Experimental evaluation of a non-conventional flat back thick airfoil concept for large offshore wind turbines

Abstract

In modern large wind turbine blades thick flat back airfoils are often used in the root part of the blades due to their structural advantages. Although in the root structural properties are more important than aerodynamic performance, thick flat back airfoils do have higher drag which limits their applicability. Numerical simulations demonstrated that a previously introduced non-conventional flat back airfoil concept, called “swallow tail”, has potential showing lower drag for similar or slightly improved lift values. In the present study the swallow tail concept is evaluated experimentally in the TU Delft low-speed wind tunnel. An existing wind tunnel model of airfoil DU97-W-300 was modified to obtain a flat back airfoil with a 10% trailing edge thickness. The model was tested in the wind tunnel with and without the swallow tail for various conditions. The results of the experiment showed that the swallow tail reduced the drag up to 40% at various angles of attack. Moreover, the maximum glide ratio increased up to 45% while both the stall and post stall behavior remained almost identical. The results suggest that the swallow tail can help to increase power output of the wind turbine without modifying the turbine’s operating conditions. Due to the lower drag thick flat back airfoil sections could possibly be located at more inboard parts of the blades.