Fish and birds experience different forces when they swim/fly near a flat surface (e.g. seabed, solid ground, still lake). We found there is a stable equilibrium altitude where they can maintain the position with extra performance benefit. Moreover, we also found the wall-effects still exist when we release the lateral freedom of hydrofoil, and the equilibrium altitude is independent of the initial position of the hydrofoil.
Besides investigating unsteady wall effects, we are also working on finite aspect ratio effects on unsteady wall effects.