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Lidar data CFD correction
The lidar data CFD correction service for complex terrain
Lidar devices are being used more and more for wind resource assessment as an alternative to meteorological masts. Once the meteorological masts are installed, the height of conventional anemometers is fixed. This therefore restricts measurements to specific places and may not allow measurements over the entire rotor length of wind turbines, which may reach up to 200 meters or more. Lidars easily overcome this drawback as they are able to measure wind speed at heights greater than what conventional measuring masts can measure.
However, remote sensors encounter measurement bias in complex terrain due to the lack of flow homogeneity across the measured volume. Since the success of a wind farm project greatly relies on the initial wind resource assessment, a correction of the lidar data using CFD simulations is necessary to reduce the uncertainty.
Meteodyn has therefore worked with several Lidar manufacturers to provide a lidar data correction service which significantly reduces this issue.
The correction process
Meteodyn provides a Lidar data correction service for WindCube® Lidars, ZX Lidars and MOLAS B300 Lidars.
STEP 1
Using the position of the Lidar, the calculated inflow angles and horizontal deflection, Meteodyn delivers a correction file such as the one displayed on the left which contains the X and Y coordinates of the Lidar on the 1st line (green), the direction sectors in degrees on the 2nd line (red), the measurement heights above ground level in meters (blue) and the correction factors (black).
STEP 2
To correct the original LIDAR data, multiply the wind speed in each direction by the correction factor for this direction at the desired height. In the example on the left, the LIDAR data at the height of 80 m and in the sector 20 degrees must be multiplied by 1.054.
The science behind the correction
1
Ground-based vertically-scanning Lidars calculate the wind speed 
as shown in the figure on the right. Based on measurements performed on a circular scanned area, the Lidar measures 
and 
. This area has a diameter size close to the size of the measured height.
The calculation for the wind speed assumes that the flow over the measurement circle is homogeneous, and therefore that 
and consequently that 
. However, this assumption breaks down when the flow is non-uniform, typical for a non-simple terrain. What is desired is the corrected wind speed 
.
2
According to Sanquer et al. (Sanquer & Woodward, 2016) the general equation for is
Therefore, 
, , , 
and 
must be obtained. is easily obtained as it is simply the half cone scan angle of the specific Lidar being used. On the other hand, an accurate wind flow model is required to obtain the wind speeds ( and ) and the inflow angles ( and ).
The correction factor is therefore
3
To obtain accurate wind speeds and inflow angles, flow features typical of non-simple terrain, such as separation and recirculation must be modeled correctly. Using a wind flow model such as Computational Fluid Dynamics (CFD), it is possible to compute the set of factors that enable the correction of Lidar result data to be similar to the one measured by a conventional anemometer at the point of interest directly above the measuring device (Bingöl, Mann, & Foussekis, 2009).
Sanquer et al. (Sanquer & Woodward, 2016) used CFD to solve the Reynolds Averaged Navier-Stokes (RANS) equations to compute the required wind speeds and inflow angles. This method solves the RANS equations in a refined mesh around the Lidar location, hence obtaining the wind speeds and inflow angles at the desired locations.
4
This method has been validated with meteorological mast data proving to be very efficient and improving the correlation of Lidar data to traditional anemometers (Sanquer et al., WindEurope Technology Workshop 2020 - WRA and O&M, June 2020). For non-simple terrains, the CFD corrections of Lidar result data should be a standard process. All that is required to perform the correction is the geographical location of the lidar, its orientation*, its window height, the result heights desired above ground level and the direction resolution.
*The orientation or direction offset only required for WindCube® Lidars is the angle of the North-South axis of the Lidar compared to the real North and counted positively in a clockwise direction.