Published online by Cambridge University Press: 04 July 2016
The note by W. T. F. Lau upon the effective “length” of two-dimensional contracting ducts has raised further points of interest.
Lau's paper presents three different criteria which can control a contraction length. Of these three, the second and third, that respectively are based upon the approach of the centre-line velocity to its asymptotic value and upon the approach of the flow to uniformity, were compared in ref. 1. The first criterion that is based upon the approach of the boundary slope to zero is not in general identical with the second.