CFD simulation of tidal-stream turbines in a compact array

An actuator-line RANS CFD model is used to simulate loads (thrust, power and flapwise bending moment) on tidal-stream turbines. Results are compared with other groups' reported laboratory measurements for an individual 1/15-scale turbine in a rectangular flume at IFREMER and a compact 3-turbine array of similar turbines in the FloWave tank at the University of Edinburgh. Provided that a 1° blade pitch adjustment is made, predicted thrust and power coefficients concur with single-turbine results from the rectangular flume. For a 3-turbine configuration, where two proximal side-by-side turbines produce accelerated bypass flow to a third turbine one diameter downstream, CFD suggests that the front turbines are unaffected by their neighbours, but downstream turbine loads are increased by 11% for thrust and 21% for power at a typical operating speed. An additional, but smaller, variation occurs with speed of the upstream turbines. Simulations suggest that overall wake momentum deficit and increased bypass flow reflect the thrust on a turbine and that the bypass-flow enhancement remains close to that established on the rotor disc for some considerable distance downstream. Vertical blockage prevents any self-similar wake of either 2-dimensional or axisymmetric form being established within the computational domain.