Abstract
The nacelle and tower affect the flow through the rotor and near-wake of horizontal-axis wind turbines. For this reason, aerodynamic models of the full geometry of wind turbines are needed to gain insight to improve turbine designs. In this work, the unsteady flow around an experimental horizontal-axis wind turbine is studied using computational fluid dynamics simulations. The emphasis is put on the effects of the nacelle, tower and rotation of the blades on the induction region and near-wake. The simulations are performed using unsteady Reynolds-Averaged Navier-Stokes equations in conjunction with the shear-stress transport k-ω model. The sliding mesh method is employed to enable the rotation of the blades. The simulations were performed at the design tip speed ratio (λ=6.67) and two off-design conditions (λ = 4.17 and 10) to assess the general validity of the numerical model. The obtained results of global loads, force distribution on the blades and tower, velocity components and angle of attack are presented in detail. It is found that the tower has little effect on the rotor loads (circular pattern), but the blades have an important effect on the tower (three-leaved rose pattern). The force distribution reveals only significant fluctuations on the blades (r/R<0.75) at λ = 4.17, whereas the tower is affected in the zone exposed to the wake behind the rotor for all three wind speeds. Furthermore, the interaction between blades is evaluated in terms of velocity and angle of attack. The correspondence between the results for different rotor blade positions reveals the effect of the ground on the rotor is negligible. The Omega method is used to visualize the behaviour of the tip and root vortices, wake expansion, coherent structures and development of the near-wake. The numerical results presented here contribute to a deeper understanding of the unsteady flow around horizontal-axis wind turbines.
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