An experimental investigation of vortex breakdown in tubes at high reynolds numbers

Abstract

This thesis deals with non-cavitating swirling flows with vortex breakdown in various tubes. Phenomenological and quantitative investigations were carried out at Reynolds numbers as high as 300,000. It was shown that a high Re(D) vortex transitions to its new state (breaks down) via a rapidly spinning spiral form, as demonstrated with 4,000 frame per second video, short exposure time (6 ns) imaging, and Digital Particle Image Velocimetry. Of the known types, the spiral emerges as the fundamental breakdown form and the axisymmetric bubble may now be regarded as a relatively low Re(D) occurrence that is bypassed at sufficiently high Re(D). Some new phenomena were observed at high Re(D): Extremely rapid spiral rotation (over 1,000 revolutions per second), core bifurcation, and changes in the sense of the spiral windings. Familiar features of breakdowns, such as the transition from a jet-like to wake-like axial velocity profile and the rapidly expanding vortex core, were observed in extensive time averaged velocity and turbulence results ascertained with Laser Doppler Velocimetry. However, a mean stagnation point and recirculation were absent in the highest Re(D) flow. The core meandering and stagnation point darting in the turbulent flow field were quantified and discussed in detail