Volume 3, issue 2 | Copyright
Wind Energ. Sci., 3, 533-543, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research articles 16 Aug 2018

Research articles | 16 Aug 2018

From standard wind measurements to spectral characterization: turbulence length scale and distribution

Mark Kelly Mark Kelly
  • Wind Energy Department, Risø Lab./Campus, Danish Technical University, Roskilde 4000, Denmark

Abstract. In wind energy, the effect of turbulence upon turbines is typically simulated using wind input time series based on turbulence spectra. The velocity components' spectra are characterized by the amplitude of turbulent fluctuations, as well as the length scale corresponding to the dominant eddies. Following the IEC standard, turbine load calculations commonly involve use of the Mann spectral-tensor model to generate time series of the turbulent three-dimensional velocity field. In practice, this spectral-tensor model is employed by adjusting its three parameters: the dominant turbulence length scale LMM (peak length scale of an undistorted isotropic velocity spectrum), the rate of dissipation of turbulent kinetic energy ε, and the turbulent eddy-lifetime (anisotropy) parameter Γ. Deviation from ideal neutral sheared turbulence – i.e., for non-zero heat flux and/or heights above the surface layer – is, in effect, captured by setting these parameters according to observations.

Previously, site-specific {LMM, ε, Γ} values were obtainable through fits to measured three-dimensional velocity component spectra recorded with sample rates resolving the inertial range of turbulence (1Hz); however, this is not feasible in most industrial wind energy projects, which lack multi-dimensional sonic anemometers and employ loggers that record measurements averaged over intervals of minutes. Here a form is derived for the shear dependence implied by the eddy-lifetime prescription within the Mann spectral-tensor model, which leads to derivation of useful forms of the turbulence length scale. Subsequently it is shown how LMM can be calculated from commonly measured site-specific atmospheric parameters, namely mean wind shear (dU∕dz) and standard deviation of streamwise fluctuations (σu). The derived LMM can be obtained from standard (10min average) cup anemometer measurements, in contrast with an earlier form based on friction velocity.

The new form is tested across several different conditions and sites, and it is found to be more robust and accurate than estimates relying on friction velocity observations. Assumptions behind the derivations are also tested, giving new insight into rapid-distortion theory and eddy-lifetime modeling – and application – within the atmospheric boundary layer. The work herein further shows that distributions of turbulence length scale, obtained using the new form with typical measurements, compare well with distributions P(LMM) obtained by fitting to spectra from research-grade sonic anemometer measurements for the various flow regimes and sites analyzed. The new form is thus motivated by and amenable to site-specific probabilistic loads characterization.

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Short summary
This paper shows how a definitive part of the commonly used Mann (1994) atmospheric turbulence model (its so-called eddy lifetime) implies that the model parameters can be directly related to typical measurements in wind energy projects. Most importantly, the characteristic turbulence length scale is found in terms of commonly measured (10 min mean) quantities (shear and standard deviation of wind speed); this estimator is found to give useful results, over different sites and flow regimes.
This paper shows how a definitive part of the commonly used Mann (1994) atmospheric turbulence...