Theperennialcupanemometer-WindSensorEN.pdfVIP

WIND ENERGY Wind Energ., 2, 59±75 (1999) Research Article The Perennial Cup Anemometer L. Kristensen,* Risù National Laboratory, Roskilde, Denmark Key words: calibration; starting speed; calibration distance; angular response; distance constant; overspeeding A short version of the history of the cup anemometer precedes a more technical discussion of the special features of this instrument. These include its extremely linear calibration and the non-linearity of its response to wind speed changes. A simple conceptual model by Schrenk is used to demonstrate this and to explain why the cup anemometer is able to start from a zero rotation rate at zero wind to one corresponding to a sudden change in the ambient wind speed to a ?nite value. The same model is used to show that the cup anemometer should be characterized by a distance constant rather than by a time constant. The bias in the measured mean wind speed due to the random variations in the three velocity components is discussed in terms of standard, semiquantitative turbulence models, and the main thesis is that this bias is overwhelmingly dominated by the ˉuctuations of the lateral wind velocity component, i.e. the wind component perpendicular to the mean wind direction, and not, as is often assumed, by the longitudinal wind velocity component. It is shown theoretically and tested experimentally that the bias due to lateral wind velocity ˉuctuations can be signi?cantly reduced by means of a special data processing of the simultaneous signals from a cup anemometer and a wind vane. This means that, with care, the overall overspeeding can be reduced to less than 1%. Copyright *c 1999 John Wiley Sons, Ltd. Introduction The cup anemometer is probably the most common instrument for measuring the wind speed at places where weather observations are routinely carried out. We see them in airports, at wind farms and at construction sitesDand for good reasons. Experience has shown that the cup anemometer is a robust and reli