A nacelle-mounted lidar system pointing downstream has been used to measure wind turbine wake dynamics. The new measurement and data analysis techniques allow estimation of quasi-instantaneous wind fi elds in planes perpendicular to the rotor axis. A newly developed wake tracking procedure delivers the instantaneous transversal wake position which is quantitatively compared with the prediction of the Dynamic Wake Meandering model. The results, shown for two 10-min time series, suggest that the conjecture of the wake behaving as a passive tracer is a fair approximation; this corroborates and expands the results of one-dimensional measurements already presented in the fi rst part of this paper. Consequently, it is now possible to separate the deterministic and turbulent parts of the wake wind fi eld, thus enabling capturing the wake in the meandering frame of reference. The results correspond, qualitatively and to some extent quantitatively, to the expectations from CFD simulations which are compared in the paper. LIDAR measurements of wake dynamics J.-J. Trujillo et al. 62with high resolution. In this regard, the light detection and ranging (lidar) technology presents great advantages due to its speed, robustness and fl exibility in measuring wind fi elds. For instance, infl ow measurements from the nacelle of a wind turbine have been successfully tested. 3,4 In that case the fast measurement at a fi xed point in front of the turbine assisted an advanced wind turbine control system. This paper describes the application of lidar to wind fi eld measurements in the wake of a 95 kW wind turbine. As in the previous example, the lidar system is installed at the nacelle but pointing downstream and implementing more complex scanning. The measurements were performed at a downstream distance of four diameters, which is considered to cover the far-wake regime. This investigation is conceived as a proof of concept of this type of measurement. New analysis techniques are developed to provide data useful for steady and dynamic wake model verifi cation. Part I of this paper 5 exposes the fi rst measurement campaign of this type along with the working principles of the instrument. Furthermore, the installation procedure on the wind turbine nacelle at the Risø test fi eld in Roskilde is also explained. Part II of the paper deals mainly with the analysis of the so-called two-dimensional wake measurements. These enable estimation of the wake wind fi eld in cross-sections perpendicular (here also named transversal) to the mean rotor axis and eventually large-scale dynamics of the wake. Here we explain fi rstly, the two-dimensional wake measurements in detail and the methods developed to capture wake dynamics; secondly, models for dynamic and deterministic wake are exposed and fi nally compared with two individual measurements.

Methods to measure the vertical flux of horizontal momentum using both continuous wave and pulsed Doppler lidar profilers are evaluated. The lidar measurements are compared to momentum flux observations performed with sonic anemometers over flat terrain at Høvsøre, Denmark, and profile-derived vertical momentum flux observations at the Horns Rev wind farm in the North Sea. Generally, the momentum fluxes are reduced because of the finite measuring volume of the instruments, and the filtering is crudely accounted for theoretically. The essential parameter for the estimation of the reduction is the ratio of the turbulence scale to the size of the measuring volume. For the continuous wave lidar the reduction can largely be compensated by averaging Doppler spectra instead of radial velocities.

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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

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