Swallow tail airfoil

1. A rotor blade for a wind turbine,

wherein the rotor blade is provided at least partly with an aerodynamic element comprising a cross section in an airflow direction with a suction side surface, a pressure side surface, and a trailing edge of a flat-back airfoil type extending between the suction side surface and the pressure side surface,

wherein the aerodynamic element further comprises an extension body attached to the trailing edge near the suction side surface of the aerodynamic element,

wherein the extension body has a top surface flush with the suction side surface and a lower surface that is remote from the pressure side surface, an exposed portion of the trailing edge extending from the lower surface of the extension body to the pressure side surface of the aerodynamic element,

wherein a first tangent is tangent to the top surface of the extension body and a second tangent is tangent to the pressure side surface at the endmost part of the trailing edge, the endmost part of the trailing edge being the endmost part of the rotor blade,

wherein the first and second tangents intersect at an intersection length downstream of the trailing edge,

wherein a length of the extension body extending from the trailing edge in the airflow direction is less than the intersection length, and

wherein the trailing edge has an end thickness T, and wherein the length l of the extension body in the airflow direction is less than the end thickness T.

2. The rotor blade of claim 1 , wherein the extension body in combination with the trailing edge forms a non-symmetrical downstream facing profile surface.

3. The rotor blade of claim 1 , wherein the length l of the extension body in the airflow direction fulfils the equation

1≤ T tan α,

wherein the thickness T of the trailing edge is perpendicular to the airflow direction, and a is an angle between a pressure side end of the trailing edge and an end rim of the extension body, the angle α being in the range between 0° and 90°.

4. The rotor blade of claim 1 , wherein the trailing edge is located at beyond 50% of a chord length of the aerodynamic element.

5. The rotor blade of claim 1 , wherein the extension body is movably attached to the trailing edge of the aerodynamic element.

6. The rotor blade of claim 1 , further comprising an actuator operatively connected to the extension body for controlling the angle of a suction side surface of the extension body with respect to the suction surface of the aerodynamic element.

7. The rotor blade of claim 1 , wherein the extension body extends over a limited length of the trailing edge of the aerodynamic element.