Effect of veer on a yawed wind turbine wake in neutral and stable atmospheric boundary layer*

Yaw control of wind turbines can improve wind farm efficiency by wake steering.  The spanwise forcing from yawing generates a counter-rotating vortex pair (CVP) behind the turbine. It induces a side-wash velocity that deflects and curls the wake. Simultaneously, wind veer occurs in atmospheric boundary layer due to Coriolis acceleration.  We study the effects of veer on the properties of the wake of a yawed turbine under conventionally neutral (CNBL) and stable (SBL) atmospheric conditions. Large Eddy Simulations of a yawed turbine under these different atmospheric conditions is performed. The lateral shear rate of veer is much stronger in SBL than in CNBL. The veer in CNBL results in a streamwise vorticity which is a linear superposition of its vorticity and the vorticity associated with the CVP. Upon removing the veer vorticity, the resultant CVP agrees well with an analytical model proposed in Shapiro et al. (JFM 903, R2, 2020) which does not include wind veer. However, the stronger veer in SBL distorts the velocity deficit and vorticity structures which significantly affects the downstream evolution of the wake. We discuss the development of analytically tractable models to include veer effects on wakes in stable and neutral conditions.