ON THE INSTABILITY MECHANISMS OF A DISK ROTATING CLOSE TO A RIGID SURFACE

The instability mechanisms of a rotating disk, coupled to a rigid surf ace through a viscous fluid film at the interface are investigated ana lytically. The fluid in the film is driven circumferentially by the vi scous shear, and it flows outwards radially under centrifugal forces. The circumferential flow component creates an equivalent viscous dampi ng rotating at one half the disk rotation speed. This film damping dis sipates all backward traveling waves where the undamped wave speeds ar e greater than one half the disk rotation speed. The radial flow compo nent creates a nonsymmetric stiffness in the disk-film system that ene rgizes any wave mode at rotation speeds above its flutter speed. Insta bilities in the disk-film system are of two types. A rotating damping instability is caused by the rotating film damping at rotation speeds above a critical value that is less than the flutter speed. A combinat ion instability is caused by the combined effect of the film stiffness and damping at rotation speeds above a threshold that is greater than the flutter speed. The maximum rotation speed of stable disk vibratio n is bounded above by the lowest onset speed of rotating damping insta bility. This speed limit is predicted for two wall enclosure designs. The maximum stable rotation speed of a 5.25-inch diameter flexible, me mory disk separated from a rigid surface by a viscous air film, is sho wn to be more than 15 times greater than the maximum speed of the disk without the air film.