Porpoising foil as a propulsion system

A potential flow solver (BEM) is used to simulate a porpoising foil in order to compare the hydrodynamic performance of the biomimetic propulsion device with a conventional propeller. The device is assumed to move in open-water. Cavitation or ventilation phenomena are not considered. The method is first verified against the Theodorsen theory. The numerical thrust-loading coefficient is in agreement with the theory for the whole range of the Strouhal number. The effect of the heave-to-chord ratio, the maximum pitch angle and the Strouhal number are then studied. The effect of the geometry is not investigated. An optimum efficiency is found for a Strouhal number from 0.2 to 0.7, and for a maximum pitch angle between 30°30° and 60°60°. The heave-to-chord ratio has no influence on the maximum efficiency. Moreover, this parametric study demonstrates that the Strouhal number plays the same role in the oscillating wing as the advance parameter in the propeller. A general approach for the comparison of the two propulsion systems is proposed. Despite their different nature, their hydrodynamic performances are similar. Finally, comparative results of unsteady state simulations are also presented since the propeller pitch variation is analogous to a change in the foil motion.

► The hydrodynamic performance of porpoising foil is studied numerically.
► Principle motion parameters are identified.
► Comparison with a conventional propeller shows similar efficiency.
► Variable pitch propeller more efficient than a change in the foil motion.