Controlling Tip Vortices and Cavitation through Tip Permeability for Tidal Turbines

Blade-tip vortices can lead to wakes, cavitation and noise, and their control remains a significant challenge for tidal and wind turbines. In the present work, we propose and investigate controlling tip vortices through local permeability. Blade-resolved Reynolds-averaged Navier-Stokes simulation has been employed on a model-scale horizontal-axis turbine, following a rigorous validation and verification process. The tip-speed ratio of the turbine varies from 4.52 to 7.54. The tip permeability is modelled by including a porous zone over the blade tip section, within which Darcy's law is applied. The results demonstrate that there is an optimal range of permeability, corresponding to a non-dimensional Darcy number, Da, of around 10^{-5}, that can substantially decrease the tip vortex intensity. The revealed flow physics show that the permeable tip treatment can effectively enlarge the vortex viscous core radius with little change to the vortex circulation. As the tip vortex intensity is significantly reduced, the permeable tip treatment can increase the minimal pressure-coefficient at the vortex core by up to 63%, which significantly alleviates the cavitation risk due to tip vortices. This approach has negligible influence on the turbine's energy-harvesting performance because the spanwise extent of the permeable tip treatment is only in the order of 0.1% turbine diameter. Our findings demonstrate this approach's great promise to break the upper tip-speed ratio limit capped by cavitation for tidal turbines. These will contribute to developing more efficient and resilient turbines.