other hand, distortion of the flow by the local terrain may reduce the turbulence
intensity. At heights above ground which 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
S
P 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).
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