An Experimental Study On The Effects Of Winglets On The Performance Of Horizontal Axis Tidal Turbines

Hydrokinetic energy contains the major uncontrolled source of renewable marine energy. The highest level of converter technology readiness offered in the last three decades is TRL8,Äì9, which is related to the first-generation horizontal axis converters. In low-depth calm waters, one of the best options to harvest tidal energy is vertical axis turbines. About 16% of the conceptual designs presented in the last 30¬†years apply this type of converter, which does not have a high level of technological readiness. In this study, a laboratory-designed vertical axis turbine has been introduced in which the effects of the number of blades, the blade profile, and attack angle on the performance of the turbine were analyzed. A 3D incompressible viscous turbulent computational finite volume approach is applied, with the spatial second-order and temporal first-order accuracies. The turbulent model k-œâ SST was used to obtain the flow inside the turbine. Rotors include two, three, and six blades with three different profiles, including NACA2421, NACA16021, and NACA0020. Computational results reveal that the turbine with three blades and an angle of attack of +8 using the NACA2421 profile has a maximum generation capacity of about 4¬†kW, with a strength factor of 0.4 and a power factor of about 20%. The capacity, however, was lower for a higher number of blades.

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.