Iván Herráez, Spain (PPRE 2005/07)
PhD at Forwind Institute / University of Oldenburg
The flow over the rotor blades of horizontal axis wind turbines is subjected to complex physical mechanisms that are still poorly understood. Even stationary, axisymmetric and uniform inflow conditions can lead to fluid dynamic phenomena not well characterized yet. The root and tip regions of the blade, where the flow is highly three-dimensional and strongly influenced by the trailing vortices, are especially prone to this problem. As a consequence, the characterization of the blade flow can be extremely challenging, what implies a high level of uncertainty in the
wind turbine design process.
This work addresses this issue by means of computational fluid dynamics (CFD) simulations. The scope is to unveil the physics of the blade aerodynamics, with a special focus on the root and tip flows. Reynolds Averaged Navier-Stokes (RANS) simulations are used in this work for all the computations in which the true geometry of the blades is simulated. Owing to the high computational cost of Large Eddy Simulations (LES), their utilization is restricted to computations in which the blade geometry is replaced by an actuator line model. All the simulations presented in this thesis are compared with measurements obtained from model wind turbines operating under controlled conditions in wind tunnels. Special emphasis is put on the validation of the flow features governing the root and tip aerodynamics. The agreement between simulations and experiments is in general very good.
Spanwise flows are shown to influence drastically the performance of the blade inboard region when it operates at high angles of attack, both with attached and separated flow. Their origin as well as their significance for the Himmelskamp effect are discussed in detail. The Himmelskamp effect is analysed in two wind turbines and in both cases the lift is enhanced
but the drag is not significantly affected. Furthermore, it is shown that the spanwise flows (and correspondingly also the Himmelskamp effect) can be disrupted by vortices trailing from the blade transition regions between different airfoil types. This, however, only occurs if the aerodynamic characteristics of two adjacent airfoils differ much from each other.
The origin and relevance of the root and tip vortices are also investigated in detail. The numerical results show how in the root and tip regions the bound vorticity is deflected from the spanwise towards the chordwise direction, what gives rise to the trailing vortices. However, this process is more gradual at the root than at the tip. Correspondingly, the formation of the root vortex extends over a comparatively large area. This makes the root vortex to present a less defined and distinctive structure than the tip vortex. The load acting on the chordwise vorticity at the tip and root is orthogonal to the blade chord.
This implies that it does not contribute to the power generation. Hence, it can be considered as a conservative load. This load is typically regarded in all calculation methods based on the blade element theory (e.g. in blade element momentum and actuator line models). However, a detailed study of its role in the turbine aerodynamics suggests that it can slightly modify the blade tip trajectory.
In order to prove this hypothesis, an actuator line model is implemented, which automatically computes and applies the conservative force. It is demonstrated that the conservative force induces a short inboard motion of the tip vortex just after release. This, in turn, reduces slightly the turbine performance.
These results show the great potential of CFD for the study of wind turbine aerodynamics.
Furthermore, they pave the way for a better characterization of the flow over the rotor blades.