Oscillating Hydrofoil Propulsion for Human-Powered Watercraft Applications
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Unlike conventional propellers, flapping wings may generate large amplitude oscillating forces, which can make them difficult to incorporate into a craft design. This is particularly true for a single, vertically oscillating hydrofoil, as part of a surface water craft where the cyclic lift of the hydrofoil disrupts the craft stability. This thesis begins by reviewing the history of human-powered watercraft with a focus on those having flapping foil propellers. This review combined with a review of the literature provides a balanced overview on how flapping wing propellers are currently designed. Current literature shows that although the mean performance of an oscillating foil has been determined in terms of the Strouhal number and the angle of attack, relatively little describes performance directly in terms of the foil motion. Hence, predicting temporal hydrodynamic forces acting on an oscillating foil is difficult. This provides motivation for research investigating the temporal performance of an oscillating foil directly in terms of its motion.
In this thesis, experimental equipment designed to measure the hydrodynamic forces on a heaving object is presented. Key features of the equipment are analysed to show how measurement accuracy is maintained. Experimental measurements of unsteady hydrodynamic forces acting on a heaving cylinder, flat plate, symmetrical foil, and an asymmetrical foil are analysed with respect to the heaving motion. Firstly, the object motion is limited to one degree of freedom; pure heaving with zero forward velocity, to investigate the start-up conditions of the oscillating hydrofoil propeller. Secondly, these results are expanded on by adding a steady forward velocity component to the object motion to investigate how the hydrodynamic forces on the object are affected by the cross-flow.
Experimental temporal hydrodynamic force measurements presented in this thesis show how the relative composition of hydrodynamic drag and inertia forces change with oscillating frequency, and forward velocity, affecting the phase, magnitude, and profile of the force cycles. This composition is also influenced by the cross-section of the oscillating object and the presence of a free surface. Current marine engineering equations for unsteady hydrodynamic forces on an object in an oscillating flow are validated for a cylinder. However, they are found to contain significant error when predicting the unsteady hydrodynamic forces on an oscillating hydrofoil. Contributions of this thesis link oscillating foil propulsion research to common marine engineering equations with the intent of making flapping wing propeller design more accessible to the general engineering community.