Coupled dynamics of a flapping foil wave powered vessel

Abstract

Wave propelled vessels utilize submerged flapping foils to convert wave energy directly into propulsion. This works by coupling the response of foils operating in a wavy flow with the flapping motion driven by the wave-induced hull motions. For platforms that are solely propelled using this method the free running forward speed is dictated by the magnitude and frequency of the ambient wave energy. Submerged flapping foils also have the potential to recover wave energy for onboard power generation, and, in this way, a vessel could be both powered and propelled by the ambient wave energy. This thesis both numerically and experimentally investigates the coupled dynamic response of a wave powered vessel, which enables the prediction of the free running forward speed and an assessment of the potential for wave energy recovery.

A hybrid numerical model has been developed to capture the free running response of a flapping foil wave powered vessel, and this model has been validated by the experimental analysis. The hybrid method combines the frequency domain strip theory approach for solving the seakeeping response with a time domain solution for the response of a spring loaded flapping foil. The numerical model also evaluates the electromechanical conversion of wave energy by modelling the power generated by a permanent magnet tubular linear generator.

Free running wave propulsion experiments were performed in both regular head and following waves using a model with spring loaded flapping foils at the bow and stern over a range of wave frequencies. The experimental setup incorporated wave energy recovery in the form of electrical power by linking the flapping foils with a power take-off device, and a series of experiments were conducted for different wave heights and frequencies. In addition to validating the numerical model, the experimental results show a notable difference in the response of the forward and aft foils in head and following waves, and confirm numerical predictions that the optimal location of the foils is at or beyond the perpendiculars of the vessel. Furthermore, the experimental and numerical analysis demonstrate that it is possible to recover wave energy from the use of submerged foils.

Numerical simulations have been carried out to provide a more detailed insight into the effect on the coupled response of: foil size and location; flapping parameters; and seakeeping characteristics. In particular, it is shown that the wave-phasing parameter and the foil spring constant is of significant importance for the efficiency of wave propulsion. Lastly, the numerical analysis provides guidance for the design of flapping foil wave powered vessels, and highlights the importance of the wavelength to vessel length ratio.

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ORCID for Nicholas Townsend: orcid.org/0000-0001-6996-3532

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