Fluctuating loads on a tidal turbine due to velocity shear and turbulence: Comparison of CFD with field data

Fluctuating loads on tidal turbines are important for fatigue analysis and there is limited information or simulation available for full-scale conditions. Here, CFD simulations have been performed for a geometry-resolved full-scale tidal-stream turbine and compared with experimental data from a 1 MW machine deployed at the EMEC test site. Initially, Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulations (LES) were performed using an inflow mean velocity profile representative of the site but low inflow turbulence. Mean blade pressures were similar for the two types of turbulence closure and yielded mean power coefficients comparable with measurements. Then, to simulate the effect of turbulence on loads, LES with synthetic turbulence prescribed at inlet was employed. For these simulations, inflow profiles of mean velocity, Reynolds stresses and length scales were determined from a precursor channel-flow simulation, with additional factoring of stresses and length scales to match hub-height conditions measured on site. Fluctuations in thrust, power and blade bending moment arise cyclically from onset mean velocity shear and the blocking effect of the support tower and over continuous spectral ranges from blade-generated turbulence, approach-flow turbulence and waves. LES simulations with realistic inflow turbulence satisfactorily reproduced the relative spectral distribution of blade bending moments in low-wave conditions.