Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Numerical Study on the Asymmetry in Wake Steering of Wind Turbine under Yawed Conditions
Dezhi Wei1, Nina Wang2, Decheng Wan1*, Sergei Strijhak3
1Computational Marine Hydrodynamics Lab (CMHL), School of Naval Architecture, Ocean and Civil Engineering,
Shanghai Jiao Tong University, Shanghai, China
2Key Laboratory of Far-shore Wind Power Technology of Zhejiang Province, Huadong Engineering Corporation Limited, Hangzhou, China
3Ivannikov Institute for System Programming of the Russian Academy of Sciences, Moscow, Russia*Corresponding author

Kirchner-Bossi et al, 2018), and increasing streamwise distance
between the consecutive turbines was suggested. What's more, some
active wake control strategies were proposed, which can be divided into
two classes, one about reducing the axial induction of the upstream
turbine (Annoni et al, 2016), and the other is the wake redirection
techniques, including pitch angle control, tilt angle adjustment andactive yaw control (Bastankhah et al, 2019; Miao et al, 2017). In
Fleming et al. (2014), they tested several wake redirection strategies
and found that active yaw control is the most promising method toimprove wind plant performance. It is implemented by intentionally
misaligning the upstream turbine rotor to the incoming wind direction,
thereby deflecting the wake and avoiding the downstream windturbines. Although the power output of the yawed turbine itself isreduced, the total power of entire wind farm can potentially increase, as
the downwind turbine is less affected by the upstream wakes.
To better apply active yaw control in practice, a detailed knowledge ofthe yawed turbine wake and its interaction with downstream turbines isrequired. Howland et al. (2016) conducted a wind tunnel measurement
to study the yawed wake morphology under uniform inflow. Theyobserved a curled wake shape and attributed its formation to a pair ofcounter-rotating vortices shed from the top and bottom of the rotor
plane. Apart from deforming the wake, Fleming et al. (2018) further
highlighted other roles of the counter-rotating vortex pair (hereafter,CVP), which were found to deflect the wake of an aligned downstream
turbine, even if it was non-yawed. This is called “secondary wakesteering” phenomenon, and is proven to be critical for the wakeevolution of turbine array under yawed conditions. Additionally, also a
number of studies focused on the possibilities of power optimization
through intentional yaw misalignment. For instance, Campagnolo et al.
(2016) experimentally studied the performance of three turbine rows,
and they observed that, for the case where the wind turbines were in theflow 4 rotor diameters apart, the total wind farm power production canincrease about 4% by using a closed-loop wake deflection controller. A
numerical simulation of Gebraad et al. (2016) on six wind turbines also
demonstrated the capability of active yaw control. In their works, a
13% increase of the combined power under yawed conditions was seen
compared to the baseline non-yawed case.

ABSTRACT
Wake redirection via active yaw control is a promising strategy toimprove the whole wind farm performance. To successfully apply suchan operational control in the real-world engineering, it is necessary tohave a precise knowledge of the yawed turbines and their wakes. In thecurrent work, a series of numerical simulations are performed by thehigh-fidelity SOWFA tool, to study the occurring asymmetry in
oppositely signed yaw angles, as well as its effects on wake-steering
performance. The simulation results demonstrate that the wake curl isasymmetric between positive and negative yaw angles, arising from theinteraction of the counter-rotating vortices with the vertical wind veer.
Some key wake properties are also observed to be asymmetric withrespect to the yaw angle direction, especially the lateral wake deflection,
which can affect the efficiency of active yaw control. What's more, aload study of wake steering is conducted, showing that the positive yaw
offsets are less harmful to the tested NREL 5MW wind turbine. In
conclude, this work emphasizes the necessity and importance ofconsidering the yaw direction in implementing yaw-based wind farmcontrol.

KEY WORDS: Opposite yaw directions; Asymmetry in wakesteering; Wake flow property; Structure loadINTRODUCTIONWind energy is a kind of renewable energy with great development
prospects (Chehouri et al, 2015). In order to maximize wind energy
extraction within the limited available land, multiple turbines arecommonly installed in an organized array in the wind farm. However, aconsequent drawback is the strong wake interference among turbines,
which has a significant impact on the wind farm performance, not only
increasing turbine loads , but also decreasing energy capture. To
mitigate the negative effects of wake interaction, many efforts havebeen made before. For instance, some scholars paid attention on
altering wind farm layout, example studies include (Park et al, 2015;

268