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

A cavitating hydrofoil falls into the self-induced oscillation under some oscillation frequencies and cavitation numbers. Especially, the violent cavitation breakdown occurs periodically while shedding the vortex cavitation when the sheet cavity lengthens near at the trailing edge. It causes the unsteady and nonlinear fluid forces acting on the hydrofoil. This nonlinear phenomenon has not been clarified as yet when the breakdown frequency shedding the vortex cavitation locks in the pitching frequency of the cavitating hydrofoil. The experiment was carried out for a pitching and cavitating NACA hydrofoil in the TMIT cavitation tunnel. The pitching lift coefficient was clarified by load cell measurement with the cavity length and the reduced frequency as parameters. Moreover, particle and bubble image processing technique for PIV is applied to this unsteady cavitation flow. Accordingly, the equivalent void fraction, the velocity vector and the vorticity map including the inner cavity are qualitatively and quantitatively visualized under the shedding vortex cavitation lock-in phase.