An Experimental Investigation of Turbulence and Unsteady Loading on Tidal Turbines

Abstract

This research addresses the need for an improved characterisation of the onset flow turbulence and the unsteady hydrodynamic blade loads on tidal turbines for the purposes of predicting fatigue life. A new, extensive set of parameters which characterise the magnitudes of the turbulent fluctuations, the anisotropy and the scales of the turbulence at a tidal energy site have been presented. A novel application of rapid distortion theory estimated the velocity fluctuations to be amplified by 15% due to the presence of the turbine. The turbulence was also predicted to be well correlated over the outer span of a turbine blade at the frequencies of interest. Together, these results enabled a set of non-dimensional parameters describing the turbulence induced forcing on a turbine blade to be established. A model-scale horizontal-axis turbine was used to investigate the unsteady blade load response in a still-water towing tank. A set of wind tunnel tests of the S814 foil were also conducted and used to demonstrate that the lift on the blades could have been degraded by 10% at the relatively low Reynolds numbers at which the turbine was tested, relative to full-scale. This was owing to dominant laminar separation bubbles. Single frequency planar oscillations of the turbine were used to quantify the contribution of hydrodynamic unsteadiness to the blade-root bending moment. For attached flow, the unsteady bending moment was found to amplify the steady loads by up to 15 %. The total hydrodynamic added mass was up to 2.7 times larger than from non-circulatory forcing and decreased with frequency. Dynamic inflow theory and a returning wake model were able to provide qualitative predictions of these results at low frequencies. At low tip-speed ratios, phenomena consistent with delayed separation and dynamic stall were characterised and the unsteady loading was up to 25% larger than the steady load. Linear superposition of the single frequency responses was also demonstrated to offer a reliable technique to model the response to a multi-frequency forcing and to a large eddy.