A BEM/RANS Interactive Method Applied to an Axial Tidal Turbine Farm

Abstract

In this study, an interactive method coupling a boundary element method (BEM) with a viscous flow solver solving the Reynolds-averaged Navier–Stokes (RANS) equations is applied to multiturbine interaction problems. The BEMis first applied to a single turbine problem to predict its performance with/without yaw in noncavitating/cavitating conditions. Improved wake alignment models, the full wake alignment and the unsteady wake alignment, are used to align the blade wake. The former is adequate for steady state with zero yaw, and the latter is used for unsteady predictions in the case of nonzero yaw in the incoming flow. The BEM results are compared with the experimental measurements and the results from full-blown RANS simulations for a range of tip speed ratios. The comparisons show satisfactory agreement between the numerical and experimental approaches. Afterward, the BEM/RANS coupling method is applied to multiturbine interaction problems with different layouts and different turbine-to-turbine offsets in an axial turbine farm. The method is shown to work well in this multiturbine interaction problem because of the capability of using a strictly Cartesian grid in the RANS method, which minimizes the artificial diffusion and improves the numerical accuracy of long-range flow development. Representation of a turbine by the body force/mass source fields in the BEM/RANS coupling approach reduces the number of cells required for 3D full-blown RANS simulations, and therefore reduces the computational cost in an efficient way. Introduction In recent years, interests in tidal energy harvesting have been growing because of the needs and perspectives of clean and renewable energy. Among all different types of tidal stream generators, the axial tidal turbine has been most frequently implemented because of its similarity to wind turbines and the analysis tools which are originally developed for propellers but equally applicable to turbines without extensive modifications. Because of the predominantly bidirectional flow in most tidal current sites, typical tidal turbine farms arrange the turbine in multiple lines that are perpendicular to the flow (Turnock et al. 2011). In such applications, turbine-to-turbine interaction needs to be studied to reduce the energy-shadow effect and improve the overall turbine farm efficiency.