Schlieren techniques for the visualization of an expansion fan/shock wave interaction

Abstract

The interaction between an expansion fan and a shock wave and the resulting flow field are of importance both from a research and an applied perspective. Interesting flow phenomena arise from this interaction which are significant in supersonic engine inlet design and aircraft store carriage and release. This paper focuses on the use of various schlieren techniques to experimentally visualize an expansion fan/shock wave interaction. The expansion fan was generated by a single wedge and the shock wave by another, at various Mach numbers. Three cut-off techniques were investigated: a knife edge, a tri-colour mask and a band lattice mask. Each of these cut-offs was placed in three orientations, parallel to the freestream flow direction, at −45° and perpendicular to the flow, to determine the effects of the directional sensitivity of the system. The different cut-off techniques and respective orientations were evaluated on the efficacy of visualizing flow features of interest, namely the expansion fan and deflection thereof, shock curvature and sliplines formed as a result of the interaction. Each technique produced visualization with varying degrees of clarity. There was a dependence on the system’s directional sensitivity when visualizing both the expansion fan deflection and the shock curvature. The visualization of the expansion fan/shock wave interaction using the band lattice mask parallel to the freestream was particularly effective as it showed all the features of interest (excepting the sliplines as these formed a region of varying entropy rather than occurring as discrete lines). Consequently, it was shown that schlieren is a valuable tool in the experimental visualization of an expansion fan/shock wave interaction.