Flow in symmetric bifurcations

Most branchings in the cardiovascular system are asymmetric, the only major exception in man being the bifurcation where the aorta divides to form the iliac arteries. This is in contrast to the bifurcating airways of the lung, for which the assumption of symmetry is more appropriate, and which have been the subject of extensive research (Pedley, 1977). Furthermore, the precise definition of an asymmetric bifurcation requires the specification of several more parameters than that of a symmetric one (e.g. the ratios of the flow-rates in, and the diameters of, the two daughter tubes, as well as the different angles of branching). There is, therefore, considerably more fluid mechanical information available on the subject of symmetric bifurcations, and this chapter begins with a survey of it (taken largely from the review by Pedley, 1977). It should be said at the start, however, that the problem is still very complicated, and most of the data have been obtained experimentally not theoretically, with steady rather than unsteady flow. Clearly much work remains to be done.

In all the investigations described in this section and the next, the geometry of the bifurcations is taken to be fully three-dimensional. There has been relatively extensive theoretical and experimental work on two-dimensional bifurcations, but since that geometry rules out all secondary motions it is unlikely to have much relevance to the cardiovascular system.