This paper demonstrates the potential of a compressible Navier–Stokes CFD method for the analysis of horizontal axis wind turbines. The method was first validated against experimental data of the NREL/NASA-Ames Phase VI (Hand, et al., 2001, “Unsteady Aerodynamics Experiment Phase, VI: Wind Tunnel Test Configurations and Available Data Campaigns,” NREL, Technical Report No. TP-500-29955) wind-tunnel campaign at 7 m/s, 10 m/s, and 20 m/s freestreams for a nonyawed isolated rotor. Comparisons are shown for the surface pressure distributions at several stations along the blades as well as for the integrated thrust and torque values. In addition, a comparison between measurements and CFD results is shown for the local flow angle at several stations ahead of the wind turbine blades. For attached and moderately stalled flow conditions the thrust and torque predictions are fair, though improvements in the stalled flow regime are necessary to avoid overprediction of torque. Subsequently, the wind-tunnel wall effects on the blade aerodynamics, as well as the blade/tower interaction, were investigated. The selected case corresponded to 7 m/s up-wind wind turbine at 0 deg of yaw angle and a rotational speed of 72 rpm. The obtained results suggest that the present method can cope well with the flows encountered around wind turbines providing useful results for their aerodynamic performance and revealing flow details near and off the blades and tower.

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This paper presents a computational investigation of the wake of the MEXICO rotor. The compressible multi-block solver of Liverpool University was employed, using a low-Mach scheme to account for the low-speed flow near the blade and in the wake. In this study, computations at wind speeds of 10, 15 and 24 m s 1 were performed, and the three components of the velocity were compared against experimental data around the rotor blade up to one and a half rotor diameters downstream. Overall, fair agreement was obtained with the computational fluid dynamics showing good vortex conservation near the blade. Vorticity values revealed discontinuities in the wake at approximately 70%R, where two different aerofoils with different zero-lift angles are blended. The results suggest that all-Mach schemes for compressible computational fluid dynamics methods can deliver good performance and accuracy over all wind speeds for flows around wind turbines, without the need to switch between incompressible and compressible flow methods.

This work explores the breakdown of the wake downstream of the Model Experiments in Controlled Conditions Project (known as the MEXICO project) wind-turbine rotor and assesses the capability of computational fluid dynamics in predicting its correct physical mechanism. The wake is resolved on a fine mesh able to capture the vortices up to eight rotor radii downstream of the blades. At a wind speed of 15 m∕s, the main frequency present in the computational fluid dynamics signals for up to four radii was the blade-passing frequency (21.4 Hz), where the vortex cores fall on a perfect spiral. Between four and five radii downstream, higher-frequency content was present, which indicated the onset of instabilities and results in vortex pairing. The effect of modeling a 120 deg azimuthally periodic domain and a 360 deg three-bladed rotor domain was studied, showing similar predictions for the location of the onset of instabilities. An increased frequency content was captured in the latter case. Empirical and wake models were also explored, they were compared with computational fluid dynamics, and a combination of kinematic and field models was proposed. The obtained results are encouraging and suggest that the wake instability of wind turbines can be predicted with computational fluid dynamics methods, provided adequate mesh resolution is used. Nomenclature= rotor radius, m R = residual Re = Reynolds number r = radial position. m S = source vector in Navier-Stokes equations T = thrust, N Tu = turbulence intensity, % u = axial velocity, m∕s u ∞ = freestream velocity, m∕s V = cell volume v = radial velocity, m∕s x = axial position, m x, y, z = Cartesian coordinates λ = tip speed ratio ϕ = pitch angle, deg ω = vorticity, 1∕s

This article presents an investigation of the relative importance of key design parameters of a horizontal axis wind turbine (HAWT) blade. Computational fluid dynamics (CFD) is used as the main tool, after validation against experimental data of the (National Aeronautics and Space Administration/National Renewable Energy Laboratory) NREL/NASA-Ames Phase VI wind tunnel campaign. Tip and root sections, blade aspect ratio, and pitch angle were analysed and all CFD calculations were performed using a compressible Reynolds-averaged Navier—Stokes solver. CFD grids of advanced multi-block topologies were used including up to 4.5 million cells. A grid convergence study indicated that a resolution of 3.4 million cells was adequate for the selected flow conditions, which correspond to an upwind wind turbine at 0° yaw angle, 7 m/s wind speed, and 72 r/min rotational speed. Various root and tip configurations were considered and the results obtained indicate that the exact representation of the root and tip geometry of an HAWT has a small but finite effect in the thrust and torque levels at working conditions. This effect is however secondary to the effects of aspect ratio and blade pitch.