Measurements of low-frequency wind spectral properties relevant to design of long-span bridges

Abstract eng:
The present paper investigates the capability of synchronized long-range Doppler wind lidars to capture the wind coherence at scanning distances up to 4.5 km. The data set considered comes from a field measurement campaign conducted from May 2016 to June 2016 in the Bjørnafjord (Norway). The longrange WindScanner system deployed consisted of three lidar units in a combination of fixed and “dynamic” Line-Of-Sight (LOS) scanning modes. In the latter configuration, the horizontal wind components were retrieved at four different locations with a sampling frequency of at least 0.25 Hz. The data are used to estimate the lateral coherence of the along-wind component for wind velocities ranging from 6 m s−1 to 14 m s−1 and lateral separations from 18 m to 177 m. Although the spatial averaging was not observed to have any significant influence on the coherence estimates, the decay coefficient measured using a simple exponential co-coherence function was larger than expected for the along-wind component and lateral separations, with values around 23 for 10 m s−1 ≤ u ≤ 14 m s−1 . The apparent predominance of stable stratification and the large scanning distances associated with large cross-wind separations may contribute to a measured coherence lower than predicted in the Norwegian handbook N400. Although the dominant wind direction was from North-West during the measurement period, a larger variety of wind directions with different properties are expected. For a more complete analysis, a wider range of wind directions and stronger wind velocities are desirable.

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