A comparison between experiments and numerical simulations on a scale model of a Horizontal-Axis current turbine

This work refers to towing-tank experiments and numerical simulations of the hydrokinetic turbines for the GEMSTAR system, designed at the University of Naples. The experimental campaign was aimed to validate the quality of the turbine design procedure, utilising low order methods such as Blade Element Momentum theory (BEMT). Discrepancies were found between BEMT results and experiments. Therefore, higher order CFD numerical models were employed to investigate on the rotor hydrodynamic behaviour, simulating the experiments. Blade section pressure distributions, obtained from 3D simulations, were compared with pressure distributions obtained from 2D calculations to explain discrepancies between BEMT and experiments. It was found that pressure distributions from 2D calculations are significantly different from the ones extracted from CFD, especially at inboard sections. As shown previously in literature for wind turbines, the presence of complex 3D phenomena arising from the combination of rotation and pressure distribution on the blades was demonstrated in water turbines. Finally, a so-called 2.5D approach was developed to improve BEMT results, with the sectional forces extracted from 3D simulations. The 2.5D BEMT was validated against the results of a test series with variable collective pitch, showing an acceptable agreement to the observed data for slight pitch angle variations.