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

An experimental study of the stretching and rolling of free shear laye rs 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 0 012 airfoil, by laser-Doppler velocimetry. The free shear layer quickl y stretches while it rolls up, forming a Series of well-defined region s of distributed vorticity referred to here as coherent vortices or vo rtical structures. The structure of the evolving velocity field was ca ptured by fight 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 visco us action at all. Vortex cores are thus created which are not ''viscou s 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. (C) 19 96 Academic Press Limited