Partial confinement effects on the performance of a flapping foil power generator

The impacts of partial confinement on the power extraction performance of a flapping foil generator at a Reynolds number of 1100 are numerically studied using an immersed boundary-lattice Boltzmann method. Four confinement levels are implemented with two thin plates of finite size symmetrically placed at the distance of 1.5, 2, 3, and 4 foil chord length from the neutral position of the flapping foil. Parametric studies on plate lengths varying from 10 to 50 foil chord lengths at the four confinement levels are conducted. The results show that the power-extraction efficiency increases nearly monotonically with the upstream plate lengths while the impact of the downstream plate lengths is much less significant, indicating that upstream confinement is the dominant factor influencing the power-extraction performance. Contrary to the performance improvement observed in studies on the effect of infinite walls, the efficiency decreases dramatically with the decrease in the distance from the plates to the foil. The reasons for the dramatically decreased performance due to confinement effects are found. First, the interactions between the boundary layer of the plates and leading edge vortices formed on the foil reduce the size of the low-pressure region on the suction surface of the foil, leading to reductions in lift forces and consequently to major reductions in the extracted power. In addition, large mass flow deficits between the finite plates are observed when the distance between the two plates is small, indicating substantial reductions in potential power that can be extracted from the inflow.