Understanding the effects of surface roughness transitions on the spatial distribution of surface shear stress and velocity is key to improving predictions of turbulent transport in the atmospheric boundary layer (ABL). This is particularly important as a boundary condition for the design of wind energy projects.
In the first part of this research, the effects of surface roughness heterogeneity on the ABL were studied in the boundary layer wind tunnel of the Saint Anthony Falls Laboratory (SAFL) at the University of Minnesota. We developed a numerical model that accurately predicts, for the first time, the change of wind velocity and surface shear stress downwind of a surface roughness transition. In this context, different surface boundary conditions for large-eddy simulation were tested downwind of a rough-to-smooth surface transition. Results show substantial differences between measured and modeled shear stress using standard boundary conditions based on the direct application of the similarity theory with local fluctuating filtered velocities.
In the second part of this research, the complex interaction between the ABL and wind turbine wake(s) was studied using model wind turbines. The structure of the turbulent flow around the wind turbines were characterized under both thermally neutral and stable stratifications. Non-axisymmetric behavior of turbulence statistics in the wake was observed in response to the non-uniformity of the incoming boundary layer flow. Nevertheless, the velocity deficit with respect to the average incoming flow was nearly axisymmetric. In the wind farm scenario, results suggest that the turbulent flow can be characterized into two main regions. In the first, located below the turbine top tip height, flow statistics appear to reach equilibrium as close as 3-4 turbines downwind of the first turbine. In the second region, located immediately above the turbine top tips, flow adjustment is slower. Here, two distinct layers were found: an internal boundary layer and an equilibrium layer, where the flow statistics are fully adjusted to the wind farm condition. Our results also show that wind turbine wakes reduce the mean surface heat flux, where a large wind farm implied the most significant change.
University of Minnesota Ph.D. dissertation. August 2010. Major: Civil Engineering. Advisor: Fernando Port e-Agel. 1 computer file (PDF); xv, 158 pages. Ill. (some col.)
Surface heterogeneity effects on the atmospheric boundary layer: parameterizations and applications to wind energy..
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