Z. G. Zhai, M. H. Wang, T. Si and X. S. Luo (2014) On the interaction of a planar shock with a light polygonal interface. Journal/Journal Of Fluid Mechanics 757 800-816. [In English]
Web link: http://dx.doi.org/10.1017/jfm.2014.516
Keywords: compressible flows, mixing, shock waves, RICHTMYER-MESHKOV INSTABILITY, FAST GAS INTERFACE, AIR/SF6 INTERFACE, FLUID LAYER, WAVES, REFRACTION, RESHOCK, EVOLUTION, CYLINDER

Abstract: The interaction of a planar shock wave with a polygonal N-2 volume surrounded by SF6 is investigated experimentally and numerically. Three polygonal interfaces (square, triangle and diamond) are formed by the soap film technique developed in our previous work, in which thin pins are introduced as angular vertexes to connect adjacent sides of polygonal soap films. The evolutions of the shock-accelerated polygonal interfaces are then visualized by a high-speed schlieren system. Wave systems and interface structures can be clearly identified in experimental schlieren images, and agree well with the numerical ones. Quantitatively, the movement of the distorted interface, and the length and height of the interface structures are further compared and good agreements are achieved between experimental and numerical results. It is found that the evolution of these polygonal interfaces is closely related to their initial shapes. In the square interface, two vortices are generated shortly after the shock impact around the left corner and dominate the flow field at late stages. In the triangular and diamond cases, the most remarkable feature is the small 'SF6 jet' which grows constantly with time and penetrates the downstream boundary of the interface, forming two independent vortices. These distinct morphologies of the three polygonal interfaces also lead to the different behaviours of the interface features including the length and height. It is also found that the velocities of the vortex pair predicted from the theory of Rudinger and Somers (J. Fluid Mech., vol. 7, 1960, pp. 161-176) agree with the experimental ones, especially for the square case. Typical free precursor irregular refraction phenomena and the transitions among them are observed and analysed, which gives direct experimental evidence for wave patterns and their transitions at a slow/fast interface. The velocities of triple points and shocks are experimentally measured. It is found that the transmitted shock near the interface boundary has weakened into an evanescent wave.