Experimental determination of the growth rate of Richtmyer-Meshkov induced turbulent mixing after reshock

The time evolution of the width of the turbulent mixing zone arising from the late development of Richtmyer-Meshkov instability is investigated in this work. This is achieved by means of the analysis of time-resolved Schlieren images obtained with a given set of shock-tube experiments. The post-reshock growth rate of the mixing zone width is found to be nearly insensitive to the development state of the mixing at the time of reshock

Study of the turbulent mixing zone induced by the Richtmyer-Meshkov instability using Laser Doppler Velocimetry and Schlieren visualizations

An experimental study of the compressible mixing generated by the Richtmyer-Meshkov instability (RMI) is carried out in a vertical shock tube by means of two-components Laser Doppler Velocimetry (2C-LDV) measurements and Time-resolved Schlieren visualizations. An attempt is made to quantify the RMI-induced air/sulphurhexafluoride (SF6) mixing by measuring turbulence levels inside the mixing zone at a given stage of its development and by extracting the growth rate of the mixing zone from the Schlieren images

Caractérisation hydrodynamique des turbines de Rushton dans une cuve agitée en régime turbulent

L’étude numérique de l’influence du nombre des pales d’une turbine de Rushton sur la structure hydrodynamique d’une cuve agitée a été entamée. Les équations de Naviers-Stokes régissant le phénomène sont résolues par une méthode de discrétisation aux volumes finis. Le modèle de turbulence utilisé est du type k–epsilon standard. Les résultats numériques issus de l’application de notre code de calcul sont présentés dans différents plans de la cuve

LDV measurements in turbulent gaseous mixing induced by the Richtmyer-Meshkov instability: statistical convergence issues and turbulence quantification

A statistical characterization of the turbulent flow produced in a vertical shock tube dedicated to the study of the Richtmyer-Meshkov instability (RMI) is carried out using Laser Doppler Velocimetry (LDV), time-resolved Schlieren images and pressure histories. The time evolution of the phase-averaged velocity field and the fluctuating velocity levels produced behind the shock wave are first investigated for different configurations of a pure air, homogeneous medium. This allows us to determine the background turbulence of the experimental apparatus. Second, the RMI-induced turbulent Air/SF6 mixing zone (TMZ) is studied both in its early stage of development and after its interaction with a reflected shock wave (reshock phenomenon). Here the gaseous interface is initially produced by a thin nitrocellulosic membrane trapped between two grids. One of the most consistent issue regarding such a process is the generation of a large number of fragments when the incident shock wave crosses the interface. These fragments are likely to corrupt the optical measurements and to interact with the flow. This work seeks to clarify the influence of these fragments on the statistical determination of the velocity field. In particular it is shown that statistical convergence cannot be achieved when the fragments are crossing the LDV measurement volume, even if a significant number of identical experiments are superimposed. Some specific locations for the LDV measurements are however identified to be more favourable than others in the Air/SF6 mixing configuration. This finally allows us to quantify the surplus of turbulence induced by the reshock phenomenon

Experimental and numerical investigation of the growth of an air/SF6 turbulent mixing zone in a shock tube

Shock-induced mixing experiments have been conducted in a vertical shock tube of 130mm square cross section located at ISAE. A shock wave traveling at Mach 1.2 in air hits a geometrically disturbed interface separating air and SF6, a gas five times heavier than air, filling a chamber of length L up to the end of the shock tube. Both gases are initially separated by a 0.5 lm thick nitrocellulose membrane maintained parallel to the shock front by two wire grids: an upper one with mesh spacing equal to either ms=1.8mm or 12.1 mm, and a lower one with a mesh spacing equal to ml=1 mm. Weak dependence of the mixing zone growth after reshock (interaction of the mixing zone with the shock wave reflected from the top end of the test chamber) with respect to L and ms is observed despite a clear imprint of the mesh spacing ms in the schlieren images. Numerical simulations representative of these configurations are conducted: the simulations successfully replicate the experimentally observed weak dependence on L, but are unable to show the experimentally observed independence with respect to ms while matching the morphological features of the schlieren pictures

Study of the gaseous mixing induced by the Richtmyer-Meshkov instability

Ce travail s’intéresse à l’analyse expérimentale du développement de la zone de mélange turbulente (ZMT) produite par une instabilité de Richtmyer-Meshkov (IRM). Les expériences sont réalisées au sein d’un tube à chocs vertical, et l’analyse s’appuie sur des mesures simultanées mettant en œuvre des techniques expérimentales de type capteurs de pression pariétaux, visualisations strioscopiques résolues en temps et mesures de vitesse par Vélocimétrie Laser Doppler (LDV). Une caractérisation de l’installation expérimentale est tout d’abord effectuée en situation homogène (air pur, sans mélange), afin de déterminer la qualité de l’écoulement de base et connaître le niveau de turbulence de fond du tube à chocs. Les configurations de mélange, principalement entre de l’air et de l’hexafluorure de soufre (SF6), sont ensuite abordées. On s’intéresse dans un premier temps aux caractéristiques globales de la zone de mélange : en particulier à l’évolution de son épaisseur et à son taux de croissance. Plusieurs configurations de mélange sont étudiées en faisant varier différents paramètres expérimentaux tels que la hauteur de la veine d’essais du tube à chocs, la forme de la perturbation initiale de l’interface entre les deux gaz et le nombre d’Atwood, dans le but de déterminer leur influence sur le développement de la ZMT. On montre ainsi une sensibilité du taux de croissance post-rechoc à plusieurs de ces paramètres. Des comparaisons avec des simulations numériques réalisées par nos partenaires du Commissariat à l’Énergie Atomique (CEA) montrent des tendances similaires entre expériences et simulations sur ce point. L’étude est ensuite complétée par une caractérisation plus locale de la ZMT, en mesurant les niveaux de turbulence en différents points de la veine d’essais à l’aide de la LDV. Après avoir quantifié les contraintes de convergence statistique imposées par l’expérience pour ce type de mesures, on donne une estimation des intensités turbulentes produites par l’écoulement de mélange à différents stades de son développement.This experimental study sheds some light on the development of the turbulent mixing zone (TMZ) arising from a Richtmyer-Meshkov instability (RMI). The experiments are conducted in a vertical shock tube, and the analysis relies on simultaneous measurements involving pressuretransducers, time-resolved Schlieren visualizations and Laser Doppler Velocimetry (LDV). In a first step, a thorough characterization of the experimental apparatus is conducted in order to qualify the basic flow configuration corresponding to homogeneous situations (pure air withoutmixing), and to evaluate the « background » turbulence level of the shock tube. Mixing configurations (mainly between air and sulfur hexafluoride, SF6) are then investigated. We first focus on a global description of the mixing zone such as the time evolution of its thickness and the corresponding growth rate. We consider several mixing configurations, varying the length of the test section, the shape of the initial interface between the two gases and the Atwood number. A clear influence of some of these parameters is shown on the the post-reshock increasing rate of the mixing zone, in good accordance with numerical results obtained from the Commissariat à l’Energie Atomique (CEA, french atomic energy commission). A more local description of the flow is then obtained in a second step by measuring the turbulence levels at different locations inside the test section thanks to the LDV technique. After quantifying the issues linked to the statistical convergence of the turbulent quantities in such specific configurations, we provide an estimation of the turbulent intensities produced by the mixing at various stages of its development

Simulation numérique de l’écoulement laminaire induit dans des cuves agitées générées par les mobiles de proximité de type mono et double vis à profils simple et modifié

Dans cet article, une étude comparative entre les caractéristiques hydrodynamiques de l’écoulement laminaire généré en cuves agitées par différents mobiles de proximité de type mono et double vis avec profils simple et modifié a été menée en utilisant la simulation numérique. Cette étude a été abordée à l’aide d’un code spécifique de dynamique des fluides numérique (CFD), basé sur la résolution des équations de Navier-Stokes par la méthode des volumes finis. Les résultats numériques obtenus montrent bien l’influence de la forme du mobile d’agitation sur le comportement de l’écoulement au sein de la cuve. En effet, on remarque que l’utilisation d’une vis à profil modifié favorise un champ de vitesse plus actif que dans le cas des configurations à profil simple. Par ailleurs, et dans le cas d’une double vis, on note que l’effet de la dissipation visqueuse et du pompage est nettement plus prépondérant, présentant ainsi une meilleure efficacité de pompage et de consommation énergétique. La comparaison de nos résultats numériques avec les résultats expérimentaux tirés de la littérature montre une bonne adéquation, ce qui prouve la validité de la méthode d’analyse adoptée

Evolution of turbulent fluctuations within an air/SF6 mixing zone induced by the Richtmyer-Meshkov instability

The Richtmyer–Meshkov instability (RMI) occurs when a shock wave impulsively accelerates a perturbed interface between two gases of different densities. The interaction of the shock wave with the perturbed interface produces vorticity through baroclinic effects, potentially leading to the development of a turbulent mixing zone (TMZ). The RMI is found in some engineering application, e.g. inertial confinement fusion (ICF) or supersonic combustion and also occurs in natural phenomena such as supernova explosion (M. Brouillette [2002]). A thorough understanding of the TMZ evolution requires quantifying the turbulence levels produced by the RMI. In this context, the present study constitutes a first step towards a deeper knowledge of the fundamental mechanisms driving such mixing. The linear and non-linear stages of the RMI-induced perturbations of the interface have been and are still widely studied. However the consecutive stages, corresponding first to the TMZ development, secondly to its interaction with a reshock, is nowadays an active field of research (S. Ukai et al. [2011]). The present study aims at completing an experimental database for the validation of numerical codes and will also help to decipher the physics underlying the above-mentioned phenomena. The experimental setup consists in a 5m long, 130mm square cross section vertical shock tube. In this investigation, the initial interface between light (air) and heavy (SF6) gases is generated by a thin 0.5μm nitrocellulose membrane. This membrane is trapped between two grids, the upper one imposing a three-dimensional sinusoidal initial perturbation of wavelength equal to 1.8mm. The length of the test section has been fixed to L=250mm. The test section is equipped with transparent walls to allow flow visualizations and laser measurements (Two-components Laser Doppler Velocimetry, hereafter denoted 2C-LDV). Pressure histories of the flow in the shock tube are obtained using five piezoelectric pressure transducers. The Mach number of the incident shock wave and the Atwood number just before mixing are fixed at 1.2 and 0.699 respectively. 2C-LDV measurements of the velocity in the center of the test section are performed at two different locations downstream the initial interface: 43mm and 135mm respectively. The experimental procedure and the characterization of the experimental test rig in terms of background turbulence levels are fully described in (G. Bouzgarrou et al. 2011). The study will focus on the evolution of the turbulent velocity fluctuations inside the TMZ, before and after reshock

Evolution of the air/SF6 turbulent mixing zone for different lengths of SF6: shock tube visualizations and 3D simulations

A turbulent mixing zone (TMZ) is created in a vertical shock tube (based in ISAE DAEP) when a Mach 1.2 shock wave in air accelerates impulsively to 70 m/s an air/SF6 interface. The gases are initially separated by a thin nitrocellulose membrane maintained at and parallel to the shock by two wire grids. The upper grid (SF6 side) of square mesh spacing hu 1.8 or 12.1 mm is expected to seed perturbation for the Richtmyer-Meshkov instability (RMI) while the lower grid with hl 1 mm is needed to prevent the membrane from bulging prior to the shot. The experiments were carried out for different lengths L of SF6 between the initial interface and the shock tube's end plate : 10, 15, 20, 25 and 30 cm. The time resolved Schlieren image processing based on space and frequency filtering yields similar evolution for the TMZ thickness. Before reshock, the thickness grows initially fast then slows down and reaches different values (10 to 14 mm) according to L. Soon after reshock, the TMZ thickness growths rate is 21 mm/ms independently of L and hu. Numerical Schlieren images gen- erated from 3D numerical simulations (performed at CEA DAM IDF) are analyzed as the experimental ones for L 15 and 25 cm and for hu 1.8 and 12.1 mm. The very weak experimental dependence on hu is not obtained by simulation as expected from dimensional reasoning. This discrepancy remains paradoxical