Wind tunnel experiments on wind turbine wakes in yaw: redefining the wake width

Abstract

Abstract. This paper presents an investigation of wakes behind model wind turbines, including cases of yaw misalignment. Two different turbines were used and their wakes are compared, isolating effects of boundary conditions and turbine specifications. Laser Doppler anemometry was used to scan full planes of wakes normal to the main flow direction, six rotor diameters downstream of the respective turbine. The wakes of both turbines are compared in terms of the time-averaged main flow component, the turbulent kinetic energy and the distribution of velocity increments. The shape of the velocity increments' distributions is quantified by the shape parameter λ2. The results show that areas of strongly heavy-tailed distributed velocity increments surround the velocity deficits in all cases examined. Thus, a wake is significantly wider when two-point statistics are included as opposed to a description limited to one-point quantities. As non-Gaussian distributions of velocity increments affect loads of downstream rotors, our findings impact the application of active wake steering through yaw misalignment as well as wind farm layout optimizations and should therefore be considered in future wake studies, wind farm layout and farm control approaches. Further, the velocity deficits behind both turbines are deformed to a kidney-like curled shape during yaw misalignment, for which parameterization methods are introduced. Moreover, the lateral wake deflection during yaw misalignment is investigated.

The Yawing Behavior of Horizontal-Axis Wind Turbines: A Numerical and Experimental Analysis

“…The yaw control has recently become a key aspect of wind farm optimization, in particular as regards the management of wakes between nearby wind turbines. For example, in Schottler [11], Trujillo [12], Schottler [13], Bromm [14], the wake deviation is studied in relation to yaw misalignment. In Schottler [13], results are reported of wind tunnel experiments on turbines in yaw and the wake flow is observed to be slightly asymmetric with respect to 0 • of yaw.…”

“…For example, in Schottler [11], Trujillo [12], Schottler [13], Bromm [14], the wake deviation is studied in relation to yaw misalignment. In Schottler [13], results are reported of wind tunnel experiments on turbines in yaw and the wake flow is observed to be slightly asymmetric with respect to 0 • of yaw. The study in Bromm [14] deals instead with field tests aimed at demonstrating the applicability of yaw control for deflecting wind turbine wakes: yaw misalignment up to 20 • with respect to the inflow direction is investigated.…”

Wind tunnel study on power output and yaw moments for two yaw-controlled model wind turbines

“…The inflow turbulence level was observed to influence the shape and deflection of the wake, in contrast to a moderate shear in the inflow. Schottler et al (2018) highlight the importance of considering non-Gaussian distributions of velocity increments in wind farm control and layout optimizations. A ring of strongly intermittent flow is shown to surround the mean velocity deficit locations, suggesting a much wider wake expansion as based on the mean velocity.…”

Wind tunnel experiments on wind turbine wakes in yaw: Effects of inflow turbulence and shear

“…This work is part of a joint experimental campaign by the NTNU Trondheim and ForWind in Oldenburg. A second paper by Schottler et al (2017a) compares the wakes of two different model wind turbines, adding two-point statistics to the evaluation.…”

“…In order to also find the lateral deflection of the turbulence peaks for yawed rotors, another first order approximation of their location proposed by Schottler et al (2017a) is applied. In this approach the expected value and standard deviation of the fitted 5 velocity profile behind a yawed rotor is calculated.…”