Blade surface pressure and drag measurement of a blade section on a 4.3 MW turbine with trailing-edge flaps

Abstract. In this paper we present the measurements of local aerodynamic sectional characteristics on a full-scale rotor blade with a novel add-on instrumentation comprising a wake rake, a pressure belt, and a five-hole Pitot tube. The general objective of the research work is to provide information on the differences between airfoil performance in wind tunnel flow and on a full-scale rotor. Although pressure belt measurements have been performed in earlier studies, this is the first campaign to use a wake rake at the full-scale level. We present the development and testing of the wake rake in the wind tunnel and on a rotating test rig, which ultimately led to its installation on the 4.3 MW turbine. A more specific objective of the campaign was to characterize the aerodynamic performance of a trailing-edge active flap system installed on one of the blades. The short measurement campaign of 2 d comprised measurements of the flaps actuated at constant time intervals of 60 s between fully retracted and activated positions with a control set point of 75 % of full deflection. The lift and drag characteristics are compared with a similar flap actuation in a wind tunnel experiment. The relative changes in both lift and drag coefficients as a function of flap actuation correlate well with wind tunnel measurements, but the absolute drag levels measured on the rotor are higher than the wind tunnel data. During the measurement campaign, it was also demonstrated that it is possible to clearly measure the increased drag from adding a roughness tape at the leading edge of the airfoil. This illustrates the potential of the measurement system to capture the effect of variations in local aerodynamic performance at full scale, even for elements at the boundary layer scale, e.g., the impact of roughness or the positioning of add-ons such as vortex generators.