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An experimental investigation into supercavity closure mechanisms

Published online by Cambridge University Press:  19 January 2016

Ashish Karn ,
Roger E. A. Arndt  and
Jiarong Hong
Ashish Karn
Affiliation:
Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55455, USA Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA
Roger E. A. Arndt
Affiliation:
Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55455, USA Department of Civil, Environmental and Geo-Engineering, University of Minnesota, Minneapolis, MN 55455, USA
Jiarong Hong*
Affiliation:
Saint Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN 55455, USA Department of Mechanical Engineering, University of Minnesota, Minneapolis, MN 55455, USA
*
†Email address for correspondence: jhong@umn.edu
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Abstract

Substantial discrepancy in the conditions for attainment of different closure modes of a ventilated supercavity has existed widely in the published literature. In this study, supercavity closure is investigated with an objective to understand the physical mechanisms determining closure formation and transition between different closure modes and to reconcile the observations from prior studies under various flow settings. The experiments are conducted in a closed-wall recirculating water tunnel to image ventilated supercavity closure using high speed and high-resolution photography and simultaneously measure pressure inside the cavity. The flow conditions are varied systematically to cover a broad range of water velocity, ventilation flow rate and cavitator size, which correspond to different Froude numbers, air entrainment coefficients and blockage ratios, respectively. In addition to the classical closure modes reported in the literature (e.g. re-entrant jet, twin vortex, quad vortex, etc.), the study has revealed a number of new closure modes that occur during the transition between classical modes, or under very specific flow conditions. Closure maps are constructed to depict the flow regimes, i.e. the range of Froude number and air entrainment coefficient, for various closure modes at different blockage ratios. From the closure map at each blockage ratio, a critical ventilation flow rate, below which the supercavity collapses into foamy cavity upon reduction of Froude number, is identified. The air entrainment coefficients corresponding to such critical ventilation rate are found to be independent of blockage ratio. It has been observed that in the process of generating a supercavity by increasing ventilation flow rate, the cavitation number gradually reduces to a minimum value and stays fixed upon further increments in the ventilation rate. Once a supercavity is formed, the ventilation rate can be decreased to a much lower value with no change in cavitation number while still maintaining a supercavity. This process is accompanied by a change in closure modes, which generally goes from twin vortex, to quad vortex, and then to re-entrant jet. In addition, the blockage effect is shown to play an important role in promoting the occurrence of twin-vortex closure modes. Subsequently, a physical framework governing the variation of different closure modes is proposed, and is used to explain mode transition upon the change of flow conditions, including the blockage effect. This framework is further extended to shed light on the occurrence of closure modes for ventilated supercavitation experiments across different types of flow facilities, the natural supercavity closure and the pulsating supercavity reported in the literature. Finally, in combination with a recent numerical study, our research discusses the role of the internal flow physics on the observed features during supercavity formation and closure-mode transition, paving the way for future investigations in this direction.

JFM classification

Drops and Bubbles: Bubble dynamics Drops and Bubbles: Cavitation Multiphase and Particle-laden Flows: Multiphase flow
Type
Papers
Information
Journal of Fluid Mechanics , Volume 789 , 25 February 2016 , pp. 259 - 284
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Copyright
© 2016 Cambridge University Press 

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Karn et al. supplementary movie

A side view of the re-entrant jet closure mechanism. Note how the liquid outside the cavity closure gushes into the cavity and the air is entrained out of the cavity in the form of toroidal vortices.

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Video 785.3 KB

Karn et al. supplementary movie

A bottom view of the re-entrant jet closure mechanism. Note how the liquid outside the cavity closure gushes into the cavity and the air is entrained out of the cavity in the form of toroidal vortices.

Download Karn et al. supplementary movie(Video)
Video 2.1 MB

Karn et al. supplementary movie

A side view of the twin vortex closure mechanism. The individual vortices can be seen more clearly in the bottom view.

Download Karn et al. supplementary movie(Video)
Video 110.5 KB

Karn et al. supplementary movie

A bottom view of the twin vortex closure mechanism. Note that the re-circulation of the two vortices are in the opposite direction.

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Video 2.6 MB

Karn et al. supplementary movie

A side view of the quad-vortex closure mechanism. The viewing angle is at an angle to the normal to show the four vortices. Both the front and rear bottom vortex have a vortex lying above it.

Download Karn et al. supplementary movie(Video)
Video 1 MB

Karn et al. supplementary movie

A side view of the hybrid quad-vortex - re-entrant jet closure mechanism. The two pairs of vortices can be seen along with the liquid gushing into the cavity. The lower pair of vortices are more stable compared to the upper pair.

Download Karn et al. supplementary movie(Video)
Video 4.7 MB

Karn et al. supplementary movie

A side view of the unstable/hybrid twin vortex - quad vortex closure mechanism. Note that in this unstable mode, one to three vortices might intermittently disappear. The lower pair of vortices are more stable compared to the upper pair.

Download Karn et al. supplementary movie(Video)
Video 7.5 MB

Karn et al. supplementary movie

A side view of the hybrid twin vortex - re-entrant jet closure mechanism. Along with the two vortices, liquid gushing into the cavity can be seen as well.

Download Karn et al. supplementary movie(Video)
Video 3.5 MB

Karn et al. supplementary movie

A side view of the interacting vortex closure mechanism. It can be clearly seen that the two vortices interact with each other to form one single thick vortex at the closure.

Download Karn et al. supplementary movie(Video)
Video 1.5 MB