Power Generation of an Array of Three-Dimensional Oscillating Hydrofoils

We present new experiments on the power generation performance of combined heaving and pitching foils for varying linear array sizes. The water channel experiments use surface-piercing foils at a Reynolds number of Re ~7,500 that have an aspect ratio of AR = 8 and pitch about their one-third chord. We examine the effect of varying the array density and the array size from one, to two, to four, to an infinite number of foils arranged side-by-side and all operating with an out-of-phase synchronization with their nearest neighbors. This was achieved by utilizing a combination of one or two foil interactions with the image effect produced by one or two solid boundaries. The various array size cases were studied and compared by examining: (one foil case) a single foil far from the walls of tunnel, (two foils case) a single foil close to a wall, (four foils case) two foils with an out-of-phase synchronization near a wall, and (infinite foils case) a single foil confined between two walls. We compare data from a single isolated foil as the effective distance between the two foil, four foil, and infinite foil cases is reduced, that is, the array density is increased. We find that there are efficiency benefits for two close foils, but a degradation in efficiency for the larger array sizes as the density is increased. We examine the time-varying forces and flow data to understand the effects of varying array size and density.