Unsteady gust response of tidal stream turbines

This paper investigates the limitations of 2D linear unsteady aerofoil theory for modelling the unsteady gust response of tidal stream turbines. The work is motivated by the fact that accurate unsteady load prediction is required to determine turbine life. Current state of the art design codes in industry use a single model, based on Theodorsen's theory, to predict the response to all types of gust. This paper shows that different types of gust require different types of model. Vortical gusts, such as due to turbulence and shear flows, should be modelled using a combination of Sears' and Horlock's theories. Pressure gusts, such as those caused by free surface waves, should be modelled using Loewy's theory. The accuracy of these models is examined using numerical predictions. It is shown that, for pressure gusts, using Theodorsen's theory can cause errors in prediction of the unsteady lift amplitude by nearly 18%. By using a calibrated version of Loewy's theory this maximum error can be reduced to less than 8%. Similarly, using the combined Sears/Horlock theory can reduce the error in predicting the response of high frequency vortical gusts from 18% to nearly 10%. The range of gusts likely to occur at real tidal sites is also examined. This has suggested that, in most likely situations, pressure gusts cause variations in loads which can be modelled quasi-steadily, but vortical gusts must be modelled using the combined Sears/Horlock theory.