  The effect of flexibility on the hydrodynamic loads and on the flow
structures generated on a rectangular foil when oscillating in pitch has been
studied. Hydrodynamic loads were measured with a 6-axes balance, and the flow
structures were investigated by using a Digital Particle Image Velocimetry
(DPIV). It is known from nature's fin based propulsion mechanisms, that
appendage stiffness plays an important role in their propulsive efficiency. We
have studied four different stiffnesses, ranging from completely rigid to
highly flexible. Optimal efficiency has been observed for an intermediate case.
In this case, a moderately stronger trailing-edge vortex system takes place. A
very high level of flexibility of the foil results in a reduction of
efficiency.
