Transitory and Periodic Flow in a Self-Oscillating Collapsible Tube: Experimental Study

Abstract

The flow through a collapsible thin-walled tube established by gravity is experimentally studied by performing two-dimensional particle image velocimetry (2D PIV) velocity measurements. Two series of experiments are performed during which the flow rate and the inlet tube pressure are kept constant but the outlet pressure is varied. In the first experiment, the velocity field is examined in a plane perpendicular to the plane of tube collapse, when the tube is in a state of weak nonperiodic tube oscillations, focusing on its two-lobe-shaped diffuser formed downstream of the tube midlength. It is verified that when the tube diffuser angle exceeds 20° from a lowering of the transmural pressure, the flow separates alternatively from the opposite walls of the tube constituting a probable triggering mechanism for the initiation of tube periodic oscillations. In the second series of experiments, the velocity field is measured in the plane of the tube collapse when the tube is in a state of periodic self-oscillation. Using the tube outlet pressure as triggering signal, phase-locked PIV measurements are performed revealing the flow evolution within a period. The two-lobed shape of the tube neck cross section oscillating streamwise for a distance of 10% of the unsupported length of the tube is verified through velocity profiles, which show two maxima off axis. Between these two maxima, which are four times the inlet bulk velocity, the flow decelerates when the tube outlet pressure is reduced in each half period, while at the same time the opposite walls of the tube approach each other. In the second half of the period during which the tube expands, negative flow appears near the tube outlet, while the velocity and the width of the two jets increase. The amplitude of the central axial velocity fluctuations is enhanced streamwise, reaching a fivefold increase compared to the inlet, especially close to the tube exit, where the tube deformation is significant.