The influence of fluid-structure interaction on cloud cavitation about a hydrofoil

The dynamics of cloud cavitation about rigid and flexible 3D hydrofoils is investigated in a cavitation tunnel. The two hydrofoils have identical undeformed geometry of tapered planform, NACA-0009 section and cantilevered setup at the hydrofoil root. The rigid model is made of stainless steel and the flexible model of carbon and glass-fibre reinforced epoxy resin with an effectively quasi-isotropic lay-up without material bend-twist coupling. Tests were conducted at a fixed incidence of 6°, a chord-based Reynolds number of 0.7×10⁶ and a cavitation number ranging from 1.0 to 0.2. Unsteady force measurements were made simultaneously with high-speed imaging to enable correlation of forces and with cavity dynamics. High-resolution force spectra at discrete cavitation numbers and separate pressure sweeps were taken to acquire spectrograms of frequency response as a function of cavitation number. Three shedding modes, designated as types 1, 2 and 3, are apparent for both rigid and flexible hydrofoils although significant differences in peak amplitudes were observed. Types 2 and 3 shedding occur at high cavitation numbers where frequency varied with cavitation number and high-speed imaging showed the dominant shedding mechanism to be due to re-entrant jet formation. The type 1 shedding that developed with reduction in cavitation number, once cavity lengths grew to about full-chord, occurred at a nominally constant frequency. In this case, the imaging showed the dominant mechanism to be shockwave formation. This behaviour has been reported upon extensively in literature although there are some new features apparent from the data. The flexibility of the composite hydrofoil was found to increase the magnitude of the force fluctuations for the low frequency type 1 mode compared to the rigid hydrofoil. However, hydrofoil flexibility was seen to dampen the fluctuating magnitude of the high-frequency type 2 and 3 modes, despite being close to the hydrofoil’s natural frequency.