We report ultrafast transient-grating experiments on heavily p-type InP at 15 K. Our measurement reveals the dynamics and diffusion of photoexcited electrons and holes as a function of their density n in the range 2  ×  1016 to 6  ×  1017 cm−3. After the first few picoseconds, the grating decays primarily due to ambipolar diffusion. While, at low density, we observe a regime in which the ambipolar diffusion is electron-dominated and increases rapidly with n, it appears to saturate at 34 cm2/s at high n. We present a simple calculation that reproduces the main results of our measurements as well as of previously published measurements that had shown diffusion to be a flat or decreasing function of n. By accounting for effect of density on charge susceptibility, we show that, in p-type semiconductors, the regime we observe of increasing ambipolar diffusion is unique to heavy doping and low temperature, where both the holes and electrons are degenerate; in this regime, the electronic and ambipolar diffusion ...

Single-mode perturbation growth in an idealized spherical implosion

Abstract This paper considers a dense imploding spherical shell, where perturbations on the inner surface grow due to hydrodynamic instabilities, geometric convergence and compression. A low-convergence implosion with a single spherical harmonic mode perturbation with mode numbers in the range from l = 5 to l = 100 and at three different amplitudes is considered. The linear theory of Epstein [1] is extended to incorporate reshock, making it applicable to the full implosion process (while perturbations are linear). Linear theory is employed to choose modes such that quantified geometric convergence and compression effects contribute significantly to perturbation growth at the lower mode number, while at the higher mode number the contribution from Rayleigh–Taylor instability dominates. Simulation results from four independent simulation codes (FLASH, HYDRA, Miranda and Flamenco) are presented. The simulation predictions are validated against linear theory pre-reshock and employed to validate the extended theory across the reshock. The simulations continue to substantially non-linear perturbation amplitudes, beyond the limits of the analytical approach, and the presented perturbation amplitudes can inform future non-linear modeling. The simulated perturbation amplitudes agree to within approximately 10% for most cases, with isolated cases having differences of greater than 50% from the simulated ensemble-mean. This occurs in the case of a low mode number perturbation with a very small amplitude, where the growth of secondary small-scale instabilities leads to substantial differences between the codes after re-shock.

Experimental Shock-Initiated Combustion of a Spherical Density Inhomogeneity

A planar shock wave that impulsively accelerates a spherical density inhomogeneity baroclinically deposits vorticity and enhances the mixing between the two fluids resulting in a complex, turbulent flow field. This is known as the classical shockbubble interaction (SBI) and has been a topic of study for several decades [1,2,3,4, 5,6,7,8,9,10,11,12], and closely related the Richtmyer-Meshkov instability (RMI) [13, 14]. While the classical SBI problem concerns a reactively neutral bubble, the present experimental study is the first of its kind in which a spherical bubble filled with a stoichiometric mixture of H2 and O2 diluted with Xe is accelerated by a planar shock wave (1.35 < M < 2.85) in ambient N2, and will be referred to as reactive shock-bubble interaction (RSBI).Experimental results for an inert spherical density inhomogeneity accelerated by a strong incident shock wave (M = 2.8) are compared with a reactive mixture of similar density. When a heavy bubble is shock accelerated in a lighter ambient gas corresponding to a large Atwood number (A > 0), the shock wave at the exterior periphery of the bubble travels faster than the interior transmitted wave, resulting in shock-focusing at the downstream pole of the bubble. The shock wave convergence results in localized temperatures and pressures an order of magnitude higher than the conditions behind the shock wave. If the bubble is composed of a reactive mixture, these localized conditions allow for a controlled, point-source ignition for the combustible mixture within the bubble. The chemical and hydrodynamic coupling is investigated. The reactive mixture is composed of a stoichiometric mixture of H2 and O2 diluted with Xe (30%, 15% and 55% by molar fraction, respectively), corresponding to A = 0.5. For the purpose of comparison, experiments are performed on an inert mixture, where the Atwood number is matched using a combination of Xe and He (58% and 42% by molar fraction, respectively). The experiments are performed at the Wisconsin Shock Tube Laboratory in a 9 m vertical shock tube with a 25.4 £ 25.4 cm 2 cross-section. A pneumatic injector is used to generate a 5 cm diameter soap bubble fllled with the gas mixture. The injector retracts ∞ushly into the side of the tube releasing the bubble into a state of free fall (Ranjan 2005, 2007). Diagnostics are performed using chemiluminescence of the OH i molecule present during the combustion process and planar Mie scattering with a frequency doubled Nd:Yag. Due to an inherently weak signal, the chemiluminescence is captured with an intensifled CCD camera, while the initial conditions are captured with a front-lit, high speed camera.