Eddy Viscosity wake model for closely spaced turbines

It is known that where turbines are closely spaced within a row, the wake losses of subsequent downwind turbines are significantly under-predicted by industry standard wake models, including the Modified PARK and standard Eddy Viscosity models in WindFarmer. This under-prediction occurs when the within-row turbine spacing is around 2D or less and where the turbines are placed in rows perpendicular to the prevailing wind directions. This type of layout is usually only chosen where there is a uni- or bi- directional wind regime.

DNV developed a modification of the Eddy Viscosity model specifically for turbine arrays with these characteristics, which has been validated using data from several wind farms [14]. For closely spaced turbines, the following alterations are made to the Eddy Viscosity model:

• The velocity deficit is allowed to add up cumulatively when combining wakes.

• The Gaussian cross-wake profile is replaced by a blunter Lindeman profile

If the within-row spacing is greater than approximately 3D, then the modified model will give the same results as the standard Eddy Viscosity model. Between 2D and 3D separation the closely spaced wake modification is gradually turned on.

Please note, if the closely-spaced wake model is used, the turbine array in question must be similar to that described above for it to fall within the realms of validity of the model. The validation for this model has been undertaken using very specific uni- and bi-directional wind regimes.

In particular there is little evidence to support that both the closely spaced eddy viscosity model and large wind farm correction should be used in combination.