![]() ![]() ![]() The first airfoils for small wind turbines were introduced by the National Renewable Energy Laboratory (NREL) the S822 and S823 airfoils were particularly designed for small stall-regulated wind turbines with a rotor diameter between 3 and 10 meters, based on the following criteria: restrained maximum lift, insensitivity to roughness and low profile drag. Currently, the majority of low Reynolds number airfoils are designed based on the latter technique, providing reduced amounts of drag and higher maximum lift-to-drag ratios (also termed as glide ratios), as compared with those of traditional airfoils that have been mainly designed for high Reynolds numbers and, therefore, they usually suffer from severe laminar separation effects when operating at low Reynolds number regimes. ![]() On the other hand, according to Giguere and Selig, the suppression of laminar separation effects could also be achieved by means of specially designed airfoils with a very gradual upper-surface pressure recovery (bubble ramp), which can decrease significantly the additional drag induced by separation bubbles. However, the particular technique, which is the only one applicable to existing airfoils, requires adequate experience in selecting the proper location and thickness of the trip, so as to maximize the reduction in bubble drag while minimizing the drag produced by the trip. One of the available methods to reduce or even eliminate bubble drag (that is drag induced by a laminar separation bubble)-as well as to delay the possible chances of separation at higher angles of attack-is related to the promotion of early transition on the upper surface (suction side) of the airfoil, through the installation of a mechanical turbulator or trip. Therefore, to optimize the aerodynamic performance of small wind turbine blades, operating at low Reynolds numbers, the effects related to laminar separation have to be minimized. However, such an unattached free shear layer (open separation area) may also be produced by the bursting of a laminar separation bubble. 2c, resulting in much higher drag ( \(D\)) levels and further reduction in the aerodynamic performance of the airfoil, compared to the supercritical flow regime. In that case, a thicker and extremely unstable wake region is produced, as shown in Fig. Now, in contrast to the supercritical regime, if the turbulent transition takes place far away from the surface of the airfoil, there is a possibility that the turbulent shear layer may not be able to reattach to the airfoil surface, creating an open separation area (subcritical flow) instead of a separation bubble. Besides, the potential premature burst of a laminar separation bubble could cause an even larger growth of the drag coefficient, which is accompanied by a sudden and severe loss of the generated lift. 2a, b this characteristic phenomenon of the supercritical flow regime is known as trailing edge stall. In addition to that, the presence of laminar separation bubble is also associated with a turbulent flow separation near the trailing edge of the airfoil, as exemplified in Fig. ![]() XFOIL analysis revealed that increasing relative thickness leads to the reduction of maximum lift-to-drag ratio \((C_\)) as well. The aerodynamic performance of RG15 family was initially evaluated by means of XFOIL code at several low Reynolds numbers ranging from 60,000 to 300,000 and angles of attack between − 6° and 14°. Six airfoils of varying relative thickness were designed by increasing the thickness distribution of RG15 airfoil up to 50% and adopting a rounded trailing edge with a diameter equal to 1% of the chord length. The present study introduces a low Reynolds number ( Re) airfoil family for the entire blade span of small wind turbines, aiming to reduce the effects related to laminar separation, improve startup response and meet acceptable levels of structural integrity. ![]()
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