Application of a LES technique to characterize the wake deflection of a wind turbine in yaw

Abstract

When a wind turbine works in yaw, the wake intensity and the power production of the turbine become slightly smaller and a defl ection of the wake is induced. Therefore, a good understanding of this effect would allow an active control of the yaw angle of upstream turbines to steer the wake away from downstream machines, reducing its effect on them. In wind farms where interaction between turbines is signifi cant, it is of interest to maximize the power output from the wind farm as a whole and to reduce fatigue loads on downstream turbines due to the increase of turbulence intensity in wakes.A large eddy simulation model with particular wind boundary conditions has been used recently to simulate and characterize the turbulence generated by the presence of a wind turbine and its evolution downstream the machine. The simplifi ed turbine is placed within an environment in which relevant fl ow properties like wind speed profi le, turbulence intensity and the anisotropy of turbulence are found to be similar to the ones of the neutral atmosphere. In this work, the model is used to characterize the wake defl ection for a range of yaw angles and thrust coeffi cients of the turbine. The results are compared with experimental data obtained by other authors with a particle image velocimetry technique from wind tunnel experiments. Also, a comparison with simple analytical correlations is carried out.

A New Analytical Wake Model for Yawed Wind Turbines

“…The contours of mean velocity and turbulence intensity as well as wake deflections are displayed in the horizontal -plane at the hub height ( = ). In Figure 6, black dashed lines represent the wake boundary denoted by the positions with the mean wind speed equal to the 95% of the free-stream velocity and the open circles denote the wake centers, which are calculated by taking the midpoint between the wake boundaries, based on the same procedure as Jimenez et al [11] and Parkin et al [6]. The black dashed lines in Figure 7 The LES results for the wind turbine wake simulation have been validated with high accuracy by comparison with the experimental data [27].…”

“…Computational fluid dynamics (CFD) has also been widely used to study wind turbine wake flows and power production optimization in a wind farm [10]. Jiménez et al [11] used large-eddy simulation (LES) to characterize the wake deflections under a range of yaw angles and thrust coefficients for a turbine modeled with a uniformly distributed actuator disk model without rotation (ADM-NR). Later, Fleming et al [12] applied LES with actuator line model (ALM) to investigate several methods for improving wind plant overall performance, and the yaw control proved to be effective with wake-redirection.…”

“…By contrast, the analytical studies for the wake under a yawed condition have not received much attention. Jiménez et al [11] presented a preliminary analysis of wakes on the leeward of a yawed turbine for the first time. He proposed a simple formula to predict the wake skew angle based on the momentum conservation and top-hat model suggested by Jensen [17] for the velocity deficit.…”

“…However, experimental validation was not sufficient as the author mentioned. Gebraad et al [5] and Howland et al [8] derived the formula of yaw induced wake center trajectory by integrating the skew angle proposed by Jiménez et al [11]. However, the model of Jiménez et al [11] overestimates the wake deflection since the assumption of top-hat for the velocity deficit is not accurate as pointed out by Ishihara et al [18].…”

“…Gebraad et al [5] and Howland et al [8] derived the formula of yaw induced wake center trajectory by integrating the skew angle proposed by Jiménez et al [11]. However, the model of Jiménez et al [11] overestimates the wake deflection since the assumption of top-hat for the velocity deficit is not accurate as pointed out by Ishihara et al [18]. Based on the theoretical analysis of the governing equations, Bastankhah and Porté-Agel [9] developed an analytical model to predict the wake deflection and the far wake velocity distributions for the yawed turbines.…”