While a number of studies have been conducted to investigate wake interference among multiple wind turbines for the optimal layout design of wind turbines in a wind farm, most previous models involved homogenous straight-line winds over simplified flat surfaces. Most onshore wind turbines are sited in wind farms with complex terrains, which are not flat. A more realistic model would account for multiple wind turbines sited in an onshore wind farm with complex terrains such as hills, valleys, ridges and escarpments. In addition, the effects of the significant variations in flow characteristics (spatial and temporal) of the surface winds over complex terrains on the power productivity, fatigue loads, and wake interference among wind turbines need to be taken into account. In this project, the PIs will perform a fundamental study to quantify the mean and turbulence characteristics of the surface winds over typical complex terrains seen in onshore wind farms and to investigate the wake interference among multiple wind turbines sited over the complex terrains. In this project the large-scale Aerodynamic/Atmospheric Boundary Layer (AABL) Wind and Wind Tunnel at Iowa State University will be used to quantify the performance of an array of wind turbines sited over a flat surface (baseline case) and complex terrains with non-homogenous surface winds. In addition to measuring dynamic wind loads and the power outputs of the wind turbines, advanced flow diagnostic techniques such as Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) will be used to determine detailed flow field characteristics such as the mean and turbulence characteristics of the surface winds and wake interference in complex terrains. The detailed flow field measurements will be correlated with the dynamic wind loads and power output measurements. The results of this project will lead to a better understanding of underlying physics and the characteristics of surface wind energy resources over complex terrains. In terms of the broader impacts, the research program will be incorporated into the Iowa State?s curricula, both undergraduate and graduate, by adding wind turbine aerodynamics experiment modules to the existing aerodynamics laboratory courses. The findings derived from the project will be disseminated broadly to contribute to the knowledge base of wind energy and wind turbine technology. Existing programs at Iowa State will be leveraged to recruit more female and minority students. "Renewable Energy and Wind Turbine Technology" seminars and demonstration experiments will be developed for K-12 teachers and students. The research, education, and outreach activities are expected to make a significant contribution to the land-grant mission of Iowa State University. The findings derived from this project will be used to develop more realistic models to predict multiple wake interactions over complex terrains and to optimize paradigms for the optimal site design of wind turbine arrays with higher power yield, better integrity, and longer durability of the turbines.