On Evolution and Acoustic Characteristics of a Compressible Vortex Ring

Abstract

Sound generated during formation of a compressible vortex ring at the open end of a shock tube and during its propagation is studied experimentally for shock Mach numbers of 1.28 to 1.61. The occurrence of different events such as primary ring formation, growth of the primary ring, secondary and tertiary vortices formation, and pinching-off are identified as predominant noise producing events. Wavelet analysis of the measured microphone signal identifies the time of occurrence of the above events which is verified using flow visualization pictures. The distributions of acoustic fluctuations are measured at a fixed distance from the exit center of the tube at different angular locations in horizontal diametrical plane and the directivity of the amplitude of sound pressures associated with the evolution of vortex ring is found. It is observed that the vortex ring's evolution sound is dominant between 25° to 50° angle from the axis of the shock tube. Experiments with short driver section produce rings with small trailing jet. Sound generated during the initial formation of these rings, after the diffraction of the incident shock at the open end of the shock tube, is dominant than the vortex ring's evolution sound. In shock tubes with larger driver section, the primary vortex ring is followed by a relatively longer trailing jet. In these cases, sound generated during formation of subsequent shear-layer vortices and their interaction with the trailing jet is also significant.