1 Vortical flow structure identification and flow transport in arteries D J Doorly, S J Sherwin, P.T. Franke and J. Peiró Aeronautics Dept., Imperial College London 1. Introduction Our interest in vortices arises for two reasons. Firstly, at moderate to large Reynolds numbers (at least 100) which characterise flow in larger arteries, vortices are quite persistent. The presence of vortices in a flow may exert a strong influence on its behaviour, although tracking vortices may be difficult as they can evolve rapidly. The effects of vortices or vortical structures are particularly evident when considering both flow stability, and the processes of mixing and transport by the flow. The object of this paper is to examine the dynamics both of vortex motion and of particle transport in arteries, and to relate these to parameters such as geometry and unsteadiness. The Lagrangian particle tracking and the vortex dynamic techniques which are described should help in understanding arterial fluid dynamics and suggest new approaches to modelling. To begin, we could ask what does the word vortex mean? A vortex describes a circulating region of a flow, and for most people the term implies the fluid is rotating in some way. Familiar examples include the flow down a bath plug hole, tornados, smoke rings, the flow behind the tips of an aircraft wing in flight, and the whirlpool eddies in the wake of a bridge pier set in a rapid stream. It appears odd to admit that fluid dynamicists have difficulties in defining precisely what constitutes a vortex, and that when pressed many prefer to discuss vortical motions, or vortical structures rather than to talk of vortices. Despite this vagueness, one often finds that discussions of vortical motions are accompanied by rotating arm or hand gestures, which appear involuntarily. At a basic level we clearly associate the word vortex with the presence of rotation of elements of the flow. Computer Methods in Biomechanics and Biomechanical Engineering, 2002 Vol.5 (3) pp. 261-275