Effect of an abrupt rough-to-smooth surface roughness transition occurring upstream or within a wind farm studied using large eddy simulation and analytical modeling

The effect of a rough-to-smooth (R-S) surface roughness transition occurring within or upstream of a wind farm is studied using large-eddy simulation (LES). The R-S transition alters the velocity deficit, turbulence intensity (TI), turbine wake recovery, and farm power generation. The wake growth rate correlates well with the added TI rather than TI. The distance between the R-S transition and the first-row turbine is shown to be a key parameter. For the parameters considered, a wind farm starting 6 turbine diameters (d) upstream to 4d downstream of the transition has 15%−19% lesser wake losses compared to a wind farm sited on a homogeneously rough surface far upstream of the transition. Similar observations hold for farms with increased row spacing or with increased turbine thrust coefficient. The farm wake losses are only weakly sensitive to the turbine hub heights. Decreasing the upstream/downstream surface roughnesses keeping their ratio fixed leads to a smaller difference between wind farms on homogeneously/heterogeneously rough surfaces. Predicting the power using analytical models for the velocity field is not trivial. Different single-turbine wake model and velocity sampling location choices are evaluated. Incorporating a model for upstream blockage effects and sampling the velocity at the turbine location are shown to be essential to predict the power accurately, to within 9% of the LES results, for most of the cases studied here.