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International audience; The propagation of a planar shock wave through a split channel is both experimentally and numerically studied. Experiments were conducted in a square cross-section shock tube having a main channel which splits into two symmetric secondary channels, for three different shock wave Mach numbers ranging from about 1.1 to 1.7. High-speed schlieren visualizations were used along with pressure measurements to analyze the main physical mechanisms that govern shock wave diffraction. It is shown that the flow behind the transmitted shock wave through the bifurcation resulted in a highly two dimensional unsteady and non-uniform flow accompanied with significant pressure loss. In parallel , numerical simulations using a personal code based on the solution of the Euler equations with a second-order Go-dunov scheme confirmed the experimental results with a good agreement. Finally, a parametric study was carried out using numerical analysis where the angular displacement of the two channels that define the bifurcation was changed from 90 • ,45 • , 20 • and 0 •. We found that the angular displacement does not significantly affect the overpressure experience in either of the two channels and that the area of the expansion region is the important variable affecting overpressure; the effect being, in the present case, a decrease of almost one half.

A new experimental investigation of the Richtmyer-Meshkov instability in a cylindrical geometry was conducted in the light/heavy configuration using a conventional shock tube equipped with a new specifically designed three-zone He/air/SF6 convergent test section. The first experimental results show the expected phases of the Richtmyer-Meshkov instability development.

The propagation of a planar shock wave through a “Y” bifurcation duct system is both experimentally and numerically studied. Experiments were conducted in an 80 × 80 mm square shock tube section at atmospheric pressure, for incident planar shock waves having Mach numbers of 1.12 and 1.36, respectively. We show that the pressure prevailing behind the reflected shock wave from the end wall of the Y-shaped duct is less than half of what exists behind a reflected shock wave from a similar straight duct under the same initial conditions. Furthermore, numerical results allow pointing out that the expansion ratio of the cross-sectional area where the shock wave propagates is a preponderant parameter in attenuating the strength of a shock wave compared to the duct geometry.

International audience; In convergent geometry, the Bell-Plesset (BP) effects modify the growth of hydrodynamic instability in comparison with the planar geometry. They account for acceleration, convergence of the flow, and compressibility of the fluids. To study these effects, the Richtmyer-Meshkov (RM) instability is examined in cylindrical geometry through shock tube experiments. To ease comparisons with models, the canonical single-mode sinusoidal perturbation at the gas interface is considered. The experimental results display a linear time-dependent growth of the amplitude of the instability. First, by cross-checking experiments and numerical simulations with the Hesione code, this peculiar growth for convergent geometry is confirmed. Second, we theoretically explain these results. We derive a new model for the growth of the perturbation in the linear regime of the instability for compressible fluids. Then, it is used to initiate a weakly nonlinear model. This model demonstrates that the linear time-dependent growth of the studied RM instability is due to the nonlinear saturation of the destabilizing BP effects at the interface. This study indicates the importance of taking into account even a slight deceleration of the interface and the compressibility of fluids by correctly describing the background velocity field, which is generated by converging shock waves.

During the last decade, investigations have been achieved to determine the physical mechanism which governs particle jet formation induced by the dispersion of a granular medium exposed to an impulsive pressure load, i.e., by a shock or a blast wave. This kind of such physical mechanism is observed during explosions or in nature as volcanic eruptions [1]. The formation of particle jets is also observed when a solid projectile impacts a particle layer [2]. Previous experiments have been conducted so far in three-dimensional spherical configurations using explosives surrounded by a granular layer [3–7]. In these experiments, the particle jet formation was obtained and clearly observed. The attempt was to correlate the particle jet distribution to the initial parameters like the particle diameter and density, the particle layer thickness and the strength of the incident shock wave in order to understand the breaking mode of a solid particle cluster. Even if it has been shown that the formation of particle jets depends both on the particle material properties and the ratio between the particle layer and the explosive mass [3], it was quite difficult to accurately relate the jet distribution with the initial conditions. More recently, cylindrical experiments were performed by Frost et al. [8] where it was easier to observe the particle jet formation. In spite of these studies, the selection mechanism is still not understood. The present experimental study was conducted in quasi two-dimensional geometry in order to ensure accurate characterization of the phenomenon. We present the concept of the two-dimensional experiments carried out. Parameters such as material density, strength of the incident shock wave, and the initial geometry of particle rings can be changed in order to find out what parameters can control the breaking mode of the particle clusters [9].

During the past 20 years, there was considerable amount of analytical, computational, and experimental works dealing with the planar version of the problem of the Richtmyer-Meshkov instability (RMI). However, this problem is often spherical, and so far there have been only few experimental studies in such configuration, except the cases of both shock-accelerated cylinders [1] and spheres [2] which can be categorized under this label. In convergent geometry, the RMI differs from the planar case due to both the geometrical effects on the instability and a different mono-dimensional (1D) motion of the interface itself. Thus, to better understand the dynamics of this instability in convergent geometry and validate models in such configuration, new experimental data are needed. Holder et al. [3] reported laboratory experiments with a convergent shock tube, using a detonable gas mixture to produce a cylindrically convergent shock. Recently, Zhai et al. [4], Apazidis et al. [5], and Biamino et al. [6] have experimentally proven the possibility of generating a converging cylindrical shock wave using a conventional shock tube. Zhai et al. proposed to design a shock tube having a curved wall test section. This specific wall converts a planar shock wave into a cylindrical one. Apazidis et al. have developed a device where the shock wave propagates inside an annular chamber and focuses after its reflection on the end-wall. Biamino et al. have used and implemented the gas lens theory revisited by Vandenboomgaerde and Aymard [7]. This theory was originally proposed by Dimotakis and Samtaney [8]. This work was the first experimental demonstration of the gas lens technique applied to the T80 conventional shock tube of IUSTI. It was shown that a planar incident shock wave propagating in air (Mis = 1.15) which refracts through a specific elliptical air/SF6 interface generates a cylindrical transmitted shock wave in a two-dimensional wedge geometry with a 30° half apex angle. We were able to diagnose the morphing and the focusing of the shock wave. This study constituted the first step toward canonical experiments on the converging RMI in shock tube environment, for which a second interface had to be implemented. Thus, a new two-dimensional wedge test section (15° half apex angle) was designed to accommodate two interfaces: a first air/SF6 interface of elliptical shape to convert the incident planar shock wave into a cylindrical converging one and a second sinusoidal SF6/air interface to study the converging RMI (see Fig. 1). In the present work, the details of this new experimental apparatus were implemented on the T80 shock tube, and the first results are reported. In particular, the evolution process of interfacial structures induced by RMI is shown.

The dispersion of particles by blast or shock waves induces the formation of coherent structures taking the shape of particle jets. In the present study, a blast wave, issued from an open shock tube, is generated at the center of a granular ring initially confined in a Hele-Shaw cell. With the present experimental setup, solid particle jet formation is clearly observed in a quasi-two-dimensional configuration. In all instances, the jets are initially generated inside the particle ring and thereafter expelled outward. Furthermore, thanks to the two-dimensional experimental configuration, a general study of the main parameters involved in these types of flows can be performed. Among them, the particle diameter, the density of the particles, the initial size of the ring, the shape of the overpressure generated and the surface friction of the Hele-Shaw cell are investigated. Empirical relationships are deduced from experimental results.

The mitigation of a planar shock wave caused by a cloud of calibrated water droplets was studied both experimentally and numerically. Experiments were carried out, with different shock wave Mach numbers ranging from 1.1 to 1.8, in a vertical shock tube coupled with a droplet generator which produced a well-characterized cloud of droplets of 120, 250 and 500 μm in diameter. By exploiting such an experimental set-up, we successfully measured the attenuation of a normal shock wave when passing through the water droplet cloud. This series of experiments allowed to identify the main parameters of this investigation and a clear dependence between the attenuation of the shock wave and terms governing the regimes of droplet breakup has been found. On the other hand, to support this experimental approach, 1D unsteady calculations were performed in similar configurations. Although the mathematical model based on an Eulerian/Eulerian approach was actually incomplete, the first comparisons between the experiments and the simulations were rather interesting and pointed out the need to improve the physical model, by taking into account the fragmentation and the vaporization of the droplets submitted to the shock wave as well as the size distribution of the water spray.

A new device has been developed to reduce the effects of the initial materialization of the gaseous interface in the context of horizontal shock tube experiments for the Richtmyer–Meshkov instability study. The thin nitrocellulosic membrane deposited on a stereolithographed grid support woven with thin wires is destroyed by thermal effect, through a powerful electrical pulse, just before the arrival of the incident shock wave. We present a first attempt realized in the light/heavy gas configuration (air/SF6) and compared with the experiments carried out without destruction. We show that the present device allows to reduce the influence of the membrane on the instability development.

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 sphere's trajectory. Based on the sphere's 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.

An attempt to extract velocity and molar fraction from a single hot-wire trace within a turbulent mixing zone induced by a shock accelerated gaseous interface has been proposed. Experiments have been conducted for negative and positive density jumps across the interface. The hot-wire signals clearly show interfaces between mixed and unmixed regions and the locations of incident and reflected shocks. With some hypotheses on the temperature, velocity and molar fraction profiles within the turbulent mixing zone have been obtained solving an inverse problem. Results show that if the molar fraction profiles follow physically coherent evolutions, those of the local velocity are strongly correlated with the choice of its variation range. So, we reasonably think that the results obtained from single wire have to remain limited to interface and shock locations. And it is only by coupling the present technique with the laser Doppler velocimetry, which we will be able to possibly obtain reliable estimates of the variations of quantities in the turbulent mixing zone.

International audience; This paper reports experimental results of planar shock waves interacting with aqueous foams in a horizontal conventional shock tube. Four incident shock wave Mach numbers are considered, ranging from 1.07 to 1.8, with two different foam columns of one meter thickness and expansion ratios of 30 and 80. High-speed flow visualizations are used along with pressure measurements to analyse the main physical mechanisms that govern shock wave mitigation in foams. During the shock/foam interaction, a precursor leading pressure jump was identified as the trace of the liquid film destruction stage in the foam fragmentation process. The corresponding pressure threshold is found to be invariant for a given foam. Regarding the mitigation effect, the results show that the speed of the shock is drastically reduced and that wetter is the foam, slower are the transmitted waves. The presence of the foam barrier attenuates the induced pressure impulse behind the transmitted shock, while the driest foam appears to be more effective, as it limits the pressure induced by the reflected shock off the foam front. Finally, it was found that the pressure histories in the two-phase gas-liquid mixture are different from those previously obtained within a cloud of droplets. The observed behavior is attributed to the process of foam fragmentation and to the modification of the flow topology past the shock. These physical phenomena occurring during the shock/foam interaction should be properly accounted for when elaborating new physical models. (C) 2015 AIP Publishing LLC.

We propose to experimentally study, in cylindrical geometry, the interaction of an initially perturbed cylindrical gaseous interface with a converging shock wave. This interaction is commonly referred as the Richtmyer–Meshkov instability (RMI) which, in the present case, is in a cylindrical geometry. In order to achieve this goal, we use a conventional shock tube which is adapted to this geometry through a specifically designed convergent test section. Here, the first results are presented for an incident planar shock wave of Mach number 1.15 propagating through an adequately elliptical $$\hbox {air}/\hbox {SF}_6$$ interface. It curves into a cylindrical transmitted shock wave and then accelerates a second sinusoidally perturbed $$\hbox {SF}_6$$ /air interface. From analyzing schlieren photos and pressure histories, we validate this original approach and exhibit the great potential of this experimental method for studying the RMI induced by focusing shock waves.

For ever two decades, the IUSTI laboratory has been known for its investigations [1] dealing with the Richtmyer-Meshkov instability (RMI). Experiments concerning the RMI have been performed in conventional shock tubes [2, 3, 4, 5]. All these experiments use a planar shock wave to generate the instability as perfectly as possible. However, the RMI also occurs in the spherical case where the convergence effects must be taken into account. As far as we know, no conventional (straight section) shock tube facility has been used to experimentally study the RMI in a spherical geometry.

For several years, investigations have been achieved to determine the physical mechanism which governs particle jet formation induced by the dispersion of a granular medium exposed to an impulsive pressure load, i.e. by a shock or a blast wave. This kind of such physical mechanism is observed during explosions or in nature as volcanic eruptions [1].

The invention of the shock tube is attributed to Paul Vieille [1] in the late 19th century. The first simplest shock tube was composed of two chambers separated by a diaphragm.With the pressure increase in the first chamber causing the diaphragm rupture, a shock wave was generated and propagated with a supersonic velocity in the second chamber.

Since several decades, the interest to study the interaction between a shock wave and a two phase medium grows. First, the influence on the propagation of a shock wave by a dusty gas then by a medium composed by solid particles were investigated

In protecting humans and installations from the destructive effects of shock and blast waves, the priority is to reduce the wave amplitude and the rate of pressure rise across the shock front

A shock tube experimental investigation and numerical simulations are undertaken to study the evolution of a perturbed interface of two different gases accelerated by a shock wave. The experimental method is based on a high-speed camera laser sheet diagnostic technique, and simulations are provided by our code CARBUR based on a finite volume discretization of Navier–Stokes’s equations. Two gas pairs are used to illustrate both the heavy/light (air/He) and the light/ heavy (air/SF6) cases. Two simultaneous large initial perturbations, one positive and one negative, are tested for an incident shock wave Mach number in air of about 1.3. The thin membrane (less than 1 μ) which materializes the initial interface between the two test gases presents 2D perturbations whose wave number is close to 1 in order to rapidly reach the non-linear regime. The development of the perturbations is captured at a frequency of 10 kHz after the interface acceleration, and the experiments are complemented with a numerical simulation to validate the interface deformations. Results show an asymmetric mutual gas penetration increasing with the absolute value of the Atwood’s number. Furthermore, they confirm that the heavier gas penetrates the lighter as thin spikes and the lighter gas penetrates the heavier as large bubbles. Moreover, we show that the spike moves faster than the bubble in the heavy/light case and slightly faster in the light/heavy one. Finally, numerical and experimental results are in agreement.

A new large cross-section shock tube has been developed in order to investigate different stages of the development of the Richtmyer-Meshkov instability and the instability-induced turbulent mixing. The main benefits of such a facility are that it allows, first, to create a free turbulent mixing with the knowledge of its initial conditions and, second, to follow different transition phases from the beginning to the fully developed turbulent stage.

An original particle accelerating technique has been developed for a shock tube. The trajectories of calibrated spherical particles $2\;{\mathrm{mm}} (\rho _p=1170\;{\mathrm{kg/m^3}})$ and $0.3\;{\mathrm{mm}} (\rho _p=500\;{\mathrm{kg/m^3}})$ in diameter have been measured by the multiple exposure shadowgraph technique coupled with a high speed drum camera. Both particle velocity and acceleration, deduced from the experimental trajectories, allow the determination of the drag coefficients for different, subsonic and supersonic, flow regimes for the particle Reynolds numbers from $5\cdot 10^2$ to $5\cdot10^4$ and the particle Mach numbers from 0.6 to 1.2. The drag coefficient values have been compared with different correlations found in the literature.

International audience; The dispersion of a cylindrical particle ring by a blast or shock wave induces the formation of coherent structures which take the form of particle jets. A blast wave, issuing from the discharge of a planar shock wave at the exit of a conventional shock tube, is generated in the center of a granular medium ring initially confined inside a Hele-Shaw cell. With the present experimental setup, under impulsive acceleration, a solid particle-jet formation is observed in a quasi-two-dimensional configuration. The aim of the present investigation is to observe in detail the formation of very thin perturbations created around the external surface of the dispersed particle layer. By means of fast flow visualization with an appropriate recording window, we focus solely on the first instants during which the external particle ring becomes unstable. We find that the critical area of the destabilization of the external ring surface is constant regardless of the acceleration of the initial layer. Moreover, we observe in detail the external front perturbation wavelength, rendered dimensionless by the initial ring perimeter, and follow its evolution with the initial particle layer acceleration. We report this quantity to be constant regardless of the evolution of the initial particle layer acceleration. Finally, we can reasonably assert that external front perturbations depend solely on the material of the particles.

International audience; When a shock wave crosses an interface between two materials, this interface becomes unstable and the Richtmyer-Meshkov instability develops. Such instability has been extensively studied in the planar case, and numerous results were presented during the previous workshops. But the Richtmyer-Meshkov (Richtmyer, 1960, ``Taylor Instability in Shock Acceleration of Compressible Fluids,'' Commun. Pure Appl. Math., 13(2), pp. 297-319; Meshkov, 1969, ``Interface of Two Gases Accelerated by a Shock Wave,''Fluid Dyn., 4(5), pp. 101-104) instability also occurs in a spherical case where the convergence effects must be taken into account. As far as we know, no conventional (straight section) shock tube facility has been used to experimentally study the Richtmyer-Meshkov instability in spherical geometry. The idea originally proposed by Dimotakis and Samtaney (2006,'' Planar Shock Cylindrical Focusing by a Perfect-Gas Lens,'' Phys. Fluid., 18(3), pp. 031705-031708) and later generalized by Vandenboomgaerde and Aymard (2011, ``Analytical Theory for Planar Shock Focusing Through Perfect Gas Lens and Shock Tube Experiment Designs,'' Phys. Fluid., 23(1), pp. 016101-016113) was to retain the flexibility of a conventional shock tube to convert a planar shock wave into a cylindrical one through a perfect gas lens. This can be done when a planar shock wave passes through a shaped interface between two gases. By coupling the shape with the impedance mismatch at the interface, it is possible to generate a circular transmitted shock wave. In order to experimentally check the feasibility of this approach, we have implemented the gas lens technique on a conventional shock tube with the help of a convergent test section, an elliptic stereolithographed grid, and a nitrocellulose membrane. First experimental sequences of schlieren images have been obtained for an incident shock wave Mach number equal to 1.15 and an air/SF6-shaped interface. Experimental results indicate that the shock that moves in the converging part has a circular shape. Moreover, pressure histories that were recorded during the experiments show pressure increase behind the accelerating converging shock wave.

An experimental investigation was conducted examining the option of using branched duct geometry for shock wave attenuation. Experiments were done in an 80 mm $$\times $$ 80 mm square section shock tube to which a 20-mm diameter pipe was added vertically. Pressures were recorded along the shock tube wall (static pressure) and at the branched pipe end wall (stagnation pressure). Experiments were repeated with a constant incident shock wave Mach number ( $$M_\mathrm{s}=1.47$$ ) and with different pipe lengths. It was found that the length of the branched pipe has a significant effect on the flow inside the branched pipe and that in the present experimental configuration, the stagnation pressure recorded at the branched pipe end wall surpasses the pressure in the main channel behind the original incident shock wave. Finally, simulations were carried out using a commercial program, Star-CCM+, to complete the description of the flow studied here. The computed pressure profiles and shock wave locations agree quite well with the present experimental data.

The behavior of an aluminum door hanging at the exit of an open shock tube at different angles, from 5\(^\circ \) to 85\(^\circ \), and thereby providing partially open space for the exiting flow, was investigated experimentally. Experiments were conducted with an incident shock wave Mach number of \(M_\mathrm{is}=1.1\) impinging on the partially opened door. Both pressure measurements in the vicinity of the door, on its center and inside the shock tube, and schlieren visualization were undertaken for studying the door movement and its maximum opening angle relative to its initial position. It was found that for an initial opening angle smaller than 25\(^\circ \) the door opened completely while for larger angles its motion is marginal. In addition, for an initial door opening angle of about 10\(^\circ \) the lowest pressures were recorded inside the shock tube behind the evolving waves after exiting of the incident shock wave. The present experimental results may be useful to numerical studies of fluid–structure interactions, e.g., in designing safety valves in jet engines. Such a device is needed for preventing rupture in the case when a sudden overpressure pulse is generated inside the aircraft engine compartment.

International audience; 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 (M-isw) 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 with the TRICLADE code show that adding short-wavelength (swl) perturbations to the single-mode interface is necessary to understand the morphology of the experimental pictures. Through these comparisons, a minimal description of the swl perturbations is proposed. The experimental growth rates are correctly predicted by the TRICLADE code. Positive agreements are also obtained by models only if they take into account the effect of a high initial amplitude. A first result is the agreement that we obtain between experiment, simulations, and models indicates that the membrane remnants no longer influence the global dynamics of the instability. This is due to the increase of M-isw from 1.15 to 1.45. Another result of this investigation is that swl perturbations disrupt the spikes which grow at the interface and prevent the appearance of roll-up structures. Numerical simulations show the importance of a diffuse transition layer at the initial interface as a stabilizing process of the swl perturbations. In our case, only such a layer would allow the roll-up structures to grow. Finally, our analysis confirms that, when they are present at the same time, the swl perturbations distort the long wavelength perturbations, and that the latter decreases the growth of the former. (C) 2014 AIP Publishing LLC.

A constant temperature hot-wire anemometry method is applied to the study of mixing zones induced by the interaction of a shock wave with Mach number 1.25 in air with air/helium (heavy/light), air/argon or air/krypton (light/heavy) initially plane interfaces. The single wire gauge is positioned at various locations along the shock tube axis. At the present stage of our investigation, although the analysis of the hot-wire signal is not achieved yet, we report the interesting concept of using hot-wire anemometry as a diagnostic method for shock tube studies of the Richtmyer-Meshkov instability. Based on this preliminary work, we discuss prospective experimental signal conversion, in order to provide some new results for this field of investigation, in particular for resolving characteristics of the turbulent mixing zone which is of most interest.

We present an overview of the diagnostic methods used in shock-tube investigations of mixing induced by Richtmyer–Meshkov instability. The different diagnostic techniques are first briefly presented, and then reviewed in a simple single table, which lists their advantages and disadvantages, their technological characteristics and domain of validity, the physical parameters measured, the laboratory in which they were developed and an assessment of their mean cost.

A turbulence transport model for the analysis of shock interface interaction-induced turbulent mixing is presented. Results given by the one-dimensional (1-D) version of this model are compared with data obtained in shock-tube experiments. Calibrations are made from an air/He interface destabilized by a 1.3 incident shock wave Mach number, taking into account the successive interactions with the different reshocks on the shock tube end wall. Then, using the same set of model constants, different gas pairs with both various Atwood and incident shock wave Mach numbers are considered, in order to point out the influence of these main parameters on the results. Mixing zone thickness time evolutions and 1-D density profiles are presented and directly compared with experimental results. Profiles of other variables such as the space integral of the turbulent kinetic energy ∫k dx, translation energy ∫ρux2 dx, and the ratio ∫ρk dx/∫ρux2 dx, given by the computations, are also shown. Using two different initializat...

A simultaneous three-directional laser absorption technique for the study of a shock-induced Richtmyer–Meshkov instability mixing zone is reported. It is an improvement of a CO 2 laser absorption technique, using three detectors during the same run, through three different directions of the test section, for the simultaneous thickness measurement of the mixing zone near the corner, near the wall and at the centre of a square-cross-section shock tube. The three-dimensional mean front and rear shapes of the mixing zone, its thickness and volume are deduced from the experimental measurements. The cases when the shock wave passes from a heavy gas to a light one, from one gas to another of similar densities and from a light gas to a heavy one, are investigated before and after the mixing zone compression by the reflected shock, for different incident shock wave Mach numbers. It is shown that the mixing zone is strongly deformed by the wall boundary layer when it becomes turbulent. Consequently, the thickness of the mixing zone is not constant along the shock tube cross-section, and the measurement of the mean volume of the mixing zone appears to be more appropriate than its mean thickness at the centre of the shock tube. The influence of the incident shock wave Mach number is also studied. When the Atwood number tends to zero, we observe a limit-like regime and the thickness, or the volume, of the mixing zone no longer varies with the incident shock wave Mach number. Furthermore, a series of experiments undertaken with an Atwood number close to zero enabled us to define a membrane-induced minimum mixing thickness, L 0 , depending on the initial configuration of the experiments. From the experimental data, a hypothesis about the mixing zone thickness evolution law with time is deduced on the basis of L 0 . The results are found to follow two very different laws depending on whether they are considered before or after the establishment of the plenary turbulent regime. However, no general trend can be determined to describe the entire phenomenon, i.e. from the initial conditions until the turbulent stage.

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.

International audience; When solid particles are impulsively dispersed by a shock wave, they develop a spatial distribution which takes the form of particle jets whose selection mechanism is still unidentified. The aim of the present experimental work is to study particle dispersal with fingering effects in an original quasi-two-dimensional experiment facility in order to accurately extract information. Shock and blast waves are generated in the carrier gas at the center of a granular medium ring initially confined inside a Hele-Shaw cell and impulsively accelerated. With the present experimental setup, the particle jet formation is clearly observed. From fast flow visualizations, we notice, in all instances, that the jets are initially generated inside the particle ring and thereafter expelled outward. This point has not been observed in three-dimensional experiments. We highlight that the number of jets is unsteady and decreases with time. For a fixed configuration, considering the very early times following the initial acceleration, the jet size selection is independent of the particle diameter. Moreover, the influence of the initial overpressure and the material density on the particle jet formation have been studied. It is shown that the wave number of particle jets increases with the overpressure and with the decrease of the material density. The normalized number of jets as a function of the initial ring acceleration shows a power law valid for all studied configurations involving various initial pressure ratios, particle sizes, and particle materials.

An experimental investigation on gaseous mixing zones originated from the Richtmyer-Meshkov instability has been undertaken in a square cross section shock tube. Mass concentration fields, of one of the two mixing constituents, have been determined within the mixing zone when the shock wave passes from the heavy gas to the light one, from one gas to an other of close density, and from the light gas to the heavy one. Results have been obtained before and after the coming back of the reflected shock wave. The diagnostic method is based on the infrared absorption of one of the two constituents of the mixing zone. It is shown that the mixing zone is strongly deformed by the wall boundary layer. The consequence is the presence of strong gradients of concentration in the direction perpendicular to the shock wave propagation. Finally, it is pointed out that the mixing goes more homogeneous when the Atwood number tends to zero.

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 ...

Shock tube experiments have been undertaken in which a shock wave accelerates normally to an interface separating two gases of different densities leading to the formation of a three-dimensional mixing zone between the two gases. Assuming the mixing zone to be homogeneous and weakly dependent on the wall effects, an integrated monodirectional absorption technique had been previously carried out and average temperature and density evolutions were determined within it. But the mixing zone is really deformed by the wall effects and in consequence it is not homogeneous. Thus, to improve the diagnostic technique, the experimental setup has been modified so that the mixing zone is divided into nine identical homogeneous imaginary regions. Temperature and species concentration are determined through each region of the mixing zone by a multidirectional laser absorption technique in order to have some new information on the influence of the wall effects on the Richtmyer-Meshkov instability study in square-cross-section shock tubes. Furthermore, a better accuracy in the measurements of the mixing zone thickness is obtained from a discussion of the concentration profiles. The consequence is that previous results achieved with integrated techniques such as global Schlieren, shadograph, or monodirectional absorption methods seem to be strongly overestimated.

International audience; Shock tube experiments are performed in order to study shock propagation along a two-gas layer in a confined geometry and to compare it to the case of a homogeneous density equivalent mixture. The analysis of the homogeneous case gives values for the adiabatic coefficient and density of the mixture of both gases, while the comparison between heterogeneous and homogeneous media with the same averaged density shows modifications of the shock front shape and velocity. In the two-gas layer, the shock propagates faster than in the homogeneous medium. The shock front is curved with a triple point which appears close to the shock-tube wall, in the slow medium, while it stays planar during its whole propagation in the homogeneous mixture. A correlation is found between the angle of curvature and the shock velocity increase. It is confirmed by two-dimensional Eulerian numerical calculations. Experiments and calculations exhibit very good agreement on all the measurements when molecular diffusion is taken into account in the numerical calculations. A sustained irregular refraction pattern of the shock front at the diffuse interface of both gases is obtained experimentally and confirmed by the calculations.

International audience; The flow field that evolves inside a duct upon the departure of a shock wave from its open exit is studied experimentally. The present investigation covers a relatively long time period, almost until the shock-induced flow inside the duct ceases. During the investigated time period, the appearance of a second shock wave is observed inside the duct. The effect of introducing a door inclined to the duct's exit plane, on the pressure field developed behind the second shock is reported. Three different inclination angles are studied, 30 degrees, 45 degrees, and 90 degrees relative to the duct's exit plane. It is shown that keeping the exit door at different angles and, thereby changing the size of the exit area, has a direct effect on the pressures prevailing inside the tube and on the exit door. The highest pressure on the door and the lowest pressure behind the second shock wave are observed when the exit door is kept at an angle of 30 degrees.

Richtmyer-Meshkov (RM) instability occurs when a interface separating two fluids of different density is impulsively accelerated in the direction of its normal. It is one of the most fundamental fluid instabilities and is of importance to the fields of astrophysics and inertial confinement fusion. RM instability experiments are normally carried out in shock tubes, where the generation of a sharp, well-controlled interface between gases is difficult, so there is a dispersion in terms of experimental results. The experiments presented here were conducted in a horizontal shock tube where the materialization of the initial interface was achieved by a thin nitrocellulosic membrane (0.5 μm thick) deposited on a stereolithographed grid support, computer-aided designed and constructed with chosen shape and dimensions. As diagnostic, we used laser sheet flow visualization to yield time-motion image sequences of the linear and the non-linear developments of the instability. In previous investigation [1], we have already shown that residual pieces from the membrane constituting the initial interface tend to delay the interpenetration in the light-to-heavy gas configuration and specifically during the linear stage of the interface evolution. In order to reduce these effects in the present experiments, we have increased the strength of the shock wave (Mach~1.5). We have also extended the test section from 0.46 m to 1.5 m which allows the instability to grow further and thus to observe the whole nonlinear phase until the transition to turbulence. The present paper summarizes the results obtained during this study undertaken for air/SF6 and air/He gas combinations (positive and negative Atwood numbers, respectively) in 2D and 3D geometries.

The pressure histories obtained when a shock wave propagates into an air-solid particle medium is well known: the overpressure jump decreases, as the shock wave propagates into the mixture and is followed by a pressure build-up corresponding to the velocity relaxation processes. In the present paper, an air-water droplet mixture interacting with a shock wave has been studied and the comportment of the pressure traces was found significantly changed in comparison to the interaction with a air-solid particle mixture. This is attributed to the ability of the droplets to deform and fragment into finer ones. This phenomenon, known as secondary atomisation, widely reviewed by Gelfand[1] and by Guildenbecher[2], affects both the pressure histories and the impulse induced by the shock wave. We have previously studied the influence of the height of cloud of droplets on shock wave propagation [3]. In the present work, we focus our attention on the influence of the droplet diameter on the attenuation of shock wave propagating into the air-water mixture. Moreover, predictions obtained by 1D numerical simulations are compared to the experimental results. The necessity to introduce a secondary atomisation model to fit the experimental behaviour is then underlined.

International audience; 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. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3657083]

International audience; Most gas dynamic computations in industrial ducts are done in one dimension with cross-section-averaged Euler equations. This poses a fundamental difficulty as soon as geometrical discontinuities are present. The momentum equation contains a non-conservative term involving a surface pressure integral, responsible for momentum loss. Definition of this integral is very difficult from a mathematical standpoint as the flow may contain other discontinuities (shocks, contact discontinuities). From a physical standpoint, geometrical discontinuities induce multidimensional vortices that modify the surface pressure integral. In the present paper, an improved 1D flow model is proposed. An extra energy (or entropy) equation is added to the Euler equations expressing the energy and turbulent pressure stored in the vortices generated by the abrupt area variation. The turbulent energy created by the flow-area change interaction is determined by a specific estimate of the surface pressure integral. Model's predictions are compared with 2D-averaged results from numerical solution of the Euler equations. Comparison with shock tube experiments is also presented. The new 1D-averaged model improves the conventional cross-section-averaged Euler equations and is able to reproduce the main flow features.

International audience; To prevent damage caused by accidental overpressure inside a closed duct (e.g. jet engine) safety valves are introduced. The present study experimentally investigates the dynamic opening of such valves by employing a door at the end of a shock tube driven section. The door is hung on an axis and is free to rotate, thereby opening the tube. The evolved flow and wave pattern due to a collision of an incident shock wave with the door, causing the door opening, is studied by employing a high speed schlieren system and recording pressures at different places inside the tube as well as on the rotating door. Analyzing this data sheds light on the air flow evolution and the behavior of the opening door. In the present work, emphasis is given to understanding the complex, unsteady flow developed behind the transmitted shock wave as it diffracts over the opening door. It is shown that both the door inertia and the shock wave strength influence the opening dynamic evolution, but not in the proportions that might be expected.

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.

A novel method to set highly accurate initial conditions has been designed in the context of shock tube experiments for the Richtmyer-Meshkov instability study. Stereolithography has been used to design the membrane supports which initially materialize the gaseous interface. The visualizations of both heavy-light and light-heavy sinusoidal interfaces were carried out with laser sheet diagnostics. Experiments are in very good agreement with theory and simulations for the heavy-light case, but probably due to the membrane effects, quickly deviate from them in the light-heavy configuration.

The evolution of a spherical gaseous interface accelerated by a plane shock wave has been investigated in a square cross section shock tube via a multiple exposure shadowgraph diagnostic. An helium bubble is introduced in air at atmospheric pressure in order to study the Richtmyer-Meshkov instability in spherical geometry for different shock wave Mach numbers (from 1.05 to 1.75). Focusing our study on the downstream bubble interface velocity and the inhomogeneity global length, we show that the bubble distortion is highly correlated with the shock wave strength.

A shock tube experimental investigation, by the use of a laser sheet diagnostic technique, is undertaken to study the Richtmyer-Meshkov instability of both air/SF6 and air/He interfaces. 2-D initial perturbations (membrane technique) with a relatively high wave number and initial amplitude product (kη0∼2) are considered in order to rapidly reach the non-linear and turbulent regimes. The development of the perturbations is captured at the frequency of 10 Khz after the interface acceleration by a Ms=l.l Mach number shock wave. For the light/heavy (air/SF6) case, we observe a quickly distortion of the initial interface with mushroom-like structures, followed by a relatively lighter vortex development before the reflected shock return, increasing the transition to turbulence. In the heavy/light (air/He) case, after a phase inversion, we found that vortex structures quickly appear and seem to be preponderant at late times after successive re-shock compressions. Finally, the light/heavy case measured perturbation ampli- tudes show good agreements with recent models, despite of the still unpredictable influence of the membrane.