Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

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Experimental Study on The Effect of Tip Clearance on The Cavitation Inception of A Hydrofoil
Churui Wan 1,2, Xinguo Dong1,2, Peng Yan1,2, Ning Li1,2
1Marine Design & Research Institute of China
2Science and Technology on Water Jet Propulsion Laboratoryvortices. The main characteristics of tip vortex field structure can beobtained by combining and splicing these measurement results.
Sourders and Platzer (1981) describe the development of tip vortices
qualitatively in three main stages. The first is the emergence of
detached tip vortices. At first, intermittent cavitation bodies areobserved downstream of tip vortices. As the pressure decreases, vortex
cavitation attaches to the tip. It can be observed that the tip vortex
cavitation is slender, stable and nearly cylindrical at this time, and there
is no obvious growth or change in the range of chord lengths
downstream. With the further decrease of pressure, tip vortex cavitation
continues to increase, resulting in a twist, and gradually connected withthe leading edge of the slice cavitation. Sinha, Katz (1999) and Sinha,Katz, Meneveau (2000) combined PIV measurement results and used
LES based on Smargrinsky sublattice model to simulate the turbulence
inside the impeller of bladed guide vane centrifugal pump, and obtained
the multi-scale turbulence structure inside the impeller. Dreyer, Decaix,Munch-Alligne, (2014) used stereo-PIV to conduct a detailed
experimental study on the tip leakage vortex of NACA0009, and gave
the vortex intensity, vortex core position and velocity distribution underdifferent working conditions. The measurement results clearly revealed
a specific tip clearance, when the vortex intensity was the largest andcavitation was most likely to occur.
Farrell and Billet (1994) found that cavitation initiation time in axialflow pumps may be delayed if tip clearance is set to an optimal value ofabout 0.2 times the maximum blade thickness. Boulon, Callenaere,Franc, Michel (1999) studied the influence of clearance size on tip
vortices generated by elliptical wing surfaces without the relative
motion of endwall and wing surface. On the contrary, they found thatthe cavitation initiation index increased with the decrease of theclearance, and in most confined cases, tip vortices did not havecavitation. Similar observations were reported by Gopalan, Katz, Liu,(2002), where arched hydrofoils were studied in water tunnel.
In our simplified method, the influence of the relative motion between
the rotating blade and the casing of the axial turbine is not taken intoaccount. It is assumed that the average eddy is not fundamentally
altered by endwall motion, as shown by Wang and Devenport (2004).
The model is a hydrofoil, one end of the hydrofoil is fixed on the shaft
and can be used to adjust the angle of attack. The clearance between theother end of the hydrofoil and the wall is adjustable. By adjusting theclearance and the angle of attack, the inception cavitation number andABSTRACT
The main incentive to study cavitation inception has been theaccompanying jump of noise. In this paper, the problem of tip leakagevortex cavitation inception is studied through experiments methods.
The model tests of cavitation inception have been carried out at SSSRI
K15 Cavitation Tunnel. From the perspective of the experimental
results, it is instructive for engineering design. In a given blade tip
working load, ie, the actual angle of attack and incoming flow aredetermined, the tip clearance vortex cavitation can be suppressed byfurther optimization of the gap between the blade and wall surface.

KEY WORDS: Cavitation Inception; Tip Clearance; Hydrofoil.
INTRODUCTION
The Reynolds number of unsteady flow in hydraulic machinery isusually above 105, and the flow is mostly in turbulent state. The internal
flow of hydraulic machinery is extremely complex, including transition,
separation, wake, clearance flow, secondary flow, reflux and othercomplex flow phenomena. In axial turbomachinery, leakage flows
occur between blade tip and casing and are driven by the pressure
difference between blade pressure and suction side. The vorticity
generated by the leakage flow is rolled up into the so-called tip leakage
vortex, which is strongly influenced near the wall surface. Because thetip of the pump blade is a vorticity concentrated region, the pressure in
the vortex center is much lower than that in other flow regions.
Therefore, cavitation initiation usually occurs in this kind of vortex.
With the further decrease of the number of cavitation, the whole coreregion of the vortex will be filled with vapor body, and tip vortex
cavitation refers to this kind of situation, resulting in sometimes tosevere erosion of the runner blades and the casing, as well as anincrease in structural vibrations. These complex flow phenomena havebeen studied by numerical simulation and experimental observation.
In the 1990s, Arndt, Song, Kjeldsen, He, Keller (2000), Fruman,
Cerrutti, Pichon, Dupont (1995) and others carried out experimental
studies on simple hydrofoil, such as measurement of tip vortex fieldstructure and observation of tip vortex cavitation phenomenon. In theseexperimental studies, double-pulse holography or laser velocimetry areused to obtain the spatial and temporal velocity distributions in tip3340