Aerodynamic Forces on a Wing Surfing in a Two-dimensional Vortex Wake*

“Surfing” on wakes, such as birds riding each other’s wake in flocks, is a well-observed natural phenomenon. The goal of this study is to understand the aerodynamic force variation on a wing surfing in an unsteady 2-D wake. Wind tunnel experiments were conducted using Particle Image Velocimetry (PIV) and force measurements with a fixed wing immersed in the wake of a pitching airfoil. Key results show that the force response of the surfing wing is aligned with the impingement of flow structures and can be effectively predicted using classic unsteady aerodynamics (Wagner, Kussner and Sears) based on measured unsteady local flow conditions (instantaneous angle of attack and speed). The theoretical predictions  compare well with direct force sensor measurements. The dependance of the overall lift fluctuations on the upstream flapping kinematics are scaled as a function of the reduced amplitude and reduced frequency of the flapping motion.  Good collapse of the data is found and deviations from scaling are explained in terms of the wake characteristics.