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Bubbly shock propagation as a mechanism for sheet-to-cloud transition of partial cavities

Published online by Cambridge University Press:  01 August 2016

Harish Ganesh ,
Simo A. Mäkiharju  and
Steven L. Ceccio
Harish Ganesh*
Affiliation:
Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA
Simo A. Mäkiharju
Affiliation:
Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, MI 48109, USA
Steven L. Ceccio
Affiliation:
Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI 48109, USA Department of Naval Architecture and Marine Engineering, University of Michigan, Ann Arbor, MI 48109, USA
*
†Email address for correspondence: gharish@umich.edu
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Abstract

Partial cavitation in the separated region forming from the apex of a wedge is examined to reveal the flow mechanism responsible for the transition from stable sheet cavity to periodically shedding cloud cavitation. High-speed visualization and time-resolved X-ray densitometry measurements are used to examine the cavity dynamics, including the time-resolved void-fraction fields within the cavity. The experimentally observed time-averaged void-fraction profiles are compared to an analytical model employing free-streamline theory. From the instantaneous void-fraction flow fields, two distinct shedding mechanisms are identified. The classically described re-entrant flow in the cavity closure is confirmed as a mechanism for vapour entrainment and detachment that leads to intermittent shedding of smaller-scale cavities. But, with a sufficient reduction in cavitation number, large-scale periodic cloud shedding is associated with the formation and propagation of a bubbly shock within the high void-fraction bubbly mixture in the separated cavity flow. When the shock front impinges on flow at the wedge apex, a large cloud is pinched off. For periodic shedding, the speed of the front in the laboratory frame is of the order of half the free-stream speed. The features of the observed condensation shocks are related to the average and dynamic pressure and void fraction using classical one-dimensional jump conditions. The sound speed of the bubbly mixture is estimated to determine the Mach number of the cavity flow. The transition from intermittent to transitional to strongly periodic shedding occurs when the average Mach number of the cavity flow exceeds that required for the generation of strong shocks.

JFM classification

Drops and Bubbles: Cavitation Drops and Bubbles: Drops and Bubbles Multiphase and Particle-laden Flows: Multiphase flow

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Type
Papers
Information
Journal of Fluid Mechanics , Volume 802 , 10 September 2016 , pp. 37 - 78
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Copyright
© 2016 Cambridge University Press 

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Ganesh supplementary movie

Figure 8 : X-ray measurements on a incipient cavity

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Ganesh supplementary movie

Figure 12: X-ray measurements on a transitory cavity.

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Ganesh supplementary movie

Figure 12: X-ray measurements on a transitory cavity 2

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Ganesh supplementary movie

Figure 17: X-ray measurements on a shedding cavity.

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Video 5.3 MB