Aerodynamic Aspects of Disk Files

Airflows between centrally clamped, rotating, rigid disks are investigated with respect to the type of flow pattern, the parameters that influence nonuniform flow, and the effects of various flow patterns on disk stability. The experimental method uses a waterflow modeling technique for the airflow. The observed flow patterns are highly unsteady. The configuration and position of the shroud and slider arm are found to be the major parameters that influence flow characteristics. A reduction of disk flutter by a factor of 12 can be achieved when the unsteady flow pattern is changed to a steady flow pattern. Introduction Read-write operations in random access disk storage devices are performed by transducers mounted on hydrodynamic slider bearings. High linear bit densities, high track densities, and reliability of operation require not only an extremely low slider flying height but also a good compliance between disk and slider axial motions. Disk surface finish, flatness, and dynamic stability are usually considered to be the most critical mechanical parameters. However, aerodynamic influences are also critical, and airflow is responsible for two separate effects: 1. The flow around spinning disks creates mechanical problems. Any kind of large-scale unsteady flow or pressure fluctuation is a random excitation source for the vibration of disk and slider-arm suspension. An excited disk or a fluttering slider arm causes nonrepeatable relative motions between the read-write head and the recorded data tracks, thereby introducing off-track errors. Severe disk flutter causes data losses that are due to slider-disk interference or slider crashes. 2 . The flow around spinning disks creates thermodynamic problems. Any nonuniform flow characteristic in the gap between disks represents a nonuniform transfer of friction-generated heat along the disk radius and the disk-pack axis. Thermal expansion gradients cause data losses that are due to inaccuracies of head position. The flow characteristic, then, is a common cause of both mechanical and thermodynamic effects. Detailed knowledge of the parameters that influence nonuniform flow is required for an adequate understanding of those 480 effects. Unsteady flows are of particular interest. E. LENNEMANN An experimental investigation is reported in this paper to provide a qualitative description of the basic flow patterns. In addition, quantitative measurements of axial disk motion show the influence of the various flow patterns on rotation stability of a rigid disk. Basic disk configurations Basic configurations of interest for various applications can be categorized in the three groups illustrated in Fig. 1: single disks, corotating disks, and centrally clamped disks. The applications usually require a stationary shroud around the spinning disks for air ducting purposes. Turbomachinery designers have a particular interest in the aerodynamic aspects of disks. Therefore torque losses, reduced efficiency and heat transfer have been the major concerns in the literature on disk aerodynamics. A number of papers have been devoted to these characteristics of both single and multiple disks [ 1-26]. Flow pattern: single disk The basic flow around a single disk is symmetric, as illustrated in Fig. 2. The laminar and turbulent flow regimes are characterized by a dimensionless Reynolds number for the disk, R , = w r 2 / v , where o is the angular velocity, r the disk radius, and v the kinematic viscosity. Previous investigators [ 1-15] have extensively analyzed the single-disk system with respect to torque characteristics and heat transfer. Because of the symmetry of the problem, a complete solution of the three-dimensional Navier-Stokes equations is possible for the laminar case. A numerical approximation of the set of differential equations has been carried out by von Karman [7]. The computed torques agreed well with experimental data up IBM J . RES. DEVELOP.