Investigation of hysteresis effect of cavitating flow over a pitching Clark-Y hydrofoil

The objective of this paper is to investigate the hysteresis effect of cavitating flow over a Clark-Y hydrofoil undergoing a transient pitching motion at Reynolds number Re = 4.55×10⁵, cavitation number σ = 1.33, pitching frequency f*= 2 Hz via combined experimental and numerical studies. A hysteresis phenomenon is observed in the hydrodynamic curve and cavity area in increasing and decreasing of the angle of attack α. The hydrodynamic curves are divided into three regions: Regions A, B and C. For Region A, the lift coefficient of downstroke is lower than that of the upstroke, and the lift coefficient curve of the downstroke is more unstable. The formation and development of counterclockwise trailing edge vortex (TEV) are responsible for the decline and fluctuation of lift during the downstroke, thus leading to the increase of the hysteresis loop. Compared with the upstroke, the hydrodynamic curve in downstroke is shifted laterally to some extent in Region B. The delay effect is the main factor leading to the shift of the hydrodynamic curve, which corresponds to the minimum hysteresis loop. In Region C, the hysteresis loop is maximum and the evolution trend of the hydrodynamic curve is peak-valley opposites. When the direction of oscillation changes, the detachment and dwell time of the cavity are advanced, thus leading to the difference of hydrodynamic curve and the increase of hysteresis loop.