Lock-in Phenomenon of Pitching Hydrofoil with Cavitation Breakdown : Unsteady Fluid Force Characteristics and Visualization of Flow Structure

The violent cavitation breakdown occurs periodically during vortex cavitation shedding when a sheet cavity elongates near the trailing edge of a hydrofoil. This causes unsteady and nonlinear fluid forces to act on the hydrofoil. This nonlinear behavior, when the breakdown frequency locks at the pitching frequency of the cavitating hydrofoil, has not yet been clarified. The unsteady lift coefficient and the flow structure of a pitching NACA hydrofoil under the lock-in phenomenon were clarified by load cell measurement and particle and bubble image processing for PIV. As the main result, in the Subcavitation region, when the elongation of the sheet cavity cannot follow the variation of the pitching angle at a highly reduced frequency, the pitching lift coefficient is offset similar to that of nonpitching oscillation. In the Transition region where cavitation breakdown frequently occurs, the reduced frequency range for the pitching lift coefficient can be characterized roughly into the Unlock-in, Quasi-lockin and Lock-in ranges according to whether or not the breakdown frequency is synchronized with the pitching frequency.