An Experimental Study of the Steadily Plunging Airfoil in Uniform-Shear Flow

Freestream shear may be found in many unsteady aerodynamic situations, such as the fighter jet landing through the air wake of an aircraft carrier and the micro air vehicle (MAV) navigating wind currents around buildings in urban environments. Despite the prevalence of shear in aeronautics, literature concerning its effects on unsteady airfoils is scarce. To address the need to understand the fundamental, complex aerodynamics of moving airfoils coupled with freestream shear, a novel experimental setup was implemented to investigate the case of the airfoil steadily plunging across a canonical uniform-shear approach flow in a water tunnel. The effect of unsteadiness on the NACA 0012 airfoil in shear is examined by using a servo motion system to plunge the airfoil from the high- to low-speed extremes of the shear zone and varying the steady plunge speed. The aerodynamic load (lift and drag coefficients), streamwise velocity component of the flow, separation and reattachment locations, and boundary layer thickness are characterized such that the flow measurements are correlated to the observed behavior of the load measurements. First, uniform flow measurements are performed that confirm the unique experimental setup reproduces the expected Galilean transformation between the stationary and steadily plunging airfoils. It is confirmed that minimal blockage, confinement, or other artifacts result from the airfoil traversing over a large fraction of the test section's width. Molecular tagging velocimetry is uniquely implemented such that tag lines are created over the entire airfoil surface, image pairs are formed with the entire airfoil in view, and flow measurements are enabled for the moving airfoil. The airfoil aerodynamics are characterized in uniform flow at the same Reynolds numbers of the shear flow at three primary cross-stream locations of interest to provide baselines for the measurements in shear. For Reynolds numbers 13,500 and 16,500, a multi-region behavior is observed in the slope of the lift coefficient curve where the observed rapid rise in lift is related to the flow switching from an open separation to a closed separation bubble. By contrast, a steady rise in lift is observed at Reynolds number 9,800 which correlates to only open separation being observed.Next, the basic effect of shear on the stationary airfoil is studied by placing the airfoil at the three primary cross-stream locations in the shear flow, which also provides baseline measurements for the plunging airfoil in shear. It is observed that the current study reproduces the negative lift at zero angle of attack that is opposite of inviscid theory but consistent with recent computational and experimental literature from our group. A common observation in the lift and drag coefficient curves for the stationary airfoil in shear is asymmetry, as exemplified by the different stall behavior between positive and negative angles of attack. A multi-region behavior is observed among the lift curves which is connected to the airfoil switching from open separation to a closed separation bubble, like for uniform flow. Except for the Reynolds number 13,500 case, there is no observed difference in the angle of attack at which the flow switches from open separation to a closed separation bubble in shear compared to uniform flow. For the highest shear, lowest Reynolds number case, only open separation is observed at positive angles of attack, like the corresponding results in uniform flow.Finally, the effect of the steadily plunging airfoil motion in shear is studied in comparison with its stationary airfoil counterpart. For the range of dimensionless shear rates (0.40-0.69) and chord Reynolds numbers (9,800-16,500) in this study, it is observed that the slope of the lift coefficient curve for the plunging airfoil begins to rapidly increase at lower effective angle of attack than for the stationary airfoil, which is found to be a result of the flow reattaching at a lower effective angle of attack for the former than for the latter. Near stall, the magnitude of the lift coefficient on the plunging airfoil is typically greater than that on the stationary airfoil, which is found to be related to the reattachment point occurring farther upstream for the former than for the latter. It is found that the airfoil must plunge as slowly as 1% of the freestream speed for the load on the plunging airfoil to be well-approximated by that on the stationary airfoil for the same effective angle of attack and freestream conditions.