Flow Conditions at the Inlet of Aspirating Pipes: Part 1—Theory

Abstract

Drastic changes occur in the dynamics of pipes conveying fluid when passing from the discharging flow case to the aspirating flow case. This can be attributed to modifications in the flow conditions which prevail at the aspirating tube inlet versus those prevailing at a discharging tube outlet. In the present paper the point is investigated in relation with the behavior of two elementary systems, namely a single degree of freedom system (1-DOF oscillator) made of a rigid pipe supported at one end by a rotational spring and a two degrees of freedom system (2-DOF coupled oscillators) made of a pair of articulated tubes. In the first system, flow-structure coupling reduces essentially to a Coriolis force while in the second, it arises via both centrifugal and Coriolis forces. The present paper is devoted to theoretical modeling and prediction of the dynamical behavior of these systems according to a few basic assumptions concerning the flow conditions at the aspirating inlet. It serves as an introduction to the companion paper by Debut et al. [1] where theoretical predictions are confronted to experiment. As a major result it is found that even “small changes” in the flow conditions at the aspirating inlet can lead to important changes in the tube dynamics which are amenable to measurement, even if the experiments are restricted to relatively low flow velocities because of limited capacity of the test loop.