Theoretical Study on Aerodynamic Damping of Disk Flutter Caused by Gap between Disk and Shroud

Abstract

That a narrow gap between a shroud and a disk reduces disk flutter amplitude is a well known and very important fact for the industry of hard disk drives. This effect has been explained by a reduction in airflow excitation because of blockage of the vertical flow from one side of the disk to the other side. However, we found a quite different phenomenon in our experiments. In the experiments, the frequency spectra of disk flutter were measured for various disk-shroud gaps. The frequency spectrum for a narrow gap showed more rounded peaks than for a wide gap, and the amplitudes of the flat area between the peaks were approximately same in the both cases. That is, the damping ratios of the disk flutter were increased by the narrow disk-shroud gap, but the mode excitations were not reduced. We derived theoretical equations of aerodynamic damping from the energy dissipation of flow induced by disk flutter at a disk-shroud gap. With reasonable parameters, these equations explained the experimental results very well, and the mechanism of the aerodynamic damping was confirmed. According to the equations, the damping ratios of disk flutter due to aerodynamic damping are inversely proportional to the cube of the disk-to-shroud spacing normalized by the outer radius of the disk. Considering a damping independent of the disk-to-shroud spacing, we derived a mathematical model of the relationship between the disk flutter amplitude and the disk-to-shroud spacing. This relationship matched the experimental results for various disk sizes.