An Investigation Into the Dynamics of a Pipe Aspirating Fluid

Abstract

Pipes aspirating fluid have applications in the filling and recovery processes for underground caverns — large subterranean cavities used to store hydrocarbons, such as natural gas and oil. This paper deals with the dynamics of a vertical cantilevered flexible pipe, immersed in fluid. Fluid is aspirated from its bottom free end up to the fixed upper end. In this study, the working fluid is assumed to be water. An existing analytical model is used to predict the dynamical behaviour of the aspirating pipe. This model is then discretized with Galerkin’s method, using Euler-Bernoulli eigen-functions for cantilevered beam as comparison functions. Once solved, the model results show a unique kind of flutter comprising three regions, denoted regions 01–03. These regions are delineated by two critical flow velocities, Ucf1 and Ucf2. In addition, two frequencies of oscillation, f1 and f2, are found to characterize the aforementioned flutter. The dominant frequency of oscillation changes from f1 to f2 as the flow velocity is increased from approximately 3 to 6 m/s — a frequency exchange phenomenon observed and reported here for the first time for this system. The analytical/numerical study was followed by a corresponding experimental study. Experiments were performed on a flexible (Silastic) pipe that was completely submerged in water. The behaviour observed experimentally was similar to the numerical study, as the aspirating fluid velocity was increased from zero to 7 m/s.