The spectrum of wind speed fluctuations encountered by a rotating blade of a wind energy conversion system

A point on a rotating wind turbine blade encounters turbulence whose characteristics are quite different from turbulence measured by a stationary anemometer. This is true for vertical as well as horizontal axis wind turbines. The spectrum of the observed turbulence is distored in several subranges of frequency in characteristic ways. The midfrequency region is depleted and the removed energy is distributed into the high frequency end of the spectrum. The resulting two-peaked continuous spectrum also contains narrow-band spikes of turbulence energy centered on the frequency of rotor rotation and multiples of that frequency. The statistics of gusts are expected to be distorted also.The rotational sampling effect is quantified using measurements of wind velocity at circular arrays of anemometers and measurements by laser and hotwire anemometers traversing crosswind circular paths. A simple theoretical model of the spectrum of turbulence which is rotationally sampled is developed. The model, which assumes homogeneous, isotropic turbulence, reproduces the observed spectral features and provides an analytic verification and extension of scaling relations using turbine and atmospheric length and time scales. All known theoretical models of the spectrum of a rotationally sampled turbulence field are compared with the spectra which are derived from rotationally measured turbulence.Implications for turbine design, siting and control are drawn. A nondimensional parametric analysis of the rotational turbulence effect suggests criteria for selecting rotor height above ground and rotor rotation rate and provides guidelines for optimizing a design once the preliminary turbine configuration has been established.