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Growth, oscillation and collapse of vortex cavitation bubbles
- JAEHYUG CHOI, CHAO-TSUNG HSIAO, GEORGES CHAHINE, STEVEN CECCIO
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- Journal:
- Journal of Fluid Mechanics / Volume 624 / 10 April 2009
- Published online by Cambridge University Press:
- 10 April 2009, pp. 255-279
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The growth, oscillation and collapse of vortex cavitation bubbles are examined using both two- and three-dimensional numerical models. As the bubble changes volume within the core of the vortex, the vorticity distribution of the surrounding flow is modified, which then changes the pressures at the bubble interface. This interaction can be complex. In the case of cylindrical cavitation bubbles, the bubble radius will oscillate as the bubble grows or collapses. The period of this oscillation is of the order of the vortex time scale, τV = 2πrc/uθ, max, where rc is the vortex core radius and uθ, max is its maximum tangential velocity. However, the period, oscillation amplitude and final bubble radius are sensitive to variations in the vortex properties and the rate and magnitude of the pressure reduction or increase. The growth and collapse of three-dimensional bubbles are reminiscent of the two-dimensional bubble dynamics. But, the axial and radial growth of the vortex bubbles are often strongly coupled, especially near the axial extents of the bubble. As an initially spherical nucleus grows into an elongated bubble, it may take on complex shapes and have volume oscillations that also scale with τV. Axial flow produced at the ends of the bubble can produce local pinching and fission of the elongated bubble. Again, small changes in flow parameters can result in substantial changes to the detailed volume history of the bubbles.
Dynamics and noise emission of vortex cavitation bubbles
- JAEHYUG CHOI, STEVEN L. CECCIO
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- Journal:
- Journal of Fluid Mechanics / Volume 575 / March 2007
- Published online by Cambridge University Press:
- 07 March 2007, pp. 1-26
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The growth and collapse of a cavitation bubble forming within the core of a line vortex was examined experimentally to determine how the dynamics and noise emission of the elongated cavitation bubble is influenced by the underlying non-cavitating vortex properties. A steady line vortex was formed downstream of a hydrofoil mounted in the test section of a recirculating water channel. A focused pulse of laser light was used to initiate a nucleus in the core of a vortex, allowing for the detailed examination of the growth, splitting and collapse of individual cavitation bubbles as they experience a reduction and recovery of the local static pressure. Images of single-bubble dynamics were captured with two pulse-synchronized high-speed video cameras. The shape and dynamics of single vortex cavitation bubbles are compared to the original vortex properties and the local static pressure in the vortex core, and an analysis was performed to understand the relationship between the non-cavitating vortex properties and the diameter of the elongated cavitation bubble. Acoustic emissions from the bubbles were detected during growing, splitting and collapse, revealing that the acoustic impulse created during collapse was four orders of magnitude higher than the noise emission due to growth and splitting. The dynamics and noise generation of the elongated bubbles are compared to that of spherical cavitation bubbles in quiescent flow. These data indicate that the core size and circulation are insufficient to scale the developed vortex cavitation. The non-cavitating vortex circulation and core size are not sufficient to scale the bubble dynamics, even though the single-phase pressure field is uniquely scaled by these parameters. A simple analytical model of the equilibrium state of the elongated cavitation bubble suggests that there are multiple possible equilibrium values of the elongated bubble radius, each with varying tangential velocities at the bubble interface. Thus, the details of the bubble dynamics and bubble–flow interactions will set the final bubble dimensions.