Dynamics of Rotors Immersed in Eccentric Annular Flow. Part 2: Experiments

Author： Grunenwald T. Axisa F. Bennett G. Antunes J.

ISSN： 0889-9746

Source： Journal of Fluids and Structures, Vol.10, Iss.8, 1996-11, pp. : 919-944

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Abstract

In order to validate the theoretical approach described in Part 1, an extensive experimental study has been performed at Instituto Tecnologico e Nuclear (I.T.N.), using a test rig presenting some improvements over previous experiments by the authors. In agreement with the theoretical model, concentric configurations always display flutter instability of the forward mode only, at velocities much higher than the critical unbalance-excited rotor velocity. Also predicted by theory, an instability of the backward mode may occur at much lower spinning velocities, for some eccentric configurations-which is a conclusion of practical significance. Therefore, rotor-annulus eccentricity is a very important parameter, when stability of the system is addressed. The quantitative agreement of both modal frequencies and damping values, with respect to the rotor spinning velocity, is quite satisfactory at lower velocities. However, it deteriorates somewhat at higher velocities, even for concentric configurations. Discrepancies are discussed, in connection with experimental difficulties (some unavoidable three-dimensional flow effects in the test rig) or other possibly pertinent phenomena (dynamic flow nonlinearities)-which were not accounted for in the theoretical model. However, these problems should not be overstressed, as theoretical instability boundaries are usually fairly close to the experimental results.