Boundary-Layer Meteorol (2018) 166:423–448
Impact of the Diurnal Cycle of the Atmospheric
Boundary Layer on Wind-Turbine Wakes: A Numerical
· Andreas Dörnbrack
Received: 29 November 2016 / Accepted: 22 September 2017 / Published online: 14 October 2017
© Springer Science+Business Media B.V. 2017
Abstract The wake characteristics of a wind turbine for different regimes occurring through-
out the diurnal cycle are investigated systematically by means of large-eddy simulation.
Idealized diurnal cycle simulations of the atmospheric boundary layer are performed with
the geophysical ﬂow solver EULAG over both homogeneous and heterogeneous terrain.
Under homogeneous conditions, the diurnal cycle signiﬁcantly affects the low-level wind
shear and atmospheric turbulence. A strong vertical wind shear and veering with height
occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas
atmospheric turbulence is much larger in the convective boundary layer and in the evening
boundary layer. The increased shear under heterogeneous conditions changes these wind
characteristics, counteracting the formation of the night-time Ekman spiral. The convective,
stable, evening, and morning regimes of the atmospheric boundary layer over a homoge-
neous surface as well as the convective and stable regimes over a heterogeneous surface are
used to study the ﬂow in a wind-turbine wake. Synchronized turbulent inﬂow data from the
idealized atmospheric boundary-layer simulations with periodic horizontal boundary condi-
tions are applied to the wind-turbine simulations with open streamwise boundary conditions.
The resulting wake is strongly inﬂuenced by the stability of the atmosphere. In both cases,
the ﬂow in the wake recovers more rapidly under convective conditions during the day than
under stable conditions at night. The simulated wakes produced for the night-time situation
completely differ between heterogeneous and homogeneous surface conditions. The wake
characteristics of the transitional periods are inﬂuenced by the ﬂow regime prior to the tran-
sition. Furthermore, there are different wake deﬂections over the height of the rotor, which
reﬂect the incoming wind direction.
Institut für Physik der Atmosphäre, DLR Oberpfaffenhofen, Weßling, Germany