Flow Conditions at the Inlet of Aspirating Pipes: Part 2—Experiments

Abstract

Following the theoretical work and experimental strategy devised by Axisa [1] in the companion paper, a test rig was designed and built in order to validate the analytical analysis of Part 1. Two configurations of partly immersed articulated pipes were tested, both for normal (discharging) and for reversed (aspirating) flows. The water-loop enabled velocities up to 3 m/s in both normal and reversed flows. The experimental results presented pertain to the following pipe configurations: (a) one articulated pipe, with either a common protruding or a rounded baffled free end; and (b) two articulated pipes with equal lengths. For all flow velocities modal identifications were performed from the measured system responses. The results obtained under normal discharging flow are in good agreement with the theoretical model originally developed by Benjamin [2], which is also reviewed in Part 1. For the single articulated pipe, the Coriolis force term leads to a steady increase of damping with flow velocity, modal frequency being significantly affected only near critical damping, as expected. For pipes with two articulations, both the Coriolis and centrifugal flow terms are significant, leading to large changes in both modal frequencies and damping, which agree with the predictions from the classical model. The most interesting results from our experiments obviously are concerned with aspirating flows. Following the discussion of Part 1, it was found that the one-pipe configuration is nearly insensitive to aspirating flows, irrespectively of the pipe termination geometry, showing that the Coriolis force term is canceled exactly by the term arising from the change in momentum of the flow entering the pipe at the free end. The experimental results from the two-pipe configuration are sensitive to the aspirating flow velocity. Among the various inflow models explored in Part 1, the one which assumes an inflow velocity directed along the tube axis, but without the tangential component of the pipe motion, proved to capture many of the features displayed by the experimental results. Actually, as the aspirating velocity increases, both identified modal frequencies of the two-pipe system, as well as the modal damping of the first mode, closely follow the theoretical predictions from this basic inflow model. However, a discrepancy was observed, concerning the modal damping trend of the second mode, which decreases slowly but steadily in our tests as the velocity increases, while the basic inlet flow model predicts a nearly constant damping value. Nevertheless, such subtle but significant behavior of system damping can be related to small variations of the basic parameters which describe the inlet flow field.