FSI investigation of a hydrofoil

Abstract

Due to the changes in electricity production where intermittent energy sources like wind and solar power increase their ratio, a stabilizing energy source is required, which hydropower plants provides. The stabilization requires that the hydropower turbines are more frequently operated outside the best efficiency point and the turbines experience increasingly transient operating conditions, which contributes to fatigue and the lifetime of the turbine decreases. One problem, which occur more often at these off-design conditions, is the von Kármán vortex shedding behind the trailing edge of the stay vane, guide vane and the runner blade, which may cause severe vibration and cracking of the turbine structure if resonance occur. In this thesis, computational fluid dynamics (CFD) and fluid-structure interactions (FSI) analyses were performed, using ANSYS CFX, ANSYS Modal and ANSYS Static Structure. The aim was to investigate if a serrated trailing edge on a hydrofoil can mitigate vortex shedding. Therefore, three hydrofoils with 6, 8 and 10 trailing edge serrations were designed and compared with each other and with a hydrofoil with a straight, oblique trailing edge. The simulations show that the shedding frequency of the hydrofoil is drastically reduces, by 81 % in this case, when using serrations, whilst the natural frequency seems to be almost independent of the TE design. Hence, using serrations will probably reduce the risk of lock-in. An investigation of the turbulent kinetic energy was performed and indicates that the serrations reduce the strength of the vortexes, even though the shedding might not be attenuated. The vortex core analysis implies that the serrations mitigate the von Kármán vortex shedding, but induce strong horseshoe vortex shedding, hence possibly explaining why the drag is only reduced for one of the serrated designs. Structural analysis found that using serrations decrease the maximum total deformation by more than 50 % and equivalent stress by at least 40 %, with the highest reduction in the hydrofoil with the lowest number of serrations. The FSI analysis concludes that the effect of the vortex shedding is lower on serrated hydrofoils, and shows that the hydrofoil with 6 serrations has the lowest impact of the vortexes.