Non-linear hydrodynamic forces on fixed axi-symmetric bodies in incident waves

Hydrodynamic forces induced by surface gravity waves on a wave energy converter directly influence its motion and power capture. Characterisation of these forces is therefore critical to optimising the hydrodynamic design of these devices. This study experimentally investigates linear and higher harmonic hydro-dynamic forces on a fixed sphere at different immersion levels. Diffraction tests are performed in a wave flume where an aluminium sphere is instrumented with a 3-axis load cell to measure the induced horizontal and vertical forces experienced while subjected to incident focused wave groups with different focus amplitudes. The force harmonics are separated using two-phase harmonic separation and subsequently analysed. The measured first harmonic excitation force is found to be consistent with numerical results across all immersion levels but exhibits weak amplitude dependency. The second and third harmonic excitation forces appear to be consistent with the expected quadratic and cubic dependence on the incident wave amplitude. The second-order super-harmonic surge force is found to be the same order of magnitude as the linear force for the largest amplitude case considered. Dependence of the first, second, and third harmonic excitation forces on the immersion level is also investigated, motivated by the small-body approximation for linear forces.