Discussion of the Results of an In-Situ Comparison of Three Full-Vector Anemometers

Extensive field measurements and the numerical modeling of dynamic responses associated with wind turbine rotor blades have pointed to strong interactions with coherent turbulent structures in the turbine inflow. These interactions are thought to be a major source of high-cycle fatigue in the primary structural components of wind turbines. The sources of such turbulent structures are not onlynatural terrain features but also the wakes from upwind turbines. Many unsteady aerodynamic processes are excited by turbulent eddies ranging in size from several rotor diameters down to the dimensions of the mean blade chord. These processes are responsible for inducing large, fluctuating loads on the turbine rotor blades. For the wind turbine generators now in use, this encompasses a spatialrange of about 0.1 to 300 m. To assess our ability to measure the coherent properties of inflow turbulence over such a wide range of spatial range, we performed a study to compare three full-vector anemometers. We believe that to identify the dominant fluid dynamic properties of such flows, the instrumentation used must be capable of good fidelity measurements over the desired spatial range. Thesonic anemometer is a primary candidate; we also wanted to compare the results associated with a well-designed mechanical instrument which is available at considerably less cost. Two sonic designs and a propeller-bivane were exposed to turbulent flows downstream of both extremely complex and moderately rolling terrain. This paper discusses some of the results of these comparisons with anemphasis on the measurements of turbulent fluctuations.