Negative lift generation in a tidal device structure

Becoming aware of the impact of global warming, all countries of the European Union have agreed to produce 20% of their electricity from renewable energy by 2020. Hence, a new market emerged to develop more efficient technologies of sustainable power production. Cranfield University has been involved since 2006 in wave and tidal energy design projects through various government agencies funded partnerships with the emerging industry of marine renewable energy. This thesis is based on one of these projects called “DeltaStream” which was developed by Tidal Energy Ltd (TEL). The tidal stream turbine is one of the most efficient concepts in marine renewable energy, because of the high predictability of tidal streams compared to wave and wind energy. Many devices are currently being developed in the UK. Most of them are moored to the seabed using traditional methods used in the offshore oil industry, which means drilling the sea floor. The goal of this work is to avoid drilling the seabed by adding hydrofoils on the structure. Indeed, by setting a hydrofoil in the tidal stream, the goal is to prove that a downward lift force is created and is efficient enough to maintain the structure at its location, avoiding mooring costs and ecological issues relative to drilling the sea floor. Computational Fluid Dynamics (CFD) models have been performed, studying the sensitivity of mesh size compared to results quality, and testing then different hydrofoils to get the best downward lift force in local conditions. The main characteristics of the simulation are unsteady and one phase. The chosen geometry is a “quasi-2D” domain in order to minimize computational requirements. A validation case has been first performed on a well-known geometry of a circular cylinder, to compare analytical results and CFD data. Small variations between results validated the model to enable us to use the model on a less-known simulation such as a hydrofoil. Different parameters influencing the efficiency of the hydrofoil in terms of lift production and drag reduction have been tested. These results have been compared with tank tests undertaken by the OENA Group in June-July 2009 in IFREMER (France). And complementary studies have been performed to compare these results and CFD results.