Airfoils of wind energy conversion systems (WECS) experience complex flow fields as they rotate through the earth's atmospheric boundary layer. Within these flow fields, unsteady boundary layers develop on the airfoil surfaces. Growth of such a viscous layer, its transition to a turbulent state, and its separation from the airfoil surface, dictate the instantaneous loads (surface shear stress and pressure distributions) which act on the rotor surface. Aerodynamic performance, structural response and fatigue-lifetime considerations all follow from the fluid physics. A diagnostic technique capable of "visualizing" the instantaneous surface shear stress pattern in dynamic flow fields, in a continuous and reversible manner, would thus prove to be a valuable research tool. The potential of liquid crystals to meet this objective has been under investigation at Sandia National Laboratories. A description of the technique and results obtained to date, from both the laboratory and on an operating vertical axis wind turbine, are given in References [1, 2]. In the present case, a cooperative research effort was jointly conducted by Sandia National Laboratories and the Solar Energy Research Institute to investigate the feasibility of applying the liquid crystal technique to horizontal axis wind turbines operating in field environments. The turbine, the airfoil, and the aerodynamic measurements obtained to date are reported at this conference in Reference [3].... In brief, these liquid-crystal tests were conducted on a three-bladed, downwind, variable-pitch, zero-yaw machine which incorporated the SERI-S809 constant-chord/zero-twist airfoil. Initial experiments were run under light wind conditions, hence the machine was motor driven and the airfoil angle of attack was essentially equal to the airfoil pitch setting. Both thermochromic (shear/temperature dependent) and shear-sensitive-only liquid crystal mixtures were employed. Liquid crystal response was recorded by a boom-mounted (downwind) video camera. Test conditions and test results are summarized in a 10-minute color video, obtainable by request from the authors. In summary, the technical feasibility and viability of the liquid-crystal technique in WECS field environments was further demonstrated. Results for the SERI-S809 airfoil showed the existence of an adverse-pressure-gradient- induced (contour-generated) laminar separation bubble near the airfoil midchord at low angles of attack. For angles of attack near 10°, this bubble occurred at the airfoil leading edge. In both cases, transition to turbulence occurred in the shear layer above the local reverse-flow region, resulting in a high-shear-stress turbulent reattachment zone immediately downstream of the bubble. At low angle of attack, the turbulent boundary layer remained attached to the airfoil surface to the trailing edge. However, at 10° angle of attack, turbulent boundary layer separation occurred at the 60 to 70% chord location.
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