|The Performance Assessment of Wave and Tidal Array Systems (PerAWaT) project, launched in October 2009 with £8m of ETI investment. The project delivered validated, commercial software tools capable of significantly reducing the levels of uncertainty associated with predicting the energy yield of major wave and tidal stream energy arrays. It also produced information that will help reduce commercial risk of future large scale wave and tidal array developments.
This report contains results and analysis of 3D RANS simulated flows through a generic full scale tidal turbine operating in various flow conditions. Conditions simulated include varying levels of shear, vertical position through the water column, turbine spacing and yaw angle. Additionally simulations of a towerless rotor are presented, including a comparison to the results of the University of Edinburgh model ofthe same towerless rotor. Results presented include thrust and power metrics as well as wake profiles. Results are analysed and contrasted with respect to perturbations in each flow condition leading to observations on their influence.
In particular shear is observed to increase the power delivered by the turbine (with mass flux through the channel held constant), as is increasing the elevation of the turbine through the shear profile. Additionally shear is observed to increase the rate of wake recovery. Reducing the lateral spacing, and therefore increasing the area blockage ratio, increases the thrust and power delivered by the turbine, consistent with existing theory. Removing the tower from the simulations increases the thrust and power experienced by the turbine, as well as removing the torque ripple. Lower power is extracted by the turbine when operating in yawed flows,and a lower thrust coefficient is observed. However, the blade loading is distinctly different from the unyawed cases, as shown in plots of azimuthally-distributed rotor torque.
Comparison to the University of Edinburgh simulation results is done on a consistent area blockage basis.
The turbine wake is parameterised by considering a force balance for a turbine operating in a confined flow, and converting the resulting ‘top-hat’ wake velocity profile into an Gaussian distribution with equivalent momentum and standard deviation. This model is valid at the end of the near-wake. The near-wake ends at a downstream location where the pressure has equalised in the cross-stream directionand the velocity profile exhibits a self-similar form. For the current set of simulations this point has been identified at a distance of five diameters downstream of the rotor plane.
Relationships between operating conditions and wake model parameters are determined.
All of the results presented here are based on unsteady blade-resolved simulations, where the simulated flow field has been averaged over one full rotor revolution.