VORTEX DYNAMICS OF A PITCHING HYDROFOIL BASED ON THE VORTICITY MOMENT THEORY

In this paper, the unsteady vortical structures and corresponding hydrodynamic characteristics of the pitching NACA66 hydrofoil are numerically simulated with the standard k- ωSST turbulence model and dynamic mesh technology. And the influence of local vortical structures on the transient lift is quantitatively obtained based on the finite-domain vorticity moment theory. The results show that during the upstroke stage, transition of laminar flow to turbulence moves from the trailing edge to the leading edge of the hydrofoil at small angle of attack. At relatively higher angle of attack, a clockwise trailing edge vortex ( defined as −TEV)appears on the suction surface firstly. It gradually increases in size and develops towards the leading edge to be fused with the clockwise leading edge vortex (defined as −LEV) there. Then the new developed −LEV interacts with the counterclockwise trailing edge vortex (defined as +TEV) until it falls off completely, which directly leads to the dynamic stall of the hydrofoil. Meanwhile, quantitative analysis based on the finite-domain vorticity moment theory shows that the attached −LEV and −TEV in the finite domain provide positive lift, while +TEV provides negative lift. At the moment when −LEV covers almost the whole suction surface, it contributes the most to the transient total lift which accounts for about 50%. It is also found that different parts of a vortex provide positive or negative lift. As for shedding vortices escaping from the finite domain, all regions of a vortex provide only consistent contribution instead, which means that a clockwise vortex provides positive lift, while a counterclockwise vortex provides negative lift. During the fluctuating stall stage, the overall contribution from the vortices out of the finite domain is quite little and has slight fluctuation, which reflects the unsteady characteristics of the vortical flow caused by the shedding and convection of large-scale vortices