Experimental and theoretical investigation of tower shadow impacts on anemometer measurements

The tower supporting an anemometer modifies the local wind field and anemometer measurements. In wind energy resource assessment, tower-induced flow modification contributes a non-negligible amount of uncertainty to the wind resource assessment. The effect of wind speed averaging period on anemometer measurement errors was investigated using high resolution sonic measurements from two sonic anemometers on a tubular tower. Measurements were post-processed into equivalent datasets that differed only by averaging periods. Averaging period only impacted the measured magnitude of the wake, while little effect was seen outside the wake region for data averaging periods between 15 s and 1 h. A model is proposed to remove tower shadow effects from anemometer data using a potential flow solution in the region outside the tower wake and assuming the tower wake is Gaussian and turbulent. An untuned version of the model reproduced the main features of tower-induced flow modification including the turbulent wake, but was not accurate enough to provide a useful correction for wind resource assessment purposes. Fitting model parameters using measured data was found to be a practical way to partially correct wind data from a pair of anemometers in which one fails or becomes unreliable.