Can coherant continuous-wave doppler lidars be utilized for in-situ instrument calibration?
Ebba Dellwik & Jakob Mann
DTU Wind Energy, Denmark
Three continuous-wave, infrared, coherent Doppler lidars with acousto-optical modulators, which allow the determination of both the sign and the magnitude of the wind speed, were accurately positioned and aligned to measure the flow inside and near a CSAT3 sonic anemometer (Campbell Scientific) measurement volume. The measurement height was 6 m, and the focus distance of the three lidars was 8 m giving a full-width half-maximum of the lidar measurement volume of approximately 9 cm, which is comparable to the path length of the sonic anemometer (11.5 cm). Both systems were sampled at 60 Hz. The goal of the experiment was to quantify the systematic flow distortion caused by the structure of the sonic anemometer for which the lidar technique could offer a novel way of calibration. We present a comparison between sonic and lidar data as a function of wind direction and attack angle.
One short-coming with the acousto-optical modulators is the inability to measure close to zero wind speed. An alternative receiver architecture, in which the received backscatter signal is split into two parts and one part is delayed a quarter of a wavelength relative to the other, was therefore implemented. In order to test the new receiver, the lidar was pointed vertically to obtain near-zero speeds and measured close to a sonic anemometer. We present the data from the field experiment and discuss the observed differences between sonic and lidar data.
Thursday, 18 September 2014, 3:30PM
3450 Mitchell Lane
Bldg. 2 Large Auditorium (Rm 1022)