Boundary Layer State and Flow Field Structure Underlying Rotational Augmentation of Blade Aerodynamic Response

Abstract

Blade rotation routinely and significantly augments aerodynamic forces during zero yaw horizontal axis wind turbine (HAWT) operation. To better understand the flow physics underlying this phenomenon, time dependent blade surface pressure data were acquired from the National Renewable Energy Laboratory (NREL). Unsteady Aerodynamics Experiment (UAE), a full-scale HAWT tested in the NASA Ames 80-by-120-foot wind tunnel. Time records of surface pressures and normal force were processed to obtain means and standard deviations. Surface pressure means and standard deviations were analyzed to identify boundary layer separation and shear layer impingement locations. Separation and impingement kinematics were then correlated with normal force behavior. Results showed that rotational augmentation was linked to specific separation and impingement behaviors, and to associated three-dimensionality in surface pressure distributions.