The transition of reflected shock waves from wedges is one of the fundamental topics of shock wave research. In a series of International Symposia on Shock Wave Interaction, this topic was discussed with emphasis on the discrepancies between experimental results and the appropriate analytical solutions. The hysteresis effect which exists in the region in which the Mach reflection appears depends on the effect that the boundary layers` growth behind the incident shock wave has on the reflected shock transition.
It is therefore not surprising that detailed experimental investigations were conducted regarding transitions of reflected shock waves over roughened wedges and from concave and convex walls. Obtained experimental results confirmed that surface irregularity decreased drastically the critical transition angle (Ref.1). In these experiments the wedges surface roughness was achieved by introducing a saw tooth shape of 90 degree tip angle as shown in Fig. 1. Four series of saw tooth roughness of 0.1 mm, 0.2 mm, 0.8 mm, and 2.0 mm in height installed on a movable wedge were investigated in a 40 mm x 80 mm shock tube test section.
Obtained shadowgraph photos for wedge roughness of 2 mm are shown in Figs.1a, b, and c. In Fig. 1a the shock wave reflection from a 270 rough-surface wedge is shown; it is a typical Mach reflection. This reflection type is about to be terminated over the 350 wedge shown in Fig.1b, and eventually transits to a regular reflection over a 420 wedge. Such peculiar regular reflection is known from 1980. It was a puzzle for many years to understand this behavior. It is known that in a Mach reflection over a wedge in shock tube flows in CO2 or SF6, the triple point sometimes tends to approach the wedge surface as if it is going to touch the wedge surface. This trend occurs because the curved reflected shock cannot intersect with the straight reflected shock wave emitted from the triple point. Figure 1c shows a similar case for Mach reflection over a wedge in CO2 or SF6 shock tube flows. In Fig. 1c, the curved reflected shock wave cannot reach the foot of the incident shock wave. In fact, we see a time delay as if the incident shock wave propagates independent of the presence of the reflected shock wave.
In 1980 when these images were reported, we did not have advanced computational capability and hence could not explain this phenomenon. We do hope to reproduce this behavior numerically. The trend we observed in 2.0 mm saw tooth roughness was also observed in 0.8 mm and 0.2 mm saw tooth roughness. This peculiar reflection pattern is observable in coherent reasonably course saw tooth roughness.
Fig. 1 Shock wave reflection from wedge having a 2 mm saw tooth surface, Ms = 1,47 at 50 kPa and 293K in air. The wedge angle was either, (a) 270; (b) 350; or (c) 420.
Reference
1 K. Takayama, J. Gotoh, G. Ben-Dor: Influence of surface roughness on shock wave transition in quasi-stationary and truly non-stationary flows, Proc. 13th ISST&W, Buffalo (1981), pp. 326-334
