The flow regime and hydrodynamic performance for a pitching hydrofoil

The objective of this paper is to study the flow regime and hydrodynamic performance for a pitching Clark-Y hydrofoil. The aims are to (1) improve the understanding of the interplay between the hydrodynamic performance, unsteady flow structures and dynamic motion of the hydrofoil, (2) study the influence of the pitching rate on the transition of different flow regimes. The experimental investigations were conducted in the looped cavitation tunnel, and the dynamic moment measurement system was applied to obtain the hydrodynamic forces. The pitching hydrofoil is controlled to rotate from α⁺ = 10° to α⁺ = 15° firstly, then goes from α⁺ = 15° to α⁻ = 5°, finally goes back to α⁺ = 10° from α⁻ = 5°. The pitching rate is varying with the Reynolds number Re = 4.4 × 10⁵. The numerical investigations were performed by solving the incompressible URANS equations using the coupled k-ω SST turbulence model and γ-Re_θ transition model. The numerical results agree well with the experimental measurements. The pitching motion affects the turbulence kinetic energy distribution around the hydrofoil, leading to the delay or acceleration of the transition between different flow patterns. During the pitching process, higher level of turbulence kinetic energy distribution causes earlier transition from laminar to turbulence. Moreover, hysteresis effect of the hydrodynamic force is observed. For the upstroke stage, the higher pitching rate promotes the laminar separation slightly and intensifies the delay of turbulence separation. For the downstroke stage, the higher pitching rate promotes the turbulence separation extensively. The first leading edge vortex (LEV) and anticlockwise trailing edge vortex (TEV) are delayed with the increase of pitching rate, which is responsible to the delay of dynamic stall. The lower pitching rate shrinks the hysteresis loops and intensifies the fluctuation of the dynamic force.