Figure 3.8: Laminar separation bubble (from [54]) Unfortunately there are few airfoils with available experimental data at low Re. The main source of data that we found is the "Low speed airfoil tests" carried out at University of Illinois at Urbana-Champaign (UIUC) for Reynolds numbers between 1 × 105 and 5 × 106 (see Summary of Low-Speed Airfoil Data - Vol. 1,2,3,4 and 5 [42, 43, 44, 29, 55]). Among the airfoils tested at UIUC, we chose to present 3 of them that seemed interesting and we compare them in this section. NREL S833, S834 and S835 The S833 airfoil family was developed by the American National Renewable Energy Laboratory (NREL) in 2001 [46]. It was specifically designed for 1-to-3 meter diameter horizontal wind turbines and was therefore optimized for lower Re than older NREL airfoil families. It consists of 3 airfoils of different thicknesses (see fig. 3.9). The root airfoil is thicker for structural reasons and the tip airfoil is thinner to be quieter. Only the S834 airfoil was tested by UIUC (see fig. 3.12) but we assumed the airfoils of the same family to have roughly the same lift and drag characteristics. See table 3.1 for other characteristics.
S833 (primary airfoil) S834 (tip airfoil) S835 (root airfoil) Figure 3.9: NREL S833 airfoil family
SG6040 and SG6043 The SG6040 and SG6043 airfoils were developed specifically for small variable-speed wind turbines by Michael Selig and Philippe Giguère in 1998 [17]. They are primarily intended for 1kW to 5kW wind turbines, which corresponds roughly to diameters of 1.5m to 3m (with CP = 0.3 and V0 = 10m/s). 43