Experiments on the trajectory and circulation of the starting vortex

Abstract

Various reasons have been given as to why the trajectories and circulations of vortices generated at sharp edges do not follow classical similarity-theory predictions for at least an initial short time. Amongst these are the effect of the particular flow geometry (e.g. duct with wedge, nozzle) distant from the salient edge (for rectilinear vortices); axisymmetry (for ring vortices); end effects (for rectilinear vortices); viscous diffusion; finite thickness of the detaching shear layer, as well as secondary vorticity caused by the interaction of the primary vortex with the edge at which it was generated. A further process that may be active is that of viscous entrainment. Experiments, in which essentially straight-line vortices were generated, indicate that of the seven possibilities mentioned, the first five do not play a significant part. All models consist of a basic flow onto which the modelled vortex is superposed. Thus either the basic flow or the vortex model are at fault. The basic flow onto which the vortex is superposed may well not be a pure edge flow, but one that is already taking on the character of an entraining jet flow. On the other hand, the vortex model fails to incorporate secondary vorticity which, particularly when rolled up, might be expected to be dynamically important.