Vortical flow. Part 1. Flow through a constant-diameter pipe

This paper describes an exploration of the behaviour and properties of swirling flow through a constant-diameter pipe. The experiments reveal a complicated transition process as the swirl intensity Omega is increased at fixed pipe Reynolds number Re approximate to 4900. For Omega less than or equal to 1.09, the vortex was steady, laminar, axisymmetric, and developed slowly with streamwise distance. The upstream velocity profiles were similar to those commonly appearing in the literature in similar apparatus. Spiral vortex breakdown appeared in the test section for 1.09 less than or equal to Omega less than or equal to 1.31 and was associated with a localized transition from jet-like to wake-like mean axial velocity profiles. Further increase in Omega caused the breakdown to move upstream of the test section. Downstream, the core of the post-breakdown flow was unsteady and recovered toward jet-like profiles with streamwise distance. At Omega = 2.68, a global transition occurred in which the mean axial velocity profiles suddenly developed an annular axial velocity deficit. At the same time, disturbances began to appear in the outer flow. Further increase in Omega eventually led to an annulus of reversed axial flow and a completely unsteady vortex.