Variable-fidelity CFD modelling of horizontal-axis tidal turbines and arrays

The assessment of a design-oriented hydrodynamics model for horizontal axis tidal turbines is addressed. The methodology is based on a Boundary Integral Equation Model with a simple viscous flow correction (BIEM-VFC) and is valid to analyse the three-dimensional flow around a turbine in arbitrary onset flow at computational costs comparable to standard blade element methods. A validation study is performed by using benchmark data from full viscous flow simulations by RANS. Numerical results by BIEM-VFC are compared with RANS predictions of turbine global performance, thrust and power, as well as local flow quantities describing pressure distributions and wall streamlines on blades and trailing vorticity path in the wakefield. The validation study demonstrates that BIEM-VFC predictions of both global and local flow quantities are comparable with those obtained from more complex full viscous flow simulations for a range of turbine operating conditions where viscosity effects are significant but blades are free from massive separation and stall. For operating conditions characterized by blade stall, BIEM-VFC fails to describe local flow details while qualitative approximations of global loads are given.