A study of aeroelastic flutter and acoustic -structure interactions of a flexible disk rotating in an enclosed compressible fluid

Namcheol Kang, Purdue University

Abstract

The vibrations of thin flexible disks rotating in the surrounding air pose significant challenges in the design of high-speed rotating disk systems especially in hard disk drives and optical storage media. In most such applications, the disk vibrations couple to the acoustic oscillations of the surrounding enclosed fluid. This thesis explains theoretically and experimentally the vibrations and dynamic stability of disks rotating in enclosed compressible fluids. This system is modeled using a rotating Kirchhoff plate for the disk and using the wave equation for the surrounding compressible fluid. The model includes the effects of a radial clearance, symmetric positioning, bulk rotating fluid flow, and damping. The discretized coupled equations of the system are cast in the compact form of a gyroscopic system. For the stationary disk in a fluid filled enclosure, in-phase and out-of-phase acoustic modes coexist. Structure-acoustic eigenvalue veering phenomena also arise. For the rotating disk system, coupled structure-acoustic traveling waves destabilize through mode coalescence leading to flutter instability. A detailed investigation of the effects of dissipation arising from acoustic and disk damping predicts previously unknown instability mechanisms for this system. In the presence of a radial clearance, the coupled rotating disk system exhibits qualitatively the same instability mechanisms, however the mode coalescence instability occurs at slightly higher speeds. The asymmetric positioning of the rotating disk in the enclosure does not change the flutter mechanisms and modifies very slightly the flutter speed. Bulk rotating fluid flow splits the acoustic oscillations into forward and backward modes. The effects of rotating damping arising from fluid viscosity are also analyzed. Finally, experiments are performed on a high-speed SpinStand with a specially designed enclosure. Several disk materials are tested, and both disk and acoustic oscillations in the enclosure are monitored at several locations and over a wide range of speeds. Several observed phenomena correlate well to the theoretical predictions. Some experimentally observed phenomena are not predicted in the present theoretical model and their investigation is recommended as topics for future studies.

Degree

Ph.D.

Advisors

Raman, Purdue University.

Subject Area

Mechanical engineering

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