Dynamics of a confined pipe aspirating fluid concurrently subjected to external axial flow: Experiments on confinement length effects

The system under consideration comprises a flexible pipe and a coaxial rigid outer tube; the pipe and tube are vertically cantilevered from their top ends in a large fluid-filled cylindrical tank. The space between the pipe and tube forms an annulus around the upper portion of the pipe. The working fluid is water. The fluid enters the annulus from its top end with flow velocity Uo and is discharged from its bottom end; the fluid exits from the tank by flowing upwards in the pipe with velocity Ui.This flow configuration represents an idealized model of one of the modi operandi of 'salt mined caverns', which are large underground cavities created to store hydrocarbons, such as natural gas and oil, in large quantities. At sufficiently high flow velocities the central pipe, referred to as the 'brine-string', vibrates, and may impact on the rigid cemented casing around it; sometimes this impact results in damage or breakage of the brine-string.Numerous experiments were performed in a laboratory-scale apparatus for several flow velocity ratios Uo/Ui, ranging from 0.02 to 0.40 (approx.), for three different outer tubes, 300, 200 and 100 mm in length; an elastomer flexible pipe approximately 441 mm long, was used in this study throughout. In all these experiments, the pipe loses stability via first-mode flutter at a sufficiently high flow velocity Ui, and at relatively lower Ui with increasing flow velocity ratio Uo/Ui and outer tube length L '. For the 300 mm outer tube, a dynamical behaviour similar to that for 200 mm outer tube, comprising two different dynamical features referred to as low and high flow velocity ratio behaviours, was observed. Also, a limit cycle was observed in the experiments for the first time, for the system involving a pipe aspirating fluid. For the 100 mm outer tube, however, only the high flow velocity ratio dynamical behaviour was observed. Finally, the effect of varying the outer tube length L ' on the foregoing dynamics is presented by comparing the experimental results for all three outer tube lengths, L ' = 100, 200 and 300 mm.