Propulsive performance of a pitching foil near the free surface

An experimental set-up has been designed to study the propulsive performance of a pitching symmetric foil near the water free surface. The system is actuated by a servo that rotates sinusoidally with different amplitudes and frequencies. Kinematics are measured using a precision potentiometer, whilst a six-axis load cell and a torque sensor provide hydrodynamic loads. A time resolved Particle Image Velocimetry system is used to study the flow generated by the moving foil in two planes, one parallel to the trailing edge and another perpendicular to the rotating shaft.

 

We will show how thrust, side force, power and efficiency, are influenced by the proximity of the foil to the free surface, by adjusting the gap between the upper edge of the foil and the water surface. The gap is varied from a condition in which there is virtually no influence of the free surface, to a point at which the upper edge of the foil is at the free surface. The interactions that take place at the free surface alter substantially the performance of the system.