Met tower shadow study

This experiment investigated the effects of tower shadowing on cup anemometer wind speed readings in the wake of common meteorological (met) tower geometries.

Met tower measurements are the industry standard for wind resource assessment for wind farm siting. Anemometers are subjected to winds from all directions. At times, the anemometers are located in the wake of the tower and therefore yield measurements which are under-representative of the true wind speed. Relying on a single anemometer at any given station can result in significant discrepancies between the measured and actual wind conditions. The objective of the experiment was to quantify the decrease in wind speed with varying wind direction experimentally using full-scale wind tunnel testing, in the Live Fire wind facility.

Testing

Tests were carried out with two 3.66 m tall met towers of 15.2 cm (6 in) and 20.3 cm (8 in) diameter, secured to a rotary table base to allow for precise rotational positioning. Two NRG #40C cup anemometers were mounted at 180 degrees to one another on horizontal booms at lengths of 1.83 m and 2.44 m (6 ft and 8 ft) radially away from the tower, to replicate the geometry of the most common commercially available boom lengths.

Three wind speeds were tested for each tower and boom configuration: 6 m/s, 8.5 m/s, and 11.5 m/s. Data was collected for angular boom positions between 30° and -30°. The results were normalized using the wind speed ratio calculated by dividing the downstream anemometer wind speed by the upstream anemometer wind speed.

The results of the experiment are similar to those in literature, in that they show a wind speed deficit in the wake of the tower. The greatest deficit is largely affected by wind speed, decreasing with increasing wind speed.

The lowest downstream wind speed does not occur directly behind the tower at 0° but rather at approximately 3-5°. This offset may be related to the specific geometry of the cup anemometer, since the measured wind speed is primarily caused by the wind speed in line with the concave side of the cup.

Future considerations for this project include performing more measurements in the wake region at small angular intervals, conducting similar experiments with a propeller type anemometer, and finding a correlation between the controlled wind tunnel tests and the uncontrolled standard 10 min average MET tower data.