WIND ENERGY Wind Energ. 2015; 18:339–349 Published online 14 January 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/we.1700 RESEARCH ARTICLE Turbulence effects on a full-scale 2.5 MW horizontal-axis wind turbine under neutrally stratified conditions Leonardo P. Chamorro 1,2 , S-J. Lee 1,3 , D. Olsen 1 , C. Milliren 1 , J. Marr 1 , R.E.A Arndt 1 and F. Sotiropoulos 1 1 Saint Anthony Falls Laboratory, Department of Civil Engineering, University of Minnesota, Minneapolis, Minnesota 55414, USA 2 Department of Mechanical Science and Engineering, University of Illinois, Urbana, Illinois 61801, USA 3 Research Institute of Marine Systems Engineering, Seoul National University, Seoul, South Korea ABSTRACT A field experiment was carried out to study the unsteady behavior of an instrumented full-scale 2.5 MW wind turbine under neutral conditions. The analysis focused on the structure of the instantaneous turbine power and strain at its foundation. A meteorological tower located 1.6 rotor diameters upstream of the turbine was used to characterize the turbulent flow. Mean velocity and temperature were steady during the 1 h period selected. The results suggest that the turbine power and foundation strain are modulated by atmospheric turbulence in a complex way. The spectral characteristics of both quan- tities exhibited three distinctive regions. Within the first region, defined by subrotor length scales, the turbine power was insensitive to the flow turbulence. In the intermediate region, with length scales up to those on the order of the atmospheric boundary layer thickness, the spectral contents of the power fluctuations ˆ P and flow ˆ U exhibit a non-linear relationship of the form ˆ P D G.f /ˆ U , where G.f / / ./f 2 is a transfer/damping function. In the third region, dominated by the very large scales of motions, the power fluctuations are found to be directly influenced by the flow. The strain also showed three regions, similar to the power fluctuations. However, it follows the structure of the inertial subrange of the turbulence at subrotor scales. Intermittent gusts were able to induce intermittent behavior on the turbine power. Finally, the flow and power correlation showed that the velocity at the hub height is the best descriptor of the flow turbulence within the rotor area. Copyright © 2014 John Wiley & Sons, Ltd. KEYWORDS atmospheric boundary layer; field experiment; power fluctuations; turbine loading; turbulence; wind energy; wind turbine Correspondence F. Sotiropoulos, Saint Anthony Falls Lab, University of Minnesota, Minnesota 55414, USA. E-mail: fotis@umn.edu Received 27 September 2012; Revised 11 August 2013; Accepted 19 November 2013 1. INTRODUCTION Power fluctuations and fatigue loads are among the most significant problems that wind turbines face through their lifetime. Atmospheric turbulence is the common driving mechanism that modulates the rich dynamics of these quantities. Because of this, the power and forces on the turbine exhibit fluctuations ranging from synoptic and diurnal to short time scales. The level of the power fluctuations determines the quality of the electricity produced by the turbine and has direct implications on the power grid and reliability of transmission systems. 1 The fatigue loads, in contrast, affect the various structural and mechanical components of the turbine. Moreover, with the monotonic increase of turbine rotor size observed in recent years and projected to continue for the near future blade loading has emerged as a critical element of the cost of energy. 2 Parametric load distribution models (e.g., Veers and Winterstein 3 ) and probabilistic methods (e.g., Moriarty et al. 4 ), among others, have been proposed to quantify the effect of turbulence-generated loads. Research efforts have also focused on smoothing the effect of the flow turbulence on the power generated by a turbine (e.g., Ran et al. and Luo et al. 5,6 ). Col- lective and, especially, individual pitch control strategies 7 have shown to be very effective in reducing turbine loads. 8 Copyright © 2014 John Wiley & Sons, Ltd. 339