Characteristics of shock tube generated compressible vortex rings at very high shock Mach numbers

Abstract

Compressible vortex rings are usually formed at the open end of a shock tube. They show exciting flow phenomena during their formation, evolution, and propagation depending on the shock Mach number (Ms) and exit flow conditions. This study considers high shock tube pressure ratio (PR) cases showing hitherto unknown, spectacular flow structures. With hydrogen as a driver section gas at high PR, a supersonic compressible vortex ring having vortex ring Mach number (Mv) >1 is obtained for the first time. The formation of multiple triple points and the corresponding slipstream shear layers and, thus, multiple counter-rotating vortex rings (CRVRs) behind the primary vortex ring at different radial locations, in addition to the usual CRVRs, appears to be a unique characteristic for high Mach number vortex rings. During the formation stage, a vortex layer of reverse circulation than that of the primary vortex ring gets generated from the outer wall of the shock tube. The instability of such a vortex layer creates another series of opposite circulation vortices, which later interfere with the primary vortex core considerably. Also, a near stationary slipstream vortex and multiple fast-moving tiny vortices of opposite circulation to the slipstream vortex are observed near the central zone. Mechanisms for the formation of these complex vortical structures are identified. The implications of these phenomena on the vortex ring's geometric and kinematic characteristics, such as ring diameter, core diameter, circulation, and translational velocity, are discussed in detail, illustrating their differences with low vortex ring Mach number cases considering 0.31 < Mv < 1.08.