On the interaction of a planar shock with a light polygonal interface

Abstract

The interaction of a planar shock wave with a polygonal N 2 volume surrounded by SF 6 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 'SF 6 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.

Experimental investigation of cylindrical converging shock waves interacting with a polygonal heavy gas cylinder

“…In this way, the gas interface has an almost ideal two-dimensional shape with a polygonal cross-section. The effects of the thin pins and fine chamfers on the shock propagation and interface deformation have been evaluated numerically and experimentally in planar shock-polygonal interface interactions Zhai et al 2014;Luo et al 2015), which predict a good performance of the soap film method in the interface formation and the successive RM instability study.…”

“…When the initial interface is formed in the test section, the pollution of the disks by of soap liquid and the arrangement of the thin pins would contaminate the schlieren images of the evolving interface, which can be seen in studies of planar shock-polygonal interface interactions Zhai et al 2014;Luo et al 2015). Moreover, the schlieren system often obtains an integrated view of the flow field along the light path.…”

“…The type of the irregular refraction is heavily dependent on the inclined angle, defined as the angle between the boundaries of the interface and On converging shock and polygonal interface interaction the incident shock wave. For a planar shock with a fixed Mach number interacting with a polygonal interface, the inclined angle remains constant at the upstream and downstream interfaces, respectively, and thus the irregular refraction system can be identified (Zhai et al 2014;Luo et al 2015). However, for a cylindrical shock case, the inclined angle always changes as the incident shock moves along the sides of the interface, and the irregular refraction system will change as well.…”

“…The instants when the reflected shock impacts on the three types of evolving interface are also estimated. Unlike the planar shock cases Zhai et al 2014;Luo et al 2015), the interface velocities change with time even in the very early stage because of the variable shock strength and the complex shock pattern during the interaction of the polygonal interface with the cylindrical shock. Also, for a planar air/SF 6 interface accelerated by an incident planar shock wave with the Mach number of 1.35, the interface will attain a constant velocity of nearly 110 m s −1 according to shock dynamics theory.…”

“…Typically, the interfaces of different shapes between two gases include spherical bubbles formed by a soap film (Haas & Sturtevant 1987;Layes, Jourdan & Houas 2003;Ranjan et al 2005;Zhai et al 2011;Si et al 2012), circular gas cylinders (Haas & Sturtevant 1987;Hosseini & Takayama 2005;Tomkins et al 2008;Luo et al 2014b), elliptical gas cylinders On converging shock and polygonal interface interaction 227 (Zou et al 2010), single-/multi-mode interfaces generated by vibration (Jacobs & Sheeley 1995;Mariani et al 2008;Long et al 2009) or by a gas curtain (Balakumar et al 2008;Orlicz et al 2009;Balasubramanian et al 2012), rectangular blocks generated by a microfilm membrane supported by a fine wire mesh (Bates, Nikiforakis & Holder 2007), and three-dimensional interfaces with a minimum surface feature formed by the soap film technique free of supporting meshes . Recently, some polygonal interfaces have been formed in our group using thin pins to restrict the soap film and the results indicate that the soap film is a flimsy but durable material during the interface formation Zhai et al 2014;Luo et al 2015).…”