Navier-Stokes CFD Analysis of a Tidal Turbine Rotor in Steady and Planar Oscillatory Flow

Initial results of an ongoing Navier-Stokes Computational Fluid Dynamics study of horizontal axis tidal current turbine hydrodynamics are presented. Part of the underlying motivation is assessing the effects of the Reynolds number on turbine performance and loads in steady and unsteady flow regimes. The study aims at a) providing initial verification and validation of Navier-Stokes CFD for steady and unsteady tidal turbine flows at tank experiment and field installation Reynolds numbers, and b) estimating the dependence of turbine performance and loads on this parameter to enable more reliable use of low-Reynolds number tank measurements for field installation analysis and design, and c) investigating crucial aspects of tank turbine hydrodynamics which are difficult to assess in the experiments. The investigation starts from a tidal current turbine towing tank experiment, compares available measured data and CFD results regarding the blade steady flow and unsteady flow due to the harmonic planar motion of the turbine, and then extends the CFD analysis to the high Reynolds numbers of typical utility-scale installations. It is found that at field-level Reynolds numbers, the blade power, force and moment coefficients are about 20 percent higher than at tank-level Reynolds numbers, and the agreement of measured and predicted loads at fairly low Reynolds numbers improves by modelling laminar-to-turbulent transition, highlighting the importance and extent of laminar flow phenomena and stall in tank experiments. Keywords— Tidal turbine hydrodynamics, Reynolds number effects, wave loads, towing tank testing, Navier-Stokes Computational Fluid Dynamics.