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Stratified Planetary Boundary Layers: from Turbulence Analysis to Wind Turbine Applications


The lowest part of the atmosphere, which is directly influenced by Earth’s surface,

is called the planetary boundary layer (PBL) or the atmospheric boundary layer (ABL).

Within a PBL, the physical quantities such as flow velocity, temperature and moisture

display rapid fluctuations, known as turbulence. In the absence of the effects of humidity

and advection, planetary boundary layers becomes stably stratified whenever the land/sea

surface is cooler than the air above. Therefore, after sunset, radiative cooling of the

ground is no longer compensated by the incoming short-wave flux from the sun, leading

to a shallow stratified boundary layer (SBL) during the night. Shear and buoyancy

compete in the SBL: shear mostly generates turbulent motions, and negative buoyancy,

which is a result of the radiative cooling, inhibits turbulence. For a strongly stable PBL,

the existing similarity theories, i.e. Monin-Obukhov theory (MOST), cannot properly

predict the turbulent fluxes, the structure and the scaling of different layers due to the

existence of complex dynamics such as low-level jets (LLJs), turbulence collapse and

global intermittency. This motivates the first part of the present research to elaborate the

dynamics of strongly stratified PBL by conducting direct numerical simulations (DNS)

of Ekman layer, a surrogate of the planetary boundary layer.

Wind energy is a clean, renewable energy source that offers many attractive

attributes including being fuel-free, inexhaustible, and cost-effective. These advantages

make the wind energy the fastest-growing energy sources in the world, and the researches

are aiming at improving the technology, integration, and lowering costs to address the

challenges to its greater use. In the second part of present research, the interaction

between the stably stratified PBL and wind turbines is examined with two different

types of simulations. In the first approach, the PBL is simulated with a large eddy

simulation (LES) model advanced in this thesis for stratified flow and provided to the

Bazilevs group (in the structural engineering department) who perform accurate fluid-

structure interaction (FSI) simulations of the wind turbines. The second approach is the

simulations of wind turbines operating in SBLs with a simpler representation, known

as the generalized actuator disk model. The FSI data obtained in the first approach are

used in obtaining the scaling and the distribution of input parameters required for the

second approach. The developed actuator disk model is calibrated with the FSI data, and

subsequently used to study the interaction of wind turbines with different regimes of

stratified PBLs.

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