Abstract
The objective of this paper was to investigate acoustic pressure waves and the transient flow structure emitted from the single bubble near an elastic boundary based on the particle image velocimetry (PIV). A combination of an electric-spark bubble generator and PIV were used to measure the temporal bubble shapes, transient flow structure, as well as the mid-span deflection of an elastic boundary. Results are presented for three different initial positions near an elastic boundary, which were compared with results obtained using a rigid boundary. A formula relating velocity and pressure was proposed to calculate the acoustic pressure contours surrounding a bubble based on the velocity field of the transient flow structure obtained using PIV. The results show the bubbles near the elastic boundary presented a “mushroom” bubble and an inverted cone bubble. Based on the PIV-measured acoustic pressure contours, a significant pressure difference is found between the elastic boundary and the underside of the bubble, which contributed to the formation of the “mushroom” bubble and inverted cone bubble. Furthermore, the bubbles had opposite migration direction near rigid and elastic boundaries, respectively. In detail, the bubble was repelled away from the elastic boundary and the bubble was attracted by the rigid boundary. The resultant force made up of a Bjerknes force and buoyancy force dominated the migration direction of the bubble.
Highlights
The occurrence of cavitation bubbles is accompanied by many serious hazards, such as noise emission [1,2], material damage [3,4], and performance drops in hydraulic machinery [5,6]
The behaviors of bubble pulsation are significantly complex, including expansion, contraction, high-speed jet, mushroom shape, and opposite migration etc. [7,8,9,10]. It often involves the basic mechanical problems of the interaction of multiple physical fields, such as a bubble and a boundary, a bubble and an acoustic pressure wave, acoustic pressure wave and boundary, and acoustic pressure wave and acoustic pressure waves, which brings significant challenges to understand the behaviors of bubble pulsation and the evolution mechanism of acoustic pressure waves
When bubble is in the free field, the acoustic pressure wave released by the bubble pulsation will gradually spread to the far field; when the bubble is pulsating near the elastic boundary, the acoustic pressure wave released by the bubble will act on the bubble itself again through the boundary reflection, which has a complex impact on the pulsation shape and collapse characteristics of the bubble [15]
Summary
The occurrence of cavitation bubbles is accompanied by many serious hazards, such as noise emission [1,2], material damage [3,4], and performance drops in hydraulic machinery [5,6]. Kröninger et al [26] investigated the velocity field in the vicinity of a laser-generated cavitation bubble by means of particle tracking velocimetry (PTV) and numerical simulation They found that the accuracy of the PTV method is good agreement with assessed the experimental data with the flow field around the bubble. Vogel and Lauterborn [27] studied the flow field around bubbles during their collapse near a solid boundary by combining particle image velocimetry (PIV) and high-speed photography. They found that velocities could be determined within a range of from 2 m/s to 30 m/s and within a field of 10 × 10 mm.
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