Estimating the wake deflection downstream of a wind turbine in different atmospheric stabilities: an LES study

Abstract

Abstract. An intentional yaw misalignment of wind turbines is currently discussed as one possibility to increase the overall energy yield of wind farms. The idea behind this control is to decrease wake losses of downstream turbines by altering the wake trajectory of the controlled upwind turbines. For an application of such an operational control, precise knowledge about the inflow wind conditions, the magnitude of wake deflection by a yawed turbine and the propagation of the wake is crucial. The dependency of the wake deflection on the ambient wind conditions as well as the uncertainty of its trajectory are not sufficiently covered in current wind farm control models. In this study we analyze multiple sources that contribute to the uncertainty of the estimation of the wake deflection downstream of yawed wind turbines in different ambient wind conditions. We find that the wake shapes and the magnitude of deflection differ in the three evaluated atmospheric boundary layers of neutral, stable and unstable thermal stability. Uncertainty in the wake deflection estimation increases for smaller temporal averaging intervals. We also consider the choice of the method to define the wake center as a source of uncertainty as it modifies the result. The variance of the wake deflection estimation increases with decreasing atmospheric stability. Control of the wake position in a highly convective environment is therefore not recommended.

A New Analytical Wake Model for Yawed Wind Turbines

“…Thus, the axial and tangential velocity at the rotor disk U n and U t are selected as the unknown parameters instead of a and a [31][32][33]. In this study, a coupled approach is adopted for the ADM-R Energies 2018, 11, 665 6 of 24 model in the yawed condition, in which U n and U t are directly obtained from the CFD simulation result of local velocities on the rotor disk U x , U y , U z by using the following transformation:…”

Brief communication: On the influence of vertical wind shear on the combined power output of two model wind turbines in yaw

“…One promising concept is the wake deflection by intentional yaw misalignment of single wind turbines. The principle of deflecting the velocity deficit behind a wind turbine was observed in field measurements by Trujillo et al (2016), in wind tunnel experiments (e.g., Medici and Alfredsson, 2006;Krogstad and Adaramola, 2012) and in numerical simulations (e.g., Jiménez et al, 2010;Gebraad et al, 2014;Vollmer et al, 2016). Further, Gebraad et al (2014) and Fleming et al (2016) applied the concept to wind farm control strategies using large-eddy simulation (LES) methods, showing a potential power increase in wind farm applications.…”

“…Further, Gebraad et al (2014) and Fleming et al (2016) applied the concept to wind farm control strategies using large-eddy simulation (LES) methods, showing a potential power increase in wind farm applications. Vollmer et al (2016) report on an asymmetric deflection of a turbine's wake with respect to its direction of yaw misalignment in numeric studies. Similarly, Bastankhah and Porté-Agel (2016) found that a wake moves upwards or downwards depending on the direction of a yaw misalignment using PIV (particle image velocimetry) measurements behind a small turbine model.…”

From wake steering to flow control

“…From the averaged flow, a power is computed, which would have been produced by an additional hypothetical at some point downstream. This approach was adapted from power calculations used in Vollmer et al (2016) This power is computed by averaging the cubed wind speed over a hypothetical rotor disk from the averaged flow. The cubed average is then converted to power through…”

“…In Vollmer et al (2016), largeeddy simulations (LES) are used to study the behavior of wake steering under varying atmospheric conditions. The authors note that the shape of the wake under yawed conditions is curled rather than circular.…”