On the structures of compressible vortex rings generated by the compressible starting jet from converging and diverging nozzles

Abstract

Compressible vortex rings (CVRs) are one fundamental supersonic flow structure, which is usually generated by a compressible starting jet from a nozzle. Changes in the nozzle geometry provide efficient passive flow control to modulate the structures of CVRs. In this paper, a shock-tube apparatus with three open-ended nozzle geometries (straight, diverging, and converging nozzles) was designed to generate CVRs. The evolution of CVRs was recorded by using the schlieren technique. Three shock Mach numbers (Ms), including Ms=1.28, 1.48, and 1.59, were designed to generate the three typical CVRs. For Ms=1.28, shock-free CVRs are generated by the three nozzles. The CVR generated by the diverging nozzle has the largest size and was followed by a trailing jet, while the CVR generated by the converging nozzle has the smallest size with a strong turbulent trailing jet. For Ms=1.48, a CVR with embedded shock is generated by the straight nozzle, and the CVR is followed by a counter-rotating vortex ring (CRVR) for Ms=1.59. When the diverging nozzle is applied, multiple shock-cells (including barrel shock, Mach disk, oblique shock) appear in the trailing jet following the CVR, but quickly disappear as the CVR evolves. In contrast, as for the converging nozzle, strong shock-cells form in the trailing jet and interact with primary CVR. In addition, the strongest CRVRs are generated owing to the interaction between trailing shock-cells/vortices and CVRs for Ms=1.59. By identifying the CVR locations, it is found that the CVRs generated by converging nozzle have the largest average propagating velocity. However, as Ms increases, the change in the averaged propagation velocity brought by converging or diverging nozzles becomes slighter.