IDENTIFICATION OF TOWER AND BOOMWAKES USING COLLOCATED ANEMOMETERS AND LIDAR MEASUREMENT

In this study the extent of tower and boom wake distortions were evaluated using collocated anemometers and Lidar measurement based on wind data from Amperbo, Namibia, where an existing latticed equilateral triangular communication tower was instrumented according to IEC specifications. Wind data analysed was 10-minute averaged, captured over a period of nine months (May to Sept. 2014). To enable further and independent investigation of flow modification within the vicinity of the tower, ZephIR 300 wind Lidar was installed at about 5.4 m from the foot of the tower. Wind data from pairs of collocated cup anemometers located at 16.88 m and 64.97 m above ground level (AGL) were analysed and compared to identify the range of directions that were affected by the waking of the entire tower physical structure. Mean speed and turbulence intensity (TI) were used in quantify the wake impact on the wind data observed using cup anemometers, showing a speed deficit of up to 49 % and order of magnitude increase in the TI for all the regions within the wake of the tower. Comparison with ZephIR 300 observed mean speed resulted in a speed deficit of up to 50 % which further confirmed the extent of tower distortion and wake boundaries. The Lidar also confirmed the speed-up effects and the asymmetric nature of the wake boundaries associated with the mounting booms. The results show that TI analysis has the potential to more accurately define the wake boundaries and wake distortion than traditional speed ratios analysis. The study shows that the severity of tower wake effects varies seasonally with winter months (June and July) recording the highest speed deficit when compared to December, a summer month. Root Mean Square Errors (RMSE) were further computed to ascertain the similarity degree of resource parameters from the two measurement techniques, resulting in peak values of RMSE in the wake affected regions. The TI approach consistently predicted larger wake boundaries than speed ratio analysis. Wind direction analysis clearly showed the 180° ambiguity of ZephIR 300 and the extent of deflection of the winds around the tower structure. Preliminary evaluation of wake impact on the resource parameter shows that removing the sectors affected by tower wakes leads to an increase in mean wind speed and a decrease in TI values.