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

Direct numerical simulation of particles dispersion in supersonic shear layers

Particles dispersion plays an important role in many industrial applications such as combustion, pollution control and also in experimental measurements like Laser Doppler Velocimetry. In this last case, particles are supposed to have the same behavior as fluid particles in order to give relevance to the experimental measure. However it has been shown (Jacquin et al. 1991) that noticeable errors can appear in the rms velocity measurement of supersonic jet or shear layer, even if care has been taken concerning particle seeding of the flow. The aim of this paper is to use direct numerical simulation of particle-gas flow to investigate this phenomenon

La troponine T ultrasensible : un nouvel outil diagnostic pour le médecin sportif?

peer reviewedIntroduction : Le risque d’accidents cardiaques ou de mort subite après effort physique intense est bien connu. Ces évènements indésirables se produisent souvent chez des sujets présentant une maladie coronarienne asymptomatique et ignorée. Néanmoins, vu ce risque, l’American Heart Association recommande de réaliser un screening cardiovasculaire chez les athlètes de tout âge. Dans cette optique, le dosage de marqueurs cardiaques de nouvelle génération, plus sensibles, comme la troponine T ultrasensible (hsTnT) peut certainement apporter des informations très intéressantes par la détection de dommages myocardiques mêmes mineurs. Matériels et méthodes : Des 20 sujets masculins volontaires âgés de 22.36±2.02 années, sédentaires, 8 ont dû être exclus (abandon, malaise à l’effort...). La VO2max a été préalablement déterminée lors d’un test à l’effort sur cycloergomètre une semaine avant le test afin de ne pas interférer avec les résultats de l’effort physique intense (EPI) pour lequel les sujets ont couru sur tapis roulant durant 1 heure à 75% de la VO2max. Quatre échantillons sanguins de 5 ml (tube hépariné-lithium) ont été prélevés : juste avant (T1), directement après (T2), 4 heures après (T3) et 24 heures après l’EPI (T4). Le dosage de hsTnT (Modular de Roche Diagnostic®) est réalisé sur du plasma par une technique d’électrocheminiluminescence. Résultats : Une augmentation statistiquement significative des résultats à T3 (p<0.01) est observée. L’élévation de la hsTnT est progressive pour atteindre un pic maximum à T3 et revenir dans les normes à T4. Le seuil critique de 0.03 ng/mL a été retenu et 75% des sujets présentent un taux supérieur à ce dernier à T3 (moyenne : 0.053 ng/mL), alors que 100% des sujets se trouvent en dessous de ce seuil à T1 (0.0041 ng/mL). Discussion - Conclusions : Ces résultats, extrêmement intéressants, suggèrent que la libération de hsTnT serait due soit à un processus physiologique de remodelage, soit à un processus irréversible de lésions au niveau des cardiomyocytes (nécrose). Il est également possible que cette élévation des troponines soit due à une libération à partir du pool cytosolique mais aussi elle peut être la conséquence de dommages membranaires potentiellement induits par le stress oxydatif. A l’issue de cette étude, nous démontrons que la hsTnT peut être un nouvel outil diagnostic dans le domaine de la cardiologie du sport

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

A new approach for the comprehensive study of a turbulent mixing zone induced by the Richtmyer-Meshkov instability

In this study we propose a new experimental and methodological approach dedicated to the analysis of the development of a turbulent mixing zone produced by the Richtmyer-Meshkov Instability. A brief description of the experimental device dedicated to the generation of the initial interface is first presented. An overview of the main results obtained by means of strioscopic, Particle Image Velocimetry (PIV) and tomoscopic time-resolved measurements are then provided. The spatio-temporal evolution of the macroscopic scale based on the thickness of the mixing zone is discussed on the basis of the time-resolved Schlieren data. A first step into a more comprehensive analysis of finer, local scales of the flow is introduced, based on the qualitative analysis of the velocity and density fields during the development of the mixing zone