Simulation and Analysis of Wind Turbine Wakes
Jens N. Sørensen
Department of Wind Energy
Technical University of Denmark
Modern wind turbines are often
clustered in wind farms in which the turbines are fully or partially influenced
by the wake of upstream located turbines. As a consequence, the wake behind the
wind turbines has a lower mean wind speed and an increased turbulence level, as
compared to the undisturbed flow outside the farm. Hence, wake interaction
leads to a decreased total production of power, caused by lower kinetic energy
in the wind, and an increase in the turbulence intensity. The turbulence
created from wind turbine wakes is mainly due to the presence of the distinct
tip and root vortices, which eventually break down and forms small scale
turbulent structures. If a wind turbine is located in a wake consisting of tip
and root vortices, the fatigue loading is more severe than in the case where
the tip vortices have already broken down by instability mechanisms. Therefore,
understanding the physical nature of the vortices and their dynamics in the
wake of a turbine is important for the optimal design of a wind farm.
Wake modeling can be carried out
using different models of varying levels of complexity. The simplest models
assume linearly expanding wakes and are described by simple integral momentum
equations, while the most complex models make use of computational fluid
dynamics (CFD) and state of the art representations of the rotor blades. For an
extensive list of different wake models, the reader is referred to Crespo et
al. [1], Vermeer et al. [2] and Sørensen [3]. In the past years,
wakes behind wind turbine blades have been studied both experimentally and
numerically, using analytical tools as well as numerical simulations based on RANS
or LES methodologies combined with actuator disc or line techniques. From these
studies it has been shown that helical wakes are inherent unstable and that the
flow inside a wind farm to a large extent is depending on the ambient
turbulence and the stability properties of the atmospheric boundary.
In
the presentation, I will give a status of state-or-the-art modeling of wind
turbine wakes, including a general description of the challenges of wake
modeling and of the various models in use today. Furthermore, I will present
mechanisms for initial breakdown of the vortex pattern in the near wake as well
as show results from studies of wake interaction and modeling of the
interaction between wind farms and the atmospheric boundary layer.
References:
2. Vermeer, L.J., Sørensen,
J.N. and Crespo, A. (2003), "Wind Turbine Wake Aerodynamics". Progress in Aerospace Sciences,
vol. 39, pp. 467-510.
3. Sørensen, J.N. (2011) ‘Aerodynamic aspects
of wind energy conversion’. Annual Review of Fluid Mechanics,
vol. 43, pp. 427-448.