The evolution of density profiles within a shock accelerated gaseous interface was investigated via a three-directional laser absorption diagnostic technique for negative, close to zero, and positive initial density jump of the interface. Results show that the direction of the shock wave acceleration could have a non-negligible effect on the mixing process development. Furthermore, the turbulent diffusion within the induced mixing zone is clearly identified from the thickening of the density profiles with time.[S0031-9007(96)02048-0] PACS numbers: 47.40.Nm, The instability mechanism which appears at an interface between two fluids of different densities when normally accelerated by a shock wave can give rise to turbulence and mixing between the two fluids. Such instability, known as the Richtmyer-Meshkov instability [1-3], is of current interest because of its importance in technological applications such as the inertial confinement fusion capsules, as well as astrophysical phenomena such as the overturn of the outer portion of the massive star collapsed core [4].The purpose of the present experimental investigation is to follow, in a shock tube environment, the partial density profile evolution of one of the two constituents of a gaseous mixing zone originated from the instability of the small random perturbations present at the interface, when a shock wave crosses through it. Three series of tests were undertaken, where the initial experimental configuration was such as, crossing the interface, the shock wave passed from one gas ͑CO 2 ͒, in which the initial pressure, temperature, and density as well as the shock wave Mach number were kept constant, into another gas, the density of which was successively increased from series one to series three (He, Ar, and Kr). This allows the study of the evolution of the density within the shock induced mixing zone when the shock wave passes from the heavy gas to the light one (CO 2 ͞He corresponds to the slow/fast interface), from one gas to another of close density ͑CO 2 ͞Ar͒, and from the light gas to the heavy one (the fast/slow interface corresponds to the CO 2 ͞Kr case), successively. Many questions concerning the development and process of the mixing are of current interest, and we hope to answer some of them with the help of the present results obtained from a suitable three-directional laser absorption diagnostic technique [5][6][7] applied to the investigations.The shock tube test section and the diagnostic setup are schematically shown in Fig. 1. Experiments were conducted in an 8 m total length double diaphragm shock tube. The test chamber was an 8.5 by 8.5 cm square cross section and its length varied from 80 to 115 cm. The movable end wall of the shock tube allowed us to select the location of the mixing zone and the reflected shock wave interaction. The initial interface between the test gases was materialized by a thin plastic membrane (1.5 mm thick) resting over a 5 3 5 square wire grid (diameter 0.2 mm and spacing 17 mm). This allowed a regular rupture of ...

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The effects of argon addition, in the range of 0-20% in a N 2 -CH 4 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 B 2 S + -» X 2 S + 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. Nomenclaturec ~ speed of light F = rotational term, I/cm G = vibrational term, I/cm h ~ Planck's constant, J s / = intensity of spontaneous emission, W/(cm 3 sr) / = rotational quantum number k = Boltzmann's constant, J/K N = number density, I/cm 3 n -electronic quantum number Gtot ~ total partition function q v .^r = Franck-Condon factor R e = electronic transition moment Sj = Honl-London factor T = temperature, K v = vibrational quantum number v = wave number, I/cm Subscripts e = electronic r = rotational v = vibrational Superscripts ' = upper state of transition " = lower state of transition

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