Light detection and ranging measurements of wake dynamics part I: one‐dimensional scanning

Abstract

The vast majority of wind turbines are today erected in wind farms. As a consequence, wake-generated loads are becoming more and more important. In this fi rst of two parts, we present a new experimental technique to measure the instantaneous wake defi cit directly, thus allowing for quantifi cation of the wake meandering, as well as the instantaneous wake expansion expressed in a meandering frame of reference. The experiment was conducted primarily to test the simple hypothesis that the wake defi cit is advected passively by the larger-than-rotor-size eddies in the atmospheric fl ow, and that the wake at the same time widens gradually, primarily because of mixing caused by small-scale atmospheric eddies. In this fi rst paper, we focus on our new measurement technique, and test if the wake meandering follows the wind direction fl uctuations, i.e. if it is advected passively in the lateral direction. The experimental results are used as a preliminary verifi cation of a wake meandering model that essentially considers the wake as a passive tracer. several tall, meteorological masts have already proven the instrument to be accurate in this respect over fl at homogeneous terrain or offshore. 4-7 Because of the fact that the present application of the instrument is new-this is the fi rst time a LIDAR has been mounted on a wind turbine to measure the downstream fl ow fi eld-more detailed information about the signal processing and working modes will be discussed extensively. We installed the system on the back of the nacelle of a small wind turbine in such a way that it faces the downwind fl ow fi eld. Techniques were developed to move the laser beam across the wake horizontally and vertically. The wind speed was measured approximately 136 times per second. The instrument was focused between one to ten rotor diameters downstream, making it possible to RESEARCH ARTICLE Wind Energ. 2010; 13:51-61

Influence of atmospheric stability on wind-turbine wakes: A large-eddy simulation study

“…As mentioned before and also shown in Figure 14a, the growth rate of the wake is different in different directions. In order to better understand the relation between the nonaxisymmetric growth of the wake and the wake dynamics, next, we explore the unsteady behaviour of the wake commonly regarded to "wake meandering" [43][44][45] under different stability conditions. The meandering can be considered as a series of wake segments in which the wake center has an offset in the lateral and vertical directions 12 .…”

Flow tilt angles near forest edges – Part 2: Lidar anemometry

“…However, as we shall see later, for measurements over ordinary terrain this may not be a valid assumption, as pointed out by Bingöl et al (2009), and the error committed on the horizontal mean wind speed may be up to 10% depending on the steepness and complexity of the terrain. A second application of the instrument in wind energy has been the investigation of flow around a wind turbine (Harris et al, 2007;Bingöl et al, 2010).…”

Natural snowfall reveals large-scale flow structures in the wake of a 2.5-MW wind turbine

“…Our ability to gain such understanding is limited by the lack of experimental techniques able to quantify turbulent flows at the utility turbine scale. Current field measurement techniques provide only point velocity characterization (sonic anemometers) or velocity profiles (LiDAR and Sodar) at a coarse spatio-temporal resolution [7][8][9] . Particle image velocimetery (PIV), based on tracking the displacement of tracers in an illuminated flow field 10,11 , is the only measurement technique capable of obtaining planar velocity distributions with the spatio-temporal resolution required to study flow-structure interactions.…”