The Effect of Planform Shapes on the Evolution of Bound Circulation of a Finite Wing Near an Interface

We investigate the evolution of the bound circulation of a finite wing near a water-air interface. An NACA 0015 airfoil with a chord of 10 cm and a span of 40 cm is used. The main objective of this research is to determine the effect of planform shapes on this evolution of the aerodynamic forces and the flow field as a wing crosses the water-air interface. The airfoil is initially submerged under water and then towed vertically at constant speed, starting from rest. During its travel, the wing crosses the water-air interface and continues its motion at constant speed before coming to stop. The forces and moments on the airfoil are measured with the help of a force-moment sensor. In addition to the planform shape of the airfoil, we also vary the angle of attack (AoA), the Reynolds number (Re), the height above and below the water level. In our investigation, we attempt to validate our setup with the published results of Weisler et al. [1]. We determine the deformation of the water-air iterface for a wide parameter space containing the angle of attack, the Reynolds number and the planform shape. We show that it is possible to significantly change the circulation variation by altering the planform. The circulation of thw wing crossing the interface depends on the degree of vorticity damping inside water. The results provide us an idea why natural flyers or swimmers fold their wing or fins while coming out of water. By varying the planform over a wider parameter space, we show why nature prefers to morph the wing at a water-air interface. We also perform planar Particle Image Velocimetry (PIV) to measure the velocity field around the wing of various shapes as it cross the interface.