Abstract
Cavitation can result in blade erosion, vibra-tion, and cavitation noise due to bubble collapse. Tidalturbines’ blade tips experience the highest flow speed, andthus, risks of cavitation increase due to low pressure insidetip vortices at high Tip Speed Ratios (TSRs). This can captheir power efficiency and lead to an upper TSR limit. Thepresent work focuses on controlling tip vortices througha novel approach: tip permeability achieved by a groovedtip design. A blade-resolved Reynolds-Averaged Navier-Stokes simulation has been carried out on a model-scalehorizontal-axis turbine. In our work, the simulation resultshave been validated with existing experimental data fromthe UK Supergen Benchmarking turbine and our wing tipvortex measurements in a water tunnel. We modelled aporous zone placed over the blade tip section, demonstrat-ing that there is an optimal permeability that can substan-tially reduce the tip vortex intensity and associated pressuredrops, and thus mitigate the risk of cavitation. Building onthis conclusion, in this study, we have developed a noveldesign with multiple grooves distributed along the bladetip chord, resulting in an equivalent local 2D permeability.The spanwise scope of the porous or grooved zone is 0.1%of the turbine diameter. It is found that the grooved tipdesign can significantly increase the minimum pressurecoefficient at the tip vortex core by up to 27% at a TSRof 6. We also explored different groove channel designs,either by pitching the grooves or making them convergent.We found that, although the effects remain significant andrelatively consistent across all design types, the tip vorticesare most effectively suppressed when the groove channelhas a convergent shape. This promising outcome suggestsa substantial reduction of the tip vortex cavitation risksand can thus enable turbines to operate at higher TSRs.Additionally, as the spanwise extent of the permeable tipis minimal, the impact on the turbine’s power and thrustcoefficients is slight.