Numerical modelling techniques to predict rotor imbalance problems in tidal stream turbines

Load fluctuations caused by the unsteady nature of tidal streams can have severe impacts on turbine components. As seen in the wind industry, turbine blades can become misaligned due to a fault in the pitch mechanism or blade deformations arising over time. These misalignments will represent a loss of power capture and perhaps even premature failure of the components if not detected in time. Computational fluid dynamic (CFD) techniques can be used to predict the performance of a turbine with a misaligned blade. However, these numerical modelling techniques quickly become computationally expensive when modelling realistic, time-varying conditions. Blade Element Momentum Theory (BEMT) offers a quicker and simpler approach, although with several limitations. In this paper BEMT is adapted to predict the performance of a three bladed tidal turbine with one or two blades offset from the optimum pitch setting. This approach is compared with a CFD model to study the effectiveness of both methods to predict power and thrust when a rotor blade has an offset. The simulations were undertaken at three flow speeds (0.9, 1.0 and 1.1 m/s). Both numerical models are compared to experimental data that was obtained at a flume tank in similar flow conditions. The results showed that both BEMT and CFD are able to predict power coefficients when there is a small offset of one rotor blade. However, the predictions were poorer when two blades had two different offsets at the same time.