Mixing at the external boundary of a submerged turbulent jet.

We study experimentally and theoretically mixing at the external boundary of a submerged turbulent jet. In the experimental study we use particle image velocimetry and an image processing technique based on the analysis of the intensity of the Mie scattering to determine the spatial distribution of tracer particles. An air jet is seeded with the incense smoke particles, which are characterized by a large Schmidt number and a small Stokes number. We determine the spatial distributions of the jet fluid characterized by a high concentration of the particles and of the ambient fluid characterized by a low concentration of the tracer particles. In the data analysis we use two approaches, whereby one approach is based on the measured phase function for the study of the mixed state of two fluids. The other approach is based on the analysis of the two-point second-order correlation function of the particle number density fluctuations generated by tangling of the gradient of the mean particle number density by the turbulent velocity field. This gradient is formed at the external boundary of a submerged turbulent jet. We demonstrate that probability density function of the phase function of a jet fluid penetrating into an external flow and the two-point second-order correlation function of the particle number density do not have universal scaling and cannot be described by a power-law function. The theoretical predictions made in this study are in qualitative agreement with the obtained experimental results.

High initial amplitude and high Mach number effects on the evolution of the single-mode Richtmyer-Meshkov instability.

Effects of high-Mach numbers and high initial amplitudes on the evolution of the single-mode Richtmyer-Meshkov shock-wave induced hydrodynamic instability are studied using theoretical models, experiments, and numerical simulations. Two regimes in which there is a significant deviation from the linear dependence of the initial velocity on the initial perturbation amplitude are defined and characterized. In one, the observed reduction of the initial velocity is primarily due to large initial amplitudes. This effect is accurately modeled by a vorticity deposition model, quantifying both the effect of the initial perturbation amplitude and the exact shape of the interface. In the other, the reduction is dominated by the proximity of the shock wave to the interface. This effect is modeled by a modified incompressible model where the shock wave is mimicked by a moving bounding wall. These results are supplemented with high initial amplitude Mach 1.2 shock-tube experiments, enabling separation of the two effects. It is shown that in most of the previous experiments, the observed reduction is predominantly due to the effect of high initial amplitudes.

Velocity scaling of a shock wave reflected off a circular cylinder.

Two different approaches are undertaken to investigate the interaction of planar shock waves with circular cylinders. Experiments are conducted in a shock-tube apparatus equipped with a schlieren-based optical system to monitor the interaction, and numerical simulations are carried out using an in-house computer code to simulate similar problems. The incident shock-wave Mach number is varied in the range 1.1-1.4. Excellent agreement is found between the simulations and the experiments in terms of shock patterns, even though the model is based on an inviscid approach. Quantitative comparisons between the experimental results for different initial conditions (shock-wave strength, cylinder diameter, and working gas) are made to find the physical parameters affecting the path of the reflected shock. An approximate universal relation is derived, which predicts the reflected-shock trajectory along the axis of symmetry as a function of the incident-shock Mach, the diameter of the cylinder, and the gas properties. This relation is valid in the vicinity of the cylinder in the range of 0.1-5 D, where D is the cylinder diameter. It is found that the reflected shock from the cylinder evolves as in the case of a reflected-shock wave from a planar wall multiplied by a reduction factor, which depends on the incident-shock Mach number and the ratio of specific heats.

Shock-wave mach-reflection slip-stream instability: a secondary small-scale turbulent mixing phenomenon.

Theoretical and experimental research, on the previously unresolved instability occurring along the slip stream of a shock-wave Mach reflection, is presented. Growth rates of the large-scale Kelvin-Helmholtz shear flow instability are used to model the evolution of the slip-stream instability in ideal gas, thus indicating secondary small-scale growth of the Kelvin-Helmholtz instability as the cause for the slip-stream thickening. The model is validated through experiments measuring the instability growth rates for a range of Mach numbers and reflection wedge angles. Good agreement is found for Reynolds numbers of Re 2 x 10(4). This work demonstrates, for the first time, the use of large-scale models of the Kelvin-Helmholtz instability in modeling secondary turbulent mixing in hydrodynamic flows, a methodology which could be further implemented in many important secondary mixing processes.

Fourier-space nonlinear Rayleigh-Taylor growth measurements of 3D laser-imprinted modulations in planar targets.

Nonlinear growth of 3D broadband nonuniformities was measured near saturation levels using x-ray radiography in planar foils accelerated by laser light. The initial target modulations were seeded by laser nonuniformities and later amplified during acceleration by Rayleigh-Taylor instability. The nonlinear saturation velocities are measured for the first time and are found to be in excellent agreement with Haan predictions. The measured growth of long-wavelength modes is consistent with enhanced, nonlinear, long-wavelength generation in ablatively driven targets.

Observation of self-similar behavior of the 3D, nonlinear Rayleigh-Taylor instability.

The Rayleigh-Taylor unstable growth of laser-seeded, 3D broadband perturbations was experimentally measured in the laser-accelerated, planar plastic foils. The first experimental observation showing the self-similar behavior of the bubble size and amplitude distributions under ablative conditions is presented. In the nonlinear regime, the modulation sigma(rms) grows as alpha(sigma)gt(2), where g is the foil acceleration, t is the time, and alpha(sigma) is constant. The number of bubbles evolves as N(t) alpha(omegat sq.rt(9) + C)(-4) and the average size evolves as (t) alpha omega(2)gt(2), where C is a constant and omega = 0.83 +/- 0.1 is the measured scaled bubble-merging rate.