The dynamics of a cantilevered pipe aspirating fluid studied by experimental, numerical and analytical methods

Abstract

This paper investigates the dynamics of a slender, flexible, aspirating cantilevered pipe, ingesting fluid at its free end and conveying it towards its clamped end. The problem is interesting not only from a fundamental perspective, but also because applications exist, notably in ocean mining. First, the need for the present work is demonstrated through a review of previous research into the topic – spanning many years and yielding often contradictory results – most recently suggesting that the system loses stability by flutter at relatively low flow velocities. In the present paper, that conclusion is refined and expanded upon by exploring the problem in three ways: experimentally, numerically and analytically. First, air-flow experiments were conducted using different elastomer pipes and intake shapes, in which the flow velocity of the fluid was varied and the frequency and amplitude of oscillation of the pipe were measured. Second, a fully coupled Computational Fluid Dynamics (CFD) and Computational Structural Mechanics (CSM) model was developed in ANSYS™ in order to simulate experiments and corroborate experimental results. Finally, using a Newtonian analytical approach, a new linear equation of motion describing the system was derived, and then solved via the Galerkin method in order to determine its stability characteristics. Heavily influenced by the CFD analysis, the proposed analytical model is different from previous ones, most notably because of the inclusion of a two-part fluid depressurisation at the intake. In general, both the actual and numerical experiments suggest a first-mode loss of stability by flutter at flow velocities comparable to those for the discharging case, which agrees with the results from the new analytical model.