Thrust measurements from a finite-span flapping wing

The thrust per unit length behind a flapping NACA0030 airfoil with an aspect ratio of three is measured and presented. Aspects of the evolution of vorticity behind the thrust-producing wing are discussed based on quantitative experiments. Multiple planes of stereoscopic particle image elocimetry measurements are conducted at several locations along the span of the wing at a Strouhal number of 0.35. Of particular interest is the effect of wingtip vortices on the structure of the flow behind the oscillating wing. Wing kinematics is responsible for the flow structure in the 2-D airfoil case. Here, the spanwise distribution of vorticity is found to be dominated, in the large scale, by a single pair of intense counter-rotating vortices. Each member of the large-scale vortex pair is constituted by two smaller corotating vortices that constructively merge in the initial stages of flow separation. Toward the wingtips, three-dimensional effects are more significant. The spatiotemporal variations of transverse and spanwise vorticity in these regions suggest severe local How deformation. Measurements reveal that flow morphology is highly complex and three-dimensional, unlike any previously observed 2-D wing-based vortex sheets. Furthermore, using 2-D particle image velocimetry data, a sinusoidal variation in thrust force, 90 deg out of phase with the airfoil motion, is measured in the midspan region of the airfoil. The largest measured thrust occurs at the maximum angles of attack, corresponding to the creation of strong leading-edge vortices.