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.

Characteristics of Noise Produced during Impingement of a Compressible Vortex Ring on a Wall

“…In this study different discrete events associated with the impingement of a vortex ring with a wall are identified from acoustics signal through wavelet based filtering method similar to [2]. The occurrences of these events for M = 1.31 & 1.55 are verified with the flow visualization pictures.…”

“…The flow field is split into three stages of evolution with respect to noise producing events. Substantial noise is produced during initial formation and growth of the vortex ring, which is discussed in Murugan & Das [2]. Depending on the position of the wall, the reflected shock from the wall interacts with the ring and acts as a major source of noise [6 -11].…”

“…1). In order to study and compare the noise generated from other events, these shock signatures are subtracted from the signal as discussed in Murugan & Das [2]. The subtracted signal is shown in Fig 3b. Discrete Wavelet Transform (DWT) based filtering, using third order Daubechies family mother wavelet is performed on the signal of Fig.…”

“…Das et al [1] studied evolution noise of vortex rings for M < 1.4. Murugan and Das [2] presented a new wavelet based method to identify different sound producing events during vortex ring's evolution from the measured sound pressure. They verified the occurrence of the events with high speed flow visualizations.…”

Experimental Investigation on Acoustic Wave Generation due to Supersonic Hot Jet Impingement on an Inclined Flat Plate

“…Figure 2 illustrates the position of the microphones relative to the nozzle exit for the free jet. It should be emphasized that while the acoustic signature consists of a transient starting phenomenon and a quasi-steady jet formation (Oertel, 1979;Lee et al, 2008;Murugan et al, 2008), the focus of this paper is on the latter one (as shown in Fig. 3).…”