Effect of chordwise flexibility on pitching foil propulsion in a uniform current

The role of the chordwise flexural stiffness in a rectangular foil undergoing pure pitching has been investigated experimentally. Digital Particle Image Velocimetry (DPIV) and load measurement with a 6-axes balance have been done to study the flow patterns and hydrodynamic forces around the flapping flexible foils. We have found a transition of thrust generation in the semi − flexible foil with θ0=72∘ for all the Re studied between the range of 0.25≤St≤0.6 where it is known that maximize the thrust by a flapping foil. As a result, the optimum flexural stiffness has been determined with a semi − flexible foil up to 69% efficiency for Re = O(104).