Visualisation of plane shock wave diffraction from the curved exit of a shock tube

Abstract

There have been numerous studies of the diffraction of a plane shock wave from the normal exit of a shock tube, including studies of tubes with square, lenticular, or other irregular exit shapes. However, in considering the practical case in which such diffraction may spontaneously occur (e.g. the propagation of a blast wave through climate conditioning ducts or of a shock wave through a tuned motor vehicle exhaust system), the more likely geometric boundary condition would be of a curved exit diffraction surface. The current study explored the diffraction of a plane shock wave from the curved exit of a shock tube with a boundary plate to remove the characteristic length of the shock tube thickness. The cases tested all had: separations between the axis of the curved plate and the shock tube axis of between 0.5 and 2.5 shock tube internal diameters; radius ratios of the exit surface to the shock tube inner diameter of 3 or 5; and Mach numbers ranging between 1.2 and 1.6. The flow field was visualised using a high-speed camera operating at 75 kfps capturing schlieren images. It was found that for the case of the exit surface tangent to the inner surface of the shock tube a vortex loop is not formed but rather a horseshoe vortex which convects downstream bound by the tangent exit plate. For higher Mach numbers it seems that the horseshoe vortex may close into a loop, but this cannot be definitely concluded from the available images. An additional feature was noted which is suspected of being the tangent view of a complex shock wave reflection due to the curved edge diffraction.

Graphical Abstract