Effects of an escarpment on flow parameters of relevance to wind turbines

Assessing potential costs and benefits of siting wind turbines on escarpments is challenging, particularly when the upstream fetch is offshore leading to more persistent wind speeds in power producing classes, but an increased importance of stable stratification under which terrain impacts on the flow may be magnified. In part because of a lack of observational data, critical knowledge gaps remain and there is currently little consensus regarding optimal models for flow characterization and turbine design calculations. We present a unique dataset comprising measurements of flow parameters conducted over a 10-14m escarpment at turbine relevant heights (from 9 to 200m) and use them to evaluate model simulations. The results indicate good agreement in terms of the wind speed decrease before the terrain feature and the increase at (and downwind of) the escarpment of similar to 3-5% at turbine hub-heights. However, the horizontal extent of the region, in which the impact of the escarpment on the mean flow is evident, is larger in the models than the measurements. A region of high turbulence was indicated close to the escarpment that extended through the nominal rotor plane, but the horizontal extent of this region was narrow (<10 times the escarpment height, H) in both models and observations. Moving onshore the profile of turbulence was more strongly influenced by higher roughness of a small forest. While flow angles close to the escarpment were very complex, by a distance of 10H, flow angles were <3 degrees and thus well within limits indicated by design standards. Copyright (c) 2016 John Wiley & Sons, Ltd.