The influence of fluid-structure interaction on cloud cavitation about a stiff hydrofoil. Part 1.

The physics associated with various cavitation regimes about a hydrofoil is investigated in a variable-pressure water tunnel using high-speed photography and synchronised force measurements. Experiments were conducted on a relatively stiff stainless steel hydrofoil at a chord-based Reynolds number, for cavitation numbers, , ranging from 0.2 to 1.2, with the hydrofoil experiencing sheet, cloud and supercavitation regimes. The NACA0009 model of tapered planform was vertically mounted in a cantilevered configuration to a six-component force balance at an incidence, , of to the oncoming flow. Tip deformations and cavitation behaviour were recorded with synchronised force measurements utilising two high-speed cameras mounted underneath and to the side of the test section. Break-up and shedding of an attached cavity was found to be due to either interfacial instabilities, re-entrant jet formation, shockwave propagation or a complex coupled mechanism, depending on . Three primary shedding modes are identified. The Type IIa and IIb re-entrant jet-driven oscillations exhibit a non-linear dependence on , decreasing in frequency with decreasing due to growth in the cavity length, and occur at higher values (Type IIa: 0.4-1.0; Type IIb: 0.7-0.9). Shockwave-driven Type I shedding occurs for lower values (0.3-0.6) with the oscillation frequency being practically independent of . The Type IIa oscillations locked in to the first sub-harmonic of the hydrofoil's first bending mode in water which has been modulated due to the reduced added mass of the vapour cavity. Supplementary movies are available with the online version of the paper.