Investigation of the acoustic oscillation self-adjustment mechanism in impinging swirling flows

Abstract

The mechanism of acoustic oscillation generation due to the formation of stable vortex structures in a medium in motion is considered with reference of the example of impinging swirling air flow. It is established that, as the swirling flow attains a limiting flow-rate velocity, the amplitude frequency characteristic of the acoustic oscillations in the hydromechanical system restructures itself. The discovered effect of the acoustic oscillation self-adjustment manifests itself in the resonance amplification of the amplitudes of the vortex chamber eigenfrequencies at the expense of the absorption of the acoustic oscillation spectrum components generated by the vortex structure of the flow.An investigation of the vortex formation effect on the acoustic oscillation generation on the sound and infrasound ranges is of great importance in developing a new, more perfect generation of different vehicles and power setups, including nuclear power plants.The phenomenon of acoustic oscillation generation in liquid and gaseous media due to the vortex formation effects was named the "vortex sound" [1]. In accordance with the conventional representations, the vortex sound generation mechanism was mainly connected with the vortex formation effect in flows past bodies (Aeolian harp [1]). At the same time, the experimentally established and fairly well studied facts include the sound oscillation generation in the so-called vortex whistles or vortex sound generators. The authors of [2-4] performed some theoretical and experimental studies which can provide the foundation for understanding the mechanism of the vortex generation of acoustic oscillations. In those studies the sound radiation process was explained basing on the ideas connected with the manifestation of the hydrodynamic instability of a swirling flow due to the precession of the region of quasi-solid rotation of the central vortex generated under the action of geometric and dynamic flow parameters. It was believed that the vortex generator, as an acoustic radiator, represents a rotating vortex.As shown in the experiments [5][6][7], the reason for the acoustic oscillation appearance in swirling flows can be the effect of the formation of a deterministic internal vortex structure of the flow. The relation between the vortex flow structure and the acoustic resonance generation effect was established using the acoustic method proposed in [5] and based on the measurement of the frequency characteristics of sound oscillations in a swirling impinging jet. The theoretical analysis using the acoustic flow approximation [5] and helical flow theory [8], together with the comparison of the experimental and calculated results, validate the suggested physical flow model which predicts the appearance of vortex resonances due to the vortex flow topology.

Studying the thermal effect in a swirled acoustic flow

“…A sharp decrease in pressure in this region, thanks to the vortex formation, provided the transition condition through the dew point (Fig. 2b) The theoretical analysis carried out in [1], using of Lissajous figures corresponding to the ratio of two clearly defined harmonic oscillations, allowed proposing a physical model explaining the mechanism of energy redistribution in an open system at resonance. The first harmonic oscillation with frequency f 1 is associated with the formation of longitudinal helical vortices whose transverse scale is determined by the width of the slit.…”

“…A redistribution mechanism for the energy of an open system at resonance, associated with the influx of air from the external medium is considered. Temperature distributions on the lower surface of the obstacle were obtained using the thermal imager, and the energy losses associated with the flow cooling due to expansion were estimated.An experimental investigation of the thermal effect in a swirled acoustic flow was carried out within the framework of studying the effect of self-regulation of acoustic oscillations in a hydromechanical system in the pre-resonance and resonance regimes [1]. A detailed description of the experimental setup is given in [2].…”

“…An experimental investigation of the thermal effect in a swirled acoustic flow was carried out within the framework of studying the effect of self-regulation of acoustic oscillations in a hydromechanical system in the pre-resonance and resonance regimes [1]. A detailed description of the experimental setup is given in [2].…”