Measuring higher-order moments with a cup anemometer

Modern, fast cup anemometers are very useful for measuring the mean-wind sp eed. The calibration is linear and its operation is well described as a lin ear, first-order filter. Discussion in the literature about the importance of the response asymmetry to increases and decreases in the streamwise turb ulent wind-velocity component seems to have ended at the conclusion that th ere is no detectable influence on the mean-wind determination from this sou rce, that is, the part of "overspeeding" due to response asymmetry can be n eglected. However, a bias on the mean-wind speed from wind-direction fluctu ations cannot always be excluded. Here the author addresses the problem of whether higher-order moments are influenced by the asymmetry and the direct ion fluctuations. Based on a theoretical analysis-a straightforward perturb ation calculation-and an experimental test, the author finds that moments u p to and including the fourth order are unaffected by the asymmetry as well as wind-direction fluctuations. The author suspects that this is true even for higher-order moments. It is argued that the cup anemometer together wi th a wind vane is well suited for measuring the horizontal components of th e turbulent wind velocity.