On the length‐scale of the wind profile

Abstract

We present the results of an analysis of simultaneous sonic anemometer observations of wind speed and velocity spectra over flat and homogeneous terrain from 10 up to 160 m height performed at the National Test Station for Wind Turbines at Høvsøre, Denmark. The mixing length, l, derived from the wind speed profile, is found to be linearly proportional to the length-scale of turbulence, derived either from the peak of the vertical velocity spectrum, (λ m ) w , or from a three-dimensional turbulence spectral model, for a range of atmospheric stability conditions, friction Rossby numbers, and within the range of observational heights ((λ m ) w ∼ 7l). Under very unstable conditions and above 100 m, the local wind shear is low, and the relation between both length-scales is slightly nonlinear. Mixing-length and wind profile models, which depend on both atmospheric stability and friction Rossby number, generally show better agreement to the observations of the length-scale and wind speed profile than the models from surface-layer theory, which show good agreement with the observations for the first 80 m only. The results from this analysis demonstrate a close connection between these two types of length-scales.

Turbulence characterization from a forward-looking nacelle lidar

“…The fit is performed on the ensemble-average spectra that have been logarithmically-averaged on the basis of the wavenumber (we will use such logarithmicallyaveraged spectra when fitting Mann parameters). These "average" Mann parameters ( = 3.00, αε 2/3 = 0.14 m 4/3 s −2 , and L = 35.38 m) are similar to those observed at a site with similar surface and turbulence characteristics (Peña et al, 2010a), but it should be noticed that these are the average of spectra for a number of atmospheric and turbulence conditions and that the Mann-model fitting procedure is normally performed over specific wind speed, turbulence, or atmospheric-stability ranges.…”

“…This model is chosen because it fits the atmospheric-turbulence velocity spectra for different surface, wind, and atmospheric-stability conditions within the first ≈ 100 m from the ground well (Peña et al, 2010a;Chougule et al, 2015) and is widely used to perform aeroelastic simulations of wind turbines. The ultimate objective of this study is to find out whether nacelle lidars can be used independently (i.e., without the need of extra measurements, e.g., from instruments on meteorological masts) to extract turbulence information from the inflow.…”

Ten Years of Boundary-Layer and Wind-Power Meteorology at Høvsøre, Denmark

“…1 with ψ m = 0) to estimate z o and found that for both winter and a three-year period, z o ≈ 0.016 m under neutral conditions. Peña et al (2010b) found nearly the same values as Gryning et al (2007) for a range of stabilities and further classified the neutral cases into wind-speed ranges; z o was found to decrease with wind speed.…”

“…Peña et al (2010c) for example, derived these parameters by fitting the one-dimensional turbulence spectra from the sonic measurements at different heights to the Mann model under near-neutral conditions and found a direct relation between the turbulence and the mixing-length scales. Later, Peña et al (2010b) derived the same parameters under different atmospheric stability conditions and found similar relations between length scales. These results demonstrate the close connection between the wind profile and the turbulence structure.…”

“…Autumn, winter, and spring roughness roses are very similar, the autumn one about two times higher than those observed during winter. Peña et al (2010a) found, under neutral conditions and within a three-month summer period, z o values that were three times higher than those in Peña et al (2010b), who used a much longer dataset.…”