The development of special-purpose airfoils for horizontal-axis wind turbines (HA WT�) began in 1984 as a joint effort between the National Renewable Energy Laboratory (NREL), formerly the Solar Energy Research Institute (SERI), and Airfoils, Incorporated. Since that time seven airfoil families have been designed for various size rotors using the Eppler Airfoil Design and Analysis Code. A general performance requirement of the new airfoil families is that they exhibit a maximum 1ift coefficient ('1,rnaJ which is relatively insensitive to roughness effects. The airfoil families address the needs of stall-regulated, variable-pitch, and variable-rpm wind turbines. For stall-regulated rotors, bett er peak-power control is achieved through the design of tip airfoils that restrain the maximum 1ift coefficient. Restrained maximum 1ift coefficient all ows the use of more swept disc area for a given generator size. Also, for stall-regulated rotors, tip airfoils with high thickness are used to accommodate overspeed control devices. For variable-pitch and variable-rpm rotors, tip airfoils having a high maximum lift coefficient lend themselves to lightweight blades with low solidity. Tip airfoils having low thickness result in less drag for blades having full-span pitch control Annual energy improvements from the NREL airfoil families are projected to be 23% to 35% for stall regulated turbines, 8% to 20% for variable-pitch turbines, and 8% to 10% for variable-rpm turbines. The improvement for stall-regulated turbines has been verified in field tests.
The objective of this study was to evaluate measured NASA Ames Unsteady Aerodynamic Experiment post-stall blade element data and to provide guidelines for developing an empirical approach that predicts post-stall aerofoil characteristics. Blade element data were analysed from the five radial stations of the baseline 5·03 m radius rotor. A lifting surface/prescribed wake performance prediction method was used to determine a reference angle of attack that corresponds to the measured blade element data. Using the measured normal and tangential force coefficients and estimated angle of attack, spanwise distributions of lift and drag performance characteristics were derived along with the circulation distributions. Guidelines for a new stall and post-stall model based on the measured trends in the aerofoil performance characteristics, along with flat plate theory, are proposed for predicting the peak and post-peak power.
The objective of this study was to provide post-stall airfoil data input guidelines for the prediction of peak and post-peak rotor power when using blade-element momentum theory. A steady-state data set from the Unsteady Aerodynamic Experiment (UAE) rotor test was used to provide guidelines for the development of a global post-stall method for the prediction of post-stall 3-D airfoil characteristics to be used with 2-D airfoil data. Based on these UAE data, methods to emulate the 3-D aerodynamics in the post-stall region were explored. Also suggested are experimental tests needed to better understand the 3-D flow physics and to quantify needed theory or empirical factors for a global post-stall approach to support blade-element momentum methods.
ICocurek TanglerThe application of lifting surface theory to the caiculation of rotor hover performance, and the development of an improved prescribed wake representation resulting from schlieren flow visualization studies of model rotor wakes are described. Qualitative comparisons between lifting line and lifting surface methods show the tendency of the lifting line to react excessively when a vortex passes close to a blade. The flow visualization studies reveal wake sensitivity to thrust coefficient, number of blades, and twist not identified in previous Investigations. The need to include a recirculation mechanism in the analytical model t o provide inflow in addition to that available from the prescribed wake structure is established. A possible source of the recirculation is demonstrated to be the result of vortex interaction and wake expansion immediately below the welldefined near wake. NOTATIONAR = Blade aspect ratio, R/c b = Number of blades c = Blade chord CI = Section lift coefficient CQ = Rotor torque coefficient, torque/pnR3-( a m 2 CT = Rotor thrust coefficient, t h r~s t / p n R ' ( n R )~ D = Aerodynamic influence coefficient k,,k2 =Axial slope of tip vortex trajectory before and after passage of the following blade n = surface panel unit normal vector r = Radial distance from axis of rotation Presented at the 32nd Annual National Forum of the American Helicopter Society, May 1976. r = Collocation point position vector R = Rotor blade tip radius V = Free stream velocity x,y,z = Rectangular coordinates zt = Axial coordinate of wake relative to tip path plane, positive up a, = Blade section effective angle of attack r = Tip vortex circulation strength 8 , = Blade linear twist, washout negative 8 ,, = Blade collective pitch a t 0.75R K = Strength of circulation distribution A =Wake contraction rate parameter p = Air density o = Rotor solidity, bc/nR I) b = Azimuth angle between blades $ w = Wake azimuth angle relative t o blade = Rotor rotational speed Subscripts i = Blade collocation point index j = Vortex box index te = Trailing edge
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