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

Abstract

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.