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On the mechanisms that sustain the inception of attached cavitation

Published online by Cambridge University Press:  26 August 2020

Omri Ram ,
Karuna Agarwal  and
Joseph Katz Open the ORCID record for Joseph Katz [Opens in a new window]
Omri Ram
Affiliation:
Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD21218, USA
Karuna Agarwal
Affiliation:
Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD21218, USA
Joseph Katz*
Affiliation:
Department of Mechanical Engineering, Johns Hopkins University, Baltimore, MD21218, USA
*
†Email address for correspondence: [email protected]
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Abstract

This experimental study addresses the longstanding question of why inception of attached cavitation on curved surfaces or hydrofoils at incidence is relatively insensitive to the concentration of free-stream nuclei. High-speed imaging and high-resolution particle image velocimetry measurements examine cavitation inception on three curved surfaces with varying pressure minima followed by regions with adverse pressure gradients. When these pressure gradients either thicken the boundary layer or cause local flow separation, thin $(50\text {--}60\ \mathrm {\mu }\textrm {m})$ low-momentum zones form close to the wall. Microbubbles trapped in these regions are generated initially from the collapse of intermittent attachment of travelling bubble cavitation. These bubbles migrate slowly upstream for a few milliseconds either under the influence of the adverse pressure gradients when the flow remains attached or carried by the recirculating flow when the boundary layer is separated. Their speed is only 2 %–4 % of the free-stream velocity, and their trajectories are erratic, indicating near-dynamic equilibrium. Owing to the low local pressure, their diameter increases by two to four times by non-condensable gas diffusion, from $10$ to $30\ \mathrm {\mu }\textrm {m}$ to the thickness of the low-momentum zone. At that time, either they are swept downstream by the free-stream flow or they become nuclei for new attached cavitation events. When the new patches collapse, new microbubbles form and the process repeats itself frequently, and independently of the free-stream nuclei. These phenomena do not occur when the adverse pressure gradients are too mild to create low-momentum zones with sufficient thickness to facilitate the slow upstream migration and growth.

JFM classification

Drops and Bubbles: Cavitation
Type
JFM Rapids
Information
Journal of Fluid Mechanics , Volume 901 , 25 October 2020 , R4
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Copyright
© The Author(s), 2020. Published by Cambridge University Press

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References

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Ram et al. supplementary movie 1

Cavitation on Model I

Download Ram et al. supplementary movie 1(Video)
Video 35.6 MB

Ram et al. supplementary movie 2

Upstream migration of micro-bubbles and cavitation inception on Model II

Download Ram et al. supplementary movie 2(Video)
Video 48.6 MB

Ram et al. supplementary movie 3

Upstream migration of micro-bubbles and cavitation inception within the Separated Region of Model III

Download Ram et al. supplementary movie 3(Video)
Video 46.4 MB