Читать книгу Wind Energy Handbook - Michael Barton Graham - Страница 47

Predicting the turbulence intensity and spectrum at a given point within an area of complex terrain is not straightforward. Hilly terrain upwind of the site in question will lead to generally higher turbulence levels, and some authors have suggested that this can be calculated from a ‘regional roughness length’ that takes the topography into account as well as the surface roughness (Tieleman 1992). On the other hand, distortion of the flow by the local terrain may reduce the turbulence intensity. At heights above ground that are of importance for wind turbines, rapid distortion theory applies, which means that the variance of the turbulent fluctuations will not change much as the flow passes over terrain features such as hills. Therefore if there is acceleration of the flow as it passes over a hill, the turbulence intensity will decrease, and the length scale will increase, resulting in a shift of the turbulence spectrum towards lower frequencies, without any change of shape (Schlez 2000). This effect is therefore easily estimated once a model such as WA^SP has been used to calculate the speed‐up factor at a particular point. However, the effect is also accompanied by a shift of turbulent energy from the longitudinal to the lateral and vertical components of turbulence, causing the turbulence on hill tops to be more isotropic (Petersen et al. 1998).