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.