PhD Project: Wind Turbine Wake in Atmospheric Turbulence

This thesis describes the different steps needed to design a steady-state
computational fluid dynamics (CFD) wind farm wake model. The ultimate
goal of the project was to design a tool that could analyze and extrapolate
systematically wind farm measurements to generate wind maps in order to
calibrate faster and simpler engineering wind farm wake models. The most
attractive solution was the actuator disc method with the steady state k-ε
turbulence model.
The first step to design such a tool is the treatment of the forces. This
thesis presents a computationally inexpensive method to apply discrete body
forces into the finite-volume flow solver with collocated variable treatment
(EllipSys), which avoids the pressure-velocity decoupling issue.
The second step is to distribute the body forces in the computational
domain accordingly to rotor loading. This thesis presents a generic flexible
method that associates any kind of shapes with the computational domain
discretization. The special case of the actuator disc performs remarkably
well in comparison with Conway’s heavily loaded actuator disc analytical
solution and a CFD full rotor computation, even with a coarse discretization.
The third step is to model the atmospheric turbulence. The standard
k-ε model is found to be unable to model at the same time the atmospheric
turbulence and the actuator disc wake and performs badly in comparison
with single wind turbine wake measurements. A comparison with a Large
Eddy Simulation (LES) shows that the problem mainly comes from the
assumptions of the eddy-viscosity concept, which are deeply invalidated in
the wind turbine wake region. Different models that intent to correct the
k-ε model’s issues are investigated, of which none of them is found to be
adequate. The mixing of the wake in the atmosphere is a deeply non-local
phenomenon that is not handled correctly by an eddy-viscosity model such
as k-ε .