X-ray measurements of growth rates at a gas interface accelerated by shock waves

Abstract

A new experimental technique to measure the density of a high atomic number gas at a shock-accelerated interface has been developed and demonstrated. It is based on the absorption of x rays by the high atomic number gas, and it was implemented in a vertical square shock tube. The object of the study was the turbulent entrainment and mixing of shock-accelerated air/xenon interfaces prepared by retracting a metal plate, initially separating the two gases, prior to the release of the shock wave. Interfaces of two types, quasi-sinusoidal and nominally flat, were examined. The amplitude of large wavelength ͑25-100 mm͒ perturbations on the interface, and the thickness of the interface were measured. An integral definition for the interface mean line was adopted, making it possible to study and time evolution of the individual Fourier modes of the perturbations. A new integral definition for the interface thickness was proposed, making it feasible to study for the first time the time evolution of the thickness of quasi-sinusoidal interfaces. Images of interfaces after interacting with a series of weak waves reverberating between the interface and the shock tube end wall were obtained. The perturbations are studied at the late stages of their evolution, when their amplitude is no longer small compared to their wavelength. Consequently, the measured growth rates of the modal amplitudes are smaller than those predicted by the impulsive model based on the small amplitude approximation. In the case of nominally flat interfaces, the thickness is observed to grow linearly at rates comparable to values previously reported.

Experimental observations of the mixing transition in a shock-accelerated gas curtain

“…They set initial conditions by stratification in a vertical shock tube experiment, but their diagnostic was still line-of-sight integration. Several other experiments [9][10][11] have generated membrane-less diffuse interfaces in vertical shock tubes. Later Jacobs et al 12 and Budzinski et al 13 developed a more precise diagnostic technique by illuminating diffuseinterface flow in a horizontal shock tube with a laser sheet.…”

Experiments on the Richtmyer–Meshkov instability: Wall effects and wave phenomena

“…The results can be partially corrected for the undesirable contribution of the wall vortices on the windows by modifying the observed optical density near the side walls such that the optical density at a given axial location averaged over the width of the test section is the same as that averaged in the bulk of the fluid. 7,21 Using the same approach, a similar correction method could be applied for extracting density information from interferometer images.…”

“…However, when using a technique that relies on an integration across the depth of the test section, such as interferometry or x-ray densitometry, 7,21 for example, some care has to be exercised in the data reduction to correct for the wall effects. Figure 18͑a͒ shows the optical density of a radiograph taken at tϭ5.82 ms after the interaction of a M s ϭ1.33 shock wave with an air/xenon interface for a long period experiment.…”

Turbulent transition of a gaseous mixing zone induced by the Richtmyer-Meshkov instability

“…To date, two main strategies have been developed to tackle this challenge. The first historically deployed strategy relies on an initial, solid or soft physical separation between the two gaseous species [2,7,[10][11][12][13][14][15]. A more recent experimental strategy aims at getting rid of any physical separation between the two gases by generating diffuse shear layers or curtains [8,[16][17][18][19].…”