Blade loading on tidal turbines for uniform unsteady flow

An improved characterisation of the unsteady hydrodynamic loads on tidal turbine blades is necessary to enable more reliable predictions of their fatigue life and to avoid premature failures. To this end, this paper presents a set of blade-root bending moment responses for a scale-model tidal turbine subjected to an unsteady planar forcing in a towing tank. In cases where the boundary layer was believed to be attached to the outer sections of the blade, the out-of-plane bending moment amplitude for unsteady flow was up to 15% greater than the corresponding load measured in steady flow and exhibited a phase-lead of up to 4.5°. Both these observations are qualitatively consistent with the effects of dynamic inflow and non-circulatory forcing. The bending moment responses for a forcing time history that comprised multiple frequencies, as well as for a discrete half-sinusoidal perturbation, were able to be reconstructed reasonably well using the responses obtained from single-frequency oscillatory flows. This suggests that blade designers could utilise relatively low fidelity techniques and conduct potentially fewer experimental tests to acquire the fatigue load spectrum.