The Eolos research facility

The field-scale experimental data presented in this paper were collected at the Eolos wind energy research station located on the University of Minnesota Outreach, Research and Education Park (UMore Park) in Rosemount, Minnesota. This 5,000 acre complex is a part of the University of Minnesota and includes eighty acres devoted to wind energy research. On site equipment consists of a fully instrumented 2.5 MW wind turbine and a meteorological tower (denoted as met tower hereafter), in addition to a mobile WindCube lidar wind profiler. The terrain surrounding the turbine and met tower primarily consists of low, rolling farm land (local elevation change of 1-3 m with respect to the turbine base elevation), shown in Figure 4.1, with a few sparse tree patches and a few two-story buildings within a 2 km radius.

Figure 4.1: Satellite photo of Eolos research facility (Google map) with the GPS loca-tions of the lidar deployment and measurements

z = 129 m

Figure 4.2: Schematic of data acquisition systems on the Eolos test site (adapted from Chamorro 2014).

Utility Scale Turbine

The Eolos wind turbine is a highly instrumented 2.5 MW Clipper Liberty C96 with a hub height z_hub=80 m and a rotor diameter D =96 m. Aside from the standard turbine supervisory control and data acquisition (SCADA) system which continuously monitors hundreds of variables on turbine operation, the Eolos turbine is equipped with twenty strain gauges placed at the connection between the base of the tower and the foundation to measure bending forces and moments. In addition, to quantify blade loading, strain gauges have been attached during turbine assembly inside each blade on both low and high pressure sides (at progressive distances from the root, specifically 0, 25, 37.5, 44.4

% of radius) and integrated into the data acquisition system together with two extra strain gauges at the leading and trailing edges on the blade root. All the data from the turbine is stored on local servers with a site-universal time stamp.

Meteorological Tower

A 130 m tall research met tower is located 160 m, approximately 1.7 D, due south of the Eolos wind turbine. The tower vertically covers the entire swept rotor area with three sonic anemometers (Campbell Scientific CSAT3) located at z = 30, 80, 129 m above ground, corresponding to approximately the turbine bottom-tip, hub height and top-tip, respectively, providing three components of wind velocity and temperature at 20 Hz for each location. Six additional instrumented arms (at z = 7, 27, 55, 77, 105, 126 m) are equipped with low frequency (1Hz) temperature, relative humidity and cup and vane anemometer sensors, see Figure 4.2. One additional CSAT3 is positioned at z =10 m for boundary layer parametrization and assessment of thermal stability conditions.

Barometric pressure for the site is recorded at the z =7 m boom.

WindCube lidar

A WindCube V1 wind profiling lidar, manufactured by LeoSphere, is used herein to pro-vide a time-resolved description (1Hz) of the incoming velocity profile to the turbine, with z ranging from 40 to 160 m. Wind speed and direction information is captured, stored every second onto an internal hard drive and synchronized with the unique time stamp of the Eolos database systems through a local network connection. The Wind-Cube V1 estimates the air velocity along four line of sight (LOS) measurements oriented 90 degrees from one another in the horizontal plane and inclined at an angle of φ = 27.8 degrees from vertical (known as the cone angle). The LOS directions with respect to a fixed reference system are determined by the WindCube specific orientation. In this study, two different WindCube alignments were tested: (i) adopting the standard ori-entation which points the lidar due north (defined here as global alignment along the cardinal directions and utilizing all four LOS measurements to calculate the three com-ponents of velocity); (ii) directing the north-south LOS plane to be perpendicular to the turbine rotor plane (denoted as local alignment, and can use the two LOS along the mean wind direction to measure the velocity in that plane). Three methods of calcu-lating the velocity components from the lidar measurements can be utilized, and are:

(i) use all four LOS measurements to calculate the spatially averaged velocity (intrinsic procedure for pulsed lidar software to resolve u, v, w); (ii) use two LOS measurements

180 degrees from one another to get averaged planar velocity (two components only);

(iii) use one LOS measurement in addition to the vertical velocity w calculated from the four beam average (allows estimation of LOS local planar velocity). Here, method (i) is employed for data taken in global lidar orientation and (iii) for data acquired in local lidar alignment. The two calculations are introduced in more detail below.

Standard method for lidar velocity calculation

The lidar software implements the following procedure to estimate the three components of velocity using all four velocity measurements along the LOS (see Equations 4.1-4.3). Note that regardless of the lidar orientation this standard method of velocity calculation is utilized and synchronized to the turbine operational data to allow correct joint statistics. The four LOS method allows the lidar to remain in a fixed location yet capture the wind direction changes that inevitably cause the turbine to yaw away from the initial alignment with the lidar.

u = RW S_N− RW SS

2sinφ ; (4.1)

v = RW S_E − RW SW

2sinφ ; (4.2)

w = RW SN + RW SS+ RW SE+ RW SW

4cosφ ; (4.3)

where, u, v and w are the velocity components in the x, y and z directions, while RW S_N, RW S_S, RW S_E and RW S_W stand for the radial wind speed (RWS) along the LOS in the north, south, east and west directions, respectively.

Quasi 2D method for lidar velocity calculation

The second method uses Equation 4.3 from method (i) in addition to Equations 4.4 and 4.5 and computes the instantaneous velocity component along the mean wind direction using the measured RWS and the spatially averaged mean vertical velocity.

M = w

sinα = RW SLOS

cosβ ; (4.4)

φ RWS M

w u

α β

LOS

Figure 4.3: Geometry used to calculate the horizontal, in-plane velocity u from the lidar measured radial wind speed (RWS) and the vertical velocity w, as computed from Equation 4.3. The line of site (LOS) beam is emitted from the lidar (lidar located at the base of LOS) and represents one of the beams emitted from the lidar.

u_LOS = w

tanα; (4.5)

where, M is the in-plane wind vector containing the u and w components of velocity (streamwise, vertical), α is the angle between the horizontal plane and the wind vector M , and β is defined as the angle between the LOS beams and the wind vector M . A schematic of the measurement is shown in Figure 4.3. Note that to calculate the streamwise velocity profile for this method, spatial averaging is performed only on the plane aligned with the mean wind direction; thus, contamination is minimized from the spanwise velocity measured along the two LOS perpendicular to the mean wind (not necessarily confined within the rotor area, and may be altered by the spanwise flow distortion due to the turbine). This method for obtaining the velocity is utilized for determining the mean profile immediately upwind of the turbine and in the turbine wake.