Flow Control Through Geometric Modifications to Improve Airfoil/Hydrofoil Performance

Geometric modification as the most effective passive flow control method has recently received wide attention due to its enormous potential in enhancing performance characteristics of airfoils or hydrofoils without expensive manufacturing and maintenance cost. Two primary passive flow control modifications, known as leading-edge tubercles and internal slots and their applications in airfoils/hydrofoils have been investigated in this dissertation. For the hydrofoil, since free surface effects cannot be neglected, the interaction between the hydrofoil-motion induced waves on the free surface and the hydrofoil has been studied as well. In the theoretical approach aspect, an empirically-based model based on an iteration scheme has been proposed for predicting the lift coefficients of twisted airfoils with leading-edge tubercles by using experimental data for untwisted airfoils. With both numerical and experimental investigations, this dissertation has discussed the application of a custom optimized-design internal slot on a NACA 634-021 airfoil blade to allow ventilation of flow through the slot from the pressure side to the suction side of the blade, in support of delaying flow separation, and stall. The combined effect of an internal slot in an airfoil and transverse leading-edge tubercles on its performance has been further studied both numerically and experimentally. Moreover, performance of a NACA 634-021 hydrofoil in motion under and in close proximity of a free surface for a large range of AoAs has been studied. Lift and drag coefficients of the hydrofoil at different submergence depths are investigated both numerically and experimentally. The results of the numerical study are in good agreement with the experimental results. The agreement confirms the new finding that for a submerged hydrofoil operating at high AoAs close to a free surface, the interaction between the hydrofoil-motion induced waves on the free surface and the hydrofoil results in mitigation of the flow separation characteristics on the suction side of the foil and delay in stall, and improvement in hydrofoil performance. A similarly submerged hydrofoil with a custom-designed internal slot has further been studied. The performance characteristics of the slotted hydrofoil in the presence of the free surface are investigated both numerically and experimentally.