L. Houas

Experimental study on a plane shock wave accelerating a gas bubble

A detailed experimental study of the interaction between a planar shock wave and an isolated spherical gas inhomogeneity is presented here. Different configurations have been considered: a shock wave moving from one gas into another, of similar density, lower density and one of higher density. Sequences of shadowgraph pictures obtained during the same run provided useful insights into several mechanisms such as shock wave reflection, refraction and focusing, distortion of the bubble interface, and vortex formation. Based on these sequences, the changes with time in the characteristic bubble sizes were plotted and the results showed that the influence of the shock wave Mach number is significantly greater in the case of light gas bubbles. The displacement of the inhomogeneity relative to the surrounding gas was determined and compared to Rudinger and Somers’ model. In all the cases studied, although the measurements were found to agree well with the theoretical predictions, in the initial acceleration phas...

Drag coefficient of a sphere in a non-stationary flow: new results

The drag coefficient of a sphere placed in a non-stationary flow is studied experimentally over a wide range of Reynolds numbers in subsonic and supersonic flows. Experiments were conducted in a shock tube where the investigated balls were suspended, far from all the tube walls, on a very thin wire taken from a spider web. During each experiment, many shadowgraph photos were taken to enable an accurate construction of the spheres trajectory. Based on the spheres trajectory, its drag coefficient was evaluated. It was shown that a large difference exists between the sphere drag coefficient in steady and non-steady flows. In the investigated range of Reynolds numbers, the difference exceeds 50%. Based on the obtained results, a correlation for the non-stationary drag coefficient of a sphere is given. This correlation can be used safely in simulating two-phase flows composed of small spherical particles immersed in a gaseous medium.

EXPERIMENTAL INVESTIGATION OF RICHTMYER-MESHKOV INSTABILITY IN SHOCK TUBE

An experimental investigation of the Richtmyer–Meshkov instability is carried out in a shock tube. The purpose of this study is to obtain information on the growth in the thickness of the turbulent mixing zone, which is induced by the impulsive acceleration of the interface between two gases of different densities. The turbulent phase of the evolution of this instability is of interest here. The thickness of the turbulent mixing zone is inferred from two different diagnostic techniques: measurements of infrared emission of CO2 and black‐and‐white or color schlieren photographs. Following an assessment of the diagnostic techniques, discussions of the main experimental difficulties as the presence of membrane fragments and the disturbances induced by the wall boundary layers, are given. Comparisons of the thickness and the thickness growth rate of the turbulent mixing zones obtained in the present experiments, with both experimental and theoretical results, are made. A tentative picture of the evolution in ...

Experimental investigation of the shock wave interaction with a spherical gas inhomogeneity

The interaction of a shock wave with a spherical gas inhomogeneity (soap bubble) is experimentally investigated using a high speed camera shadowgraph diagnostic. Negative, close to zero, and positive density jumps across the bubble interface are studied for weak incident shock waves. For each case, the bubble length and height evolutions have been determined, as well as the generated vortex diameter and pair spacing from only one run. We point out that in all cases, after the shock bubble compression phase, the bubble and surrounding gaseous mixing length is linear with time.

Shock-tube analysis of argon influence in Titan radiative environment

The effects of argon addition, in the range of 0-20% in a N2-CH4 mixture on the nonequilibrium radiation emitted behind a normal shock wave, have been investigated in a free-piston-driven shock tube. The intensity of spontaneous emission, for the B2S+ —» X 2S+ electronic transition of CN molecules, is measured at a shock velocity of 5700 m/s propagating in a 200-Pa test gas mixture. Rotational and vibrational temperature profiles in the shock layer are obtained by matching three spectral lines simultaneously recorded in the A*> = 0 band with theoretical spectra calculations. The results show that the nonequilibrium radiation overshoot weakly increases with argon addition, whereas the equilibrium intensity value is not affected. The characteristic relaxation time of radiation is also affected so as to be reduced by argon addition. The vibrational relaxation time for CN molecules is also determined from the temperature profiles, but the accuracy is difficult to assess since the temperatures are weakly dependent functions of ratios of intensities.

Shock induced Rayleigh–Taylor instability in the presence of a boundary layer

The aim of this work is an experimental study of the development of perturbations of a gaseous interface impulsively accelerated by a plane shock wave. The experiments are performed in a double diaphragm shock tube, where the second diaphragm is a very thin Mylar film which can be initially bulged because of a pressure difference between the two gases. The shape of the leading front of the contact zone is measured at three locations along the tube using a transversal array of heat transfer gauges. After the shock passage, the evolution of the interface is sensitive to vorticity production and boundary layer effects so that the impulsive Rayleigh–Taylor theory is inadequate for the description of this evolution. In particular, the predicted perturbation reversal when the shock wave passes from the heavy gas to the light one may not occur because of the boundary layer effect.

An experimental and numerical investigation of the dependency on the initial conditions of the Richtmyer-Meshkov instability

The nonlinear evolution of 2D single-mode Richtmyer-Meshkov instabilities is investigated through experiments in shock tube and numerical simulations. In our shock tube, the interface is materialized by a thin membrane attached to a stereo-lithographed grid. The purpose of this study is to compare experimental and numerical results, verify that using a higher Mach number for the incident shock wave (Misw) than in a previous study [C. Mariani, M. Vandenboomgaerde, G. Jourdan, D. Souffland, and L. Houas, “Investigation of the Richtmyer-Meshkov instability with stereolithographed interfaces,” Phys. Rev. Lett. 100, 254503 (2008)] drastically reduces the deleterious effects of the membrane remnants, explore the effect of a high initial amplitude at the interface on the growth of the perturbation, and understand the lack of roll-up structures in the nonlinear phase of the instability. Using grayscale gradient rather than gray level, a new processing of the raw pictures is developed. Numerical simulations run wi...

Experimental investigation of the propagation of a planar shock wave through a two-phase gas-liquid medium

We conducted a series of shock tube experiments to study the influence of a cloud of water droplets on the propagation of a planar shock wave. In a vertically oriented shock tube, the cloud of droplets was released downwards into the air at atmospheric pressure while the shock wave propagated upwards. Two shock wave Mach numbers, 1.3 and 1.5, and three different heights of clouds, 150 mm, 400 mm, and 700 mm, were tested with an air-water volume fraction and a droplet diameter fixed at 1.2% and 500 mu m, respectively. From high-speed visualization and pressure measurements, we analyzed the effect of water clouds on the propagation of the shock wave. It was shown that the pressure histories recorded in the two-phase gas-liquid mixture are different from those previously obtained in the gas-solid case. This different behavior is attributed to the process of atomization of the droplets, which is absent in the gas-solid medium. Finally, it was observed that the shock wave attenuation was dependent on the exchange surface crossed by the shock combined with the breakup criterion

Nonequilibrium determination of temperature profiles by emission spectroscopy

The intensity of the spontaneous radiation emitted behind a normal shock wave is measured in a free-piston shock tube, at a velocity of 5.8 km/s. The shocked test gas is a N2CH4 mixture at an initial pressure of 2 mbar. The time-dependent spontaneous emission of CN molecules, for the B2Σ+ → X2Σ+ electronic transition, is recorded on three different wavelengths in the same shot. The rotational and vibrational temperature evolutions are deduced from a comparison between measured and calculated intensities in the Δv = 0 band.

On the compression process in a free-piston shock-tunnel

Preliminary results in the Marseille free-piston shock-tunnel facility are presented. The compression of the driver gas by the piston is studied experimentally for two different geometries of the end of the compression tube. Peak pressures obtained with the end of the compression tube closed, and with bursting of the diaphragm separating the high pressure from the low pressure chamber, are compared with calculated values in the cases of N2 and He as driver gases. A phenomenon of accoustic resonance has been uncovered, generating strong pressure oscillations which, if not properly dealt with, could impair the quality of the useful flow in such a facility.