Date of Award

Spring 1-1-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Aerospace Engineering Sciences

First Advisor

Sedat Biringen

Second Advisor

Peter Sullivan

Third Advisor

Ken Jansen

Fourth Advisor

Tom Manteuffel

Fifth Advisor

Stuart Marlatt

Abstract

With the recent push for renewable energy sources, wind energy has emerged as a candidate to replace some of the power produced by traditional fossil fuels. Recent studies, however, have indicated that wind farms may have a direct effect on local meteorology by transporting water vapor away from the Earth's surface. Such turbulent transport could result in an increased drying of soil, and, in turn, negatively affect the productivity of land in the wind farm's immediate vicinity.

This numerical study will analyze four scenarios with the goal of understanding turbulence transport in the wake of a turbine: the neutrally-stratified boundary layer with system rotation, the unstably-stratified atmospheric boundary layer, and wind turbine simulations of these previous two cases. For this work, the Ekman layer is used as an approximation of the atmospheric boundary layer and the governing equations are solved using a fully-parallelized direct numerical simulation (DNS). The in-depth studies of the neutrally and unstably-stratified boundary layers without introducing wind farm effects will act to provide a concrete background for the final study concerning turbulent transport due to turbine wakes.

Although neutral stratification rarely occurs in the atmospheric boundary layer, it is useful to study the turbulent Ekman layer under such conditions as it provides a limiting case when unstable or stable stratification are weak. In this work, a thorough analysis was completed including turbulent statistics, velocity and pressure autocorrelations, and a calculation of the full turbulent energy budget.

The unstably-stratified atmospheric boundary layer was studied under two levels of heating: moderate and vigorous. Under moderate stratification, both buoyancy and shearing contribute significantly to the turbulent dynamics. As the level of stratification increases, the role of shearing is shown to diminish and is confined to the near-wall region only. A recent, multi-equation closure model, used to model the third and fourth velocity-temperature moments, performed well for the unstable cases. Optimal model coefficients found for the DNS data are shown to agree with atmospheric observations as well as LES data. Finally, the effects of top-down diffusion (entrainment-induced flux at the temperature inversion) and bottom-up diffusion (non-zero surface flux) were studied and improvements to correlation functions are suggested.

This thesis concludes by analyzing the neutral and unstable cases under the effects of wind turbine wakes. A unique means of converting a periodic simulation into a spatially evolving flow in the wake of a turbine is demonstrated; present results under neutral stratification are shown to agree with wind tunnel experiments under similar conditions. By introducing a scalar (humidity) into the flow field, the effect of a turbine wake on scalar transport in a wind farm is uncovered. The results show a clear drying effect under both neutral and unstable stratification given a wet surface. An investigation of energy and flux budgets gives guidance as to why such a phenomena occurs.

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