The influence of bend-twist coupling on the dynamic response of cavitating composite hydrofoils

Cavity shedding frequencies and structural response of a rigid metal hydrofoil and two flexible composite hydrofoils are investigated in a cavitation tunnel. All three hydrofoils have identical geometry with a trapezoidal planform, span of 0.3 m, 0.12 m root chord tapering to 0.06 m tip chord and constant NACA0009 section. The composite hydrofoils have -30° and +30° nominal fibre alignment to give positive and negative bend-twist coupling respectively. The hydrofoils were tested at a fixed chord based Reynolds number of 800,000, a fixed incidence of 6◦ and a range of cavitation numbers from inception to a supercavity. The cavity shedding frequencies and coupled structural response of each hydrofoil were measured using highspeed photography and dynamic force measurement using a six-component force balance. Cavity lengths on the rigid and flexible hydrofoils are compared with classical analytical theory. Both composite hydrofoils exhibited lock-in behaviour: the +30° hydrofoil at frequencies of 1/2 and 3/2 of the first natural frequency and the -30° hydrofoil at 1/2 of the first natural frequency only. At low cavitation numbers, all three hydrofoils display lock-in at the lower frequency indicating that cavity shedding dynamics dominates over any FSI response in this region.