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Ventilated cavities on a surface-piercing hydrofoil at moderate Froude numbers: cavity formation, elimination and stability
Published online by Cambridge University Press: 29 June 2016
Abstract
The atmospheric ventilation of a surface-piercing hydrofoil is examined in a series of towing-tank experiments, performed on a vertically cantilevered hydrofoil with an immersed free tip. The results of the experiments expand upon previous studies by contributing towards a comprehensive understanding of the topology, formation and elimination of ventilated flows at low-to-moderate Froude and Reynolds numbers. Fully wetted, fully ventilated and partially ventilated flow regimes are identified, and their stability regions are presented in parametric space. The stability of partially and fully ventilated regimes is related to the angle of the re-entrant jet, leading to a set of criteria for identifying flow regimes in a laboratory environment. The stability region of fully wetted flow overlaps those of partially and fully ventilated flows, forming bi-stable regions where hysteresis occurs. Ventilation transition mechanisms are classified as formation and elimination mechanisms, which separate the three steady flow regimes from one another. Ventilation formation requires air ingress into separated flow at sub-atmospheric pressure from a continuously available air source. Ventilation washout is caused by upstream flow of the re-entrant jet. The boundary denoting washout of fully ventilated flow is expressed as a semi-theoretical scaling relation, which captures past and present experimental data well across a wide range of Froude and Reynolds numbers.
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- © 2016 Cambridge University Press
References
Harwood et al. supplementary movie
Two-perspective video (suction surface below water, looking down along trailing edge) of spontaneous ventilation formation (inception, followed by stabilization) on the surface-piercing hydrofoil. The hydrofoil accelerates from rest at α=15 deg, ARh=1.0. Ventilation formation is initiated by leading-edge stall very early in the run.
Harwood et al. supplementary movie
Two-perspective video of tip-vortex-induced ventilation formation on the surface-piercing hydrofoil at α=15 deg, Fnh=3.5, ARh=1.0. Bubble coalescence occurs at the trailing edge of the free tip, where air is entrained in the tip vortex.
Harwood et al. supplementary movie
Three-perspective video (suction-surface above water, suction-surface below water, and looking down along the trailing edge) of perturbation-induced ventilation formation on the surface-piercing hydrofoil at α=10 deg, Fnh=2.75, ARh=1.0. The perturbation in question is the high-pressure air-jet located at the junction of the free surface and hydrofoil leading edge. Formation follows activation of the air-jet, with inception and stabilization occurring in rapid succession.
Harwood et al. supplementary movie
Two-perspective video of ventilation elimination (washout, followed by reattachment) on the surface piercing hydrofoil undergoing deceleration at α=15 deg, ARh=1.0. Washout is initiated by the re-entrant jet, which destabilizes the FV cavity, causing large-scale cavity shedding and a transition to PV flow.
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