Linear and non-linear wave theories coupled with a modified BEM model for quantifying dynamic loads of a tidal stream turbine

Abstract

Dynamic loads resulted from wave-current interaction is a serious problem facing tidal current turbines (TCTs), which may lead to catastrophic results. The main issue in such environments is the accelerated fatigue, which likely results from the cyclic loading especially in the axial direction. For investigating the current effects on the wave field and evaluating the resulted dynamic loads, a modified Blade Element Momentum (BEM) model was presented and coupled with linear and non-linear wave theories.Firstly, a validation study has been performed between the present modified BEM model coupled with linear wave theory and previous numerical and experimental results. A previously introduced dynamic inflow model was reemployed in the present BEM model during this validation to assess its applicability and accuracy. Very good agreements were seen between the present model and the experimental data at different wave properties and operational conditions, which emphasized the precision of the present unsteady BEM model and its corrections for further investigations.Comparing with other operational conditions, the intrinsic wave period and wave height had significant effects on the resulted hydrodynamic loading ranges of both the blade root bending moments. By the use of linear wave theory, the resulted out-of-plane bending moment was under predicted. Linear wave and second-order models had similar bending moments at all wave and flow conditions. Due to considering the current effects on wave properties, Dean's, third-order and fifth-order models had dynamic loadings differed in phase, frequency and amplitude from the experimental data. Fifth-order model was finally considered the best choice for simulating the wave-current velocity at different wave conditions and immersion depths.An additional complete time series runs were performed to assess the effect of dynamic stall correction inclusion in the present BEM model on the resulted hydrodynamic loads. It had minor effects on the bending moments, while had noticeable influence on the turbine thrust.