USING LES TO UNDERSTAND WAKE EVOLUTION DURING THE DIURNAL CYCLE Jordan Nielson Graduate student, Department of Mechanical Engineering, University of Texas, San Antonio, TX, USA, 78248 Email: mmr315@my.utsa.edu, Kiran Bhaganagar Associate Professor, Department of Mechanical Engineering, 1 UTSA Boulevard, San Antonio, Texas, 78248 Email: Kiran.bhaganagar@utsa.edu, Telephone: 210- 458-6496 ABSTRACT Numerical study using actuator-line method based Large Eddy Simulation (LES) has been performed to understand the role of atmospheric stability on the wake effects of horizontal-axis full-scale 5-MW wind turbine (WT). The paper will specifically focus on using specific instances in the diurnal cycle corresponding to stable, neutral and unstable ABL state to gain understanding on the transient aerodynamics of a wind turbine throughout the diurnal cycle. Capturing accurate Atmospheric Boundary Layer (ABL) characteristics is key factor in improving the accuracy of WT model predictions as turbulence developed in the ABL has potential to adversely affect the fatigue lifetime and performance of wind turbines. ABL simulations for the diurnal cycle are performed to isolate the key ABL metrics such as surface momentum flux, boundary layer height, surface temperature flux, wind shear, and temperature gradient that influence the wake evolution of WT. Precursor ABL inflow is generated for the WT simulations. The positive heat flux on the surface causes high vertical velocity fluctuations described with streaks and updraft motions during the day while surface cooling rates result in increased shear and strong temperature gradients during the night. The surface temperature, geostrophic wind velocity, heating/cooling rates, and period of the diurnal cycle are varied in different simulations to compare turbulent statistics and the helical vortices of the wind turbine wake. The results have revealed surface temperature and surface flux are the important ABL metrics that have a strong effect on altering the turbulence in the WT wake. In addition, instabilities related to WT blade rotation exhibit sensitivity to ABL metrics. The positive heat flux shows higher mixing and causes large wake movement in the day-time conditions. The results aid in quantifying the movement of the wake at different times of the diurnal cycle. During night-time conditions mixing is low, causing slower wake recovery times. This is the first study to clearly isolate the key ABL metrics that influence the full-scale WT near- wake effects The study has implications in improving the predictions of WT power loss due to wake deficits. Further, this study sets an important direction on future modeling studies in identifying the ABL conditions in a diurnal cycle that influence the WT wake evolution. INTRODUCTION Full-scale WT operate in atmospheric boundary layers, which is a part of the atmosphere that interacts with the earth. The State of Atmospheric boundary layer (ABL) significantly influence the wake generated by the WT, in particular, the differences in the turbulent kinetic energy (TKE), competing roles of TKE production mechanisms, namely, wind shear vs. thermal buoyancy production, wind shear and temperature gradients. . However, to-date, most of the numerical studies are based on idealistic flow conditions and do not account for ABL effects. Further, to simulate all the instances of a diurnal cycle is computationally intensive. What is urgently needed is to understand the key physics of the ABL and the differences that arise due to atmospheric stability. For this purpose, we need to identify the metrics of the ABL state during different stability conditions, namely, stable, unstable or convective and neutral state. Recent work [1, 2] has clearly suggested response of wind turbine wake evolution is Proceedings of the ASME 2015 International Mechanical Engineering Congress and Exposition IMECE2015 November 13-19, 2015, Houston, Texas IMECE2015-52045 1 Copyright © 2015 by ASME