Abstract
Aspects of the evolution of vorticity behind a thrust-producing flapping wing are discussed based on quantitative experiments. Multiple planes of stereoscopic particle image velocimetry measurements are conducted at several locations along the span of a NACA0030-type 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 wing since there is a interrelationship between the wing motion and the flow structure. The spanwise distribution of vorticity is found to be dominated, in the large scale, by a single pair of intense counterrotating vortices. Over one complete oscillation cycle, the characteristics of the flow in the plane of symmetry is similar to a reverse Kármán vortex street. Each member of the large scale vortex pair is constituted by two smaller co-rotating vortices that constructively merge in the initial stages of flow separation. Toward the wingtip the effect of wing three-dimensionality is more significant. The spatio-temporal variations of transverse and spanwise vorticity in these regions suggest severe local flow deformation. Measurements reveal that flow morphology is highly complex and three-dimensional unlike any previously observed, 2D-wing based vortex sheets.