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 130 mm 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 μm thick nitrocellulose membrane maintained parallel to the shock front by two wire grids: an upper one with mesh spacing equal to either m_s = 1.8 mm or 12.1 mm, and a lower one with a mesh spacing equal to m_l = 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 m_s is observed despite a clear imprint of the mesh spacing m_s 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 m_s while matching the morphological features of the schlieren pictures.