Assessment of PIV-based unsteady load determination of an airfoil with actuated flap

Abstract

For complex experimental setups involving movable structures it is not trivial to directly measure unsteady loads. An alternative is to deduce unsteady loads indirectly from measured velocity fields using Noca's method. The ultimate aim is to use this method in future work to determine unsteady loads for fluid–structure interaction problems. The focus in this paper is first on the application and assessment of Noca's method for an airfoil with an oscillating trailing edge flap. To our best knowledge Noca's method has not been applied yet to airfoils with moving control surfaces or fluid–structure interaction problems. In addition, wind tunnel corrections for this type of unsteady flow problem are considered.The experiment is performed in a closed wind tunnel with a wing with a chord of 0.5m and a 0.2c trailing edge flap at a Reynolds number of Re=700000. The reduced frequencies of the flap are k=0.1 and k=0.2, whereas the mean flap deflections and amplitudes are 1° or 3°. Velocity fields are obtained with planar particle image velocimetry (PIV) and Noca's method is evaluated at multiple contours along the airfoil. The resulting unsteady loads are compared with loads obtained with Kutta–Joukowski's theorem applied to the experimental data and 2-D panel simulations with mimicked wind tunnel walls.Conclusion is that Noca's approach is relatively sensitive to the contour location and shows small offsets in the force coefficients. Using the experimental data, Noca's momentum flux equation applied to a set of contours gives a mean solution of the unsteady loads with an error bandwidth on average 6.39% of its mean value. The mean aerodynamic forces are slightly underpredicted, on average of about 5%. Among others, a higher resolution of the experimental data and more accurate approximations of velocity gradients will improve the force prediction. Phase and amplitude of the lift confirm 2-D panel computations including modeled wind tunnel walls and a gap correction.