THE FORMATION AND INTERNAL STRUCTURE OF COHERENT VORTICES IN THE WAKE OF A PITCHING AIRFOIL

Abstract An experimental study of the stretching and rolling of free shear layers released at the trailing edge of a pitching airfoil was conducted. Data were obtained in a water tunnel, in the wake of a pitching NACA 0012 airfoil, by laser-Doppler velocimetry. The free shear layer quickly stretches while it rolls up, forming a series of well-defined regions of distributed vorticity referred to here as coherent vortices or vortical structures. The structure of the evolving velocity field was captured by tight measuring grids. The velocity profiles associated with the coherent vortices exhibit what is often called a viscous core. It is shown that the vortical cores observed are not the result of viscous action at all. Vortex cores are thus created which are not “viscous cores” but rather “inviscid cores.” It is also demonstrated that the large-scale characteristics of the vortical field are invariant to the laminar or turbulent character of the feeding shear layer.