Synchronous shock wave emergence from intersecting ducts

Abstract

Various studies have been conducted on the dynamics of the flow field resulting from the emergence of a shock wave from a duct. These studies have included differences in duct crosssection and even the interaction of shock waves from several tubes simultaneously. However, the simple case of two shock waves of similar strength emerging from orthogonal ducts, such as might be the case in the event of a blast in HVAC ducting, has not been well considered. In this study a shock tube was bifurcated to produce two waves of equal strength and close synchronisation which could then interact in an open test section. It was found that a complex reflected shock wave system forms where the two shock waves interact. This interaction was visualised using high-speed shadowgraph at a speed of 75 000 frames per second for incident shock wave Mach numbers of 1.15, 1.3, and 1.4. Related to these shock waves are the vortex lines shed at the diffraction edges, which are strongly influenced by the close proximity near the shared corner and show significant narrowing as a result. When the two shock waves are not perfectly synchronised, there is also a slipstream which develops from the shared corner and, for strong shock waves, a vortex structure bound by the shear layer. The exact shape of this vortex structure is still being interrogated. A complementary numerical study using the commercial code, ANSYS Fluent, to understand the topology of the shock waves and vortices produced better was undertaken though this also requires further refinement. Another interesting feature noted in the case of unsynchronised shock wave interaction is jetting from between the shear layers produced by the irregular reflection of the two shock waves from each other and also the possibility of a Kelvin-Helmholtz instability of the shear layer, which is not nominally plane unlike in previous observations thereof.