Characterisation of Recirculation Zones in Complex Terrain Using Multi-Lidar Measurements

Wind fields in complex terrain show a higher complexity compared to sites with simpler geometries. The characteristics of complex flows must be understood to account for them during site validation for wind energy projects to ensure that turbines can withstand the local flow conditions. Flow recirculation can have a significant impact on the success of wind energy projects since it represents one of the main contributors to turbulence generation.

This study focuses on the description of recirculation zones occurring on lee sides of ridges. An extensive dataset of observations of flow over complex terrain is available from the Perdigão-2017 field campaign over a period of three months. The measurement site was selected because of the unique terrain feature of two nearly parallel ridges with a valley-to-ridge-top height difference of about 200 m and a ridge-to-ridge distance of 1.4 km.

Eight long-range WindScanners, scanning Doppler lidars deployed by DTU, probed the flow field in vertical planes by performing range-height-indicator(RHI) scans at several positions along the two ridges. In addition, data from several 20-100 m high meteorological masts equipped with sonic anemometers are available. With the above-mentioned lidar setup, we achieved vertical scans of the recirculation zone at three positions along two parallel ridges, as seen in Figure 1. We develop methods to identify flow recirculation zones in the scans, as well as different characterising features of these zones. We find that recirculation zones, with reverse flow wind speeds greater than 0.5 m s-1, occur over 50% of the time. Atmospheric conditions, such as atmospheric stability and wind speed, affect the occurrence of recirculation: recirculation appears to occur more frequently during periods with wind speeds above 8 m s-1. Moreover, the topography along the different transects has a significant influence on the shape of recirculation zones. Recirculations within the valley affect the wind field at the downwind ridge in magnitudes significant for wind resource assessment.