| Popis: |
The wake behind the 2 MW wind turbine situated on flat country at Näsudden, Sweden has been mapped in a field experiment where several measuring techniques were employed. A high resolution sodar appeared to be a very powerful tool which enabled detailed studies of both mean and turbulence wake profiles to be carried out. Measurements with this system were made with three antennae in three different modes of operation and at several distances in the range 2D-3.6D from the turbine (D being the rotor diameter, 75 m). Additional detailed data were obtained with turbulence instruments at three levels at a 145 m tower, situated 3D downwind, during conditions with the wind blowing from the turbine towards the tower. Finally good quality data of the centre line velocity deficit and of the longitudinal turbulence intensity were obtained with Tala kites at distances of up to 10.5D. The measurements show good agreement with data on the thrust coefficient derived from turbine characteristics. The centre line velocity deficit was found to vary with distance as x-1.1, in general agreement with wind tunnel simulations, but the constant of proportionality was found to be about 40% lower than in those simulations. Detailed data on the turbulence structure in the wake have been obtained. For downwind distances of a few diameters, the vertical profile of turbulence intensity is found to resemble the corresponding profiles found in the laboratory, with two sharp peaks at about rotor radius distance from the centre. The general level of turbulence intensity as an average at the centre of the wake is found to be about a factor of two higher than found in wind tunnel simulations. Turbulence spectra measured in the wake with the sodar and with tower mounted instruments show significantly increased fluctuation levels for frequencies above about 0.004 Hz and decreased energy at lower frequencies. © 1988. |
