Effect of shear layer unsteadiness on the aerodynamics of a pitching airfoil

It is common practice to utilize a uniform approach flow in most of the problems in aerodynamics. However, in numerous circumstances the complex approach flows found in nature can be significantly non-uniform and even include spatially non-uniform temporal fluctuations. Motivated by these non-uniform unsteady flows, this experimental study investigates the effects of non-uniform approach flow unsteadiness on the aerodynamic performance of an airfoil.To isolate the effects of unsteadiness from those of the mean non-uniform approach flow, two non-uniform (shear) flows with a matching mean velocity profile are generated in a water tunnel facility. One of these profiles is made through a canonical two-stream shear layer, which is known to contain vortical structures and exhibit non-uniform velocity fluctuations. A matching shear velocity profile with uniform low level fluctuations is generated utilizing a modified shaped honeycomb shear generation method based on the original model proposed by Kotansky (1966). These shear flows demonstrate the same mean behavior and only differ in their fluctuation profiles.Behavior and development of both shear flows are examined through measurements of their streamwise velocity profile at multiple downstream locations utilizing single component molecular tagging velocimetry. The steady and unsteady shear flows are found to produce the same mean velocity profile with different classes of velocity fluctuations. The steady shear flow demonstrates a uniform low level of fluctuating velocity profile while the unsteady shear layer velocity fluctuations mimic the signature fluctuating velocity profile of plane mixing layers, with a high level of fluctuations in the center and a gradual decrease in fluctuations moving away from the shear layer center line.A NACA0012 airfoil is positioned at the center of each shear flow and the average aerodynamic forces on the stationary airfoil are directly measured across a wide range of angles of attack. The resulting lift and drag coefficient curves are compared for each of these shear flows as well as the reference uniform flow. The unsteady shear layer is found to generate a positive lift at zero angle of attack, in contrast to the negative lift observed under the same condition in the steady shear flow. Furthermore, in the presence of the unsteady shear flow, the linear region of the lift curve around zero angle of attack shows a larger slope and extends over a wider range of angles of attack compared to those of the steady shear and uniform flow lift curves. The unsteady shear flow results in a smaller magnitude of drag at small angles of attack compared to the other two flow conditions.Force measurements are also performed with the airfoil set to sinusoidally pitch around its quarter chord over a range of oscillation frequencies at the center of both shear flows and the reference uniform flow. The mean lift results show that at small oscillation frequencies the steady and unsteady shear flows produce opposite sign lift forces (negative lift in steady shear flow), but both result in positive lift coefficients of similar magnitudes at higher frequencies. The presence of shear and its unsteadiness seems to only weakly affect the mean and fluctuations of the streamwise force, with almost no effect observed on lift fluctuations.A closer look at the flow around the surface of the stationary airfoil reveals interesting differences between the behavior of the airfoil boundary layer in the presence of steady versus unsteady shear flow. Single component molecular tagging velocimetry is used to measure the streamwise velocity of the flow near the surface of the airfoil at multiple angles of attack in each shear flow. While the steady shear layer results confirm the presence of laminar separation on the suction side of the airfoil at the angles of attack investigated here, no sign of laminar separation or a reverse flow region is found when the airfoil is placed in the unsteady shear layer.The wake flow behind the pitching airfoil is also visualized through molecular tagging flow visualization to qualitatively examine how these shear flows affect the wake flow behavior. It is observed that both steady and unsteady shear flows result in the wake flow deflecting towards the high speed side of the flow at high oscillation frequencies, with more cycle-to-cycle variation and perturbations observed in the presence of unsteady shear flow.