Aqueous Flow Limitation in a Tapered-Stiffness Collapsible Tube

ISSN： 0889-9746

Source： Journal of Fluids and Structures, Vol.14, Iss.8, 2000-11, pp. : 1195-1214

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Abstract

To find out whether the relation between flow-limited flow-rate and upstream transmural pressure was nonunique, as has been reported for air flow through a tapered-stiffness tube, and for comparison with a completed investigation of flow limitation in a uniform tube, flow limitation was observed in a tapered-stiffness tube. The tube was made by removal of material from the outside of a segment of the previous uniform tubing; thus the stiffness was on average less than that of the uniform tube. Therefore, quantitative differences in behaviour were expected, but in addition significant qualitative differences were found. Whereas in the uniform tube, large-amplitude oscillations were almost entirely confined to a transition from peak pre-collapse flow-rate to the largely pressure-drop-independent flow-limited flow-rate, the latter state here included operating points of all oscillatory types. The dramatic reduction in flow-rate at the transition was absent, and instead multiple (up to six) operating points occurred at a single value of upstream head and upstream transmural pressure. The plotting of control–space diagrams revealed a unique region of weak oscillations corresponding to the tube throat being located at an intermediate and time-varying point along the tube, with collapse as far as that point only. These oscillations were extremely variable in waveshape and frequency, often displayed intermittency, and depended sensitively on the precise operating point conditions. When in this mode, the tube upstream of the final collapse position exhibited small standing waves of area, so that up to four just-perceptible minima were seen.