On the decay of 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 foil near the water free surface. The system is actuated by a servo that rotates sinusoidally at a fixed amplitude and frequency. The experiment allows to adjust 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.

Kinematics are measured using a precision potentiometer, whilst a six-axis load cell and a torque sensor provide hydrodynamic loads. The robotic system hangs from an air bearing that ensures that there is no friction between the supporting structure and the system. Thanks to this arrangement, the thrust force can be measured redundantly using a single axis precision load cell, showing excellent agreement with the output of the six-axis balance. 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.

 

Results are presented in the form of dimensionless parameters of thrust, side force, power and efficiency, which are heavily influenced by the proximity of the foil to the free surface, showing a nonlinear behaviour with submergence depth. The vortex formation when the system is near the free surface is substantially modified, with the appearance of axial flow and the loss of symmetry in the wake.