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TECHNICAL PAPERS

# Flow Instability and Disk Vibration of Shrouded Corotating Disk System

[+] Author and Article Information
S. Kanagai

LEXUS Body Engineering Division, Toyota Motor Corporation, 1 Toyota-cho, Toyota, Aichi 471-8571, Japankana@earth.tec.toyota.co.jp

J. Suzuki

Machinery and Assembly Shop, Mitsubishi Heavy Industries, Ltd., 1-1-1 Wadasaki-cho, Hyogo-ku, Kobe 652-8585, Japanjyo-suzuki@mhi.co.jp

S. Obi

Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama 223-8522, Japanobsn@mech.keio.ac.jp

S. Masuda

Department of Mechanical Engineering, Keio University, 3-14-1 Hiyoshi, Kouhoku-ku, Yokohama 223-8522, Japanbzp01264@nifty.ne.jp

J. Fluids Eng 129(10), 1306-1313 (Apr 25, 2007) (8 pages) doi:10.1115/1.2776958 History: Received January 28, 2005; Revised April 25, 2007

## Abstract

This paper focuses on the interaction between the flow unsteadiness and disk vibration of shrouded corotating disk system to identify the nature of the flow-induced vibration of disks in the wide range of rotation speed below critical. Special attention is paid to the role of the vortical flow structure on the disk vibration and vice versa. The water test rig for optical measurement and the air test rig for hot-wire and vibration measurements are employed, both being axisymmetric models of $3.5in.$ hard disk drive. Before investigating fluid-solid interaction, the velocity and vorticity fields between disks are examined by employing a particle image velocimetry, in order to check the flow within our own apparatus to have the same characteristics as those commonly accepted. In the course of this preliminary experiment, it is found that “vortical structures” reported in the previous papers based on the flow visualization are actually “vortices” in the sense that it exhibits closed streamlines with concentrated vorticity at its center when seen from an observer rotating with the structure itself. The measurements of out-of-plane displacement of the disk employing different disk materials reveal that disk vibration begins to occur even in low subcritical speed range, and amplitude of nonrepeatable run out (NRRO) can be uniquely correlated by using the ratio between the rotating speed and the critical speed. The power spectral densities of disk vibration showed that the disk vibrates as a free vibration triggered by, but not forced by, the flow unsteadiness even in the high subcritical speed range. The disk vibration has negligible effect on the vortical flow structure suggesting the soundness of the rigid disk assumption employed in the existing CFD . However, RRO has significant influence on the flow unsteadiness even if the disks are carefully manufactured and assembled. Since the RRO is unavoidable in the real disk system, the flat disk assumption should be considered more carefully.

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## Figures

Figure 1

Geometry and coordinate system of unobstructed corotating disks enclosed in cylindrical container (not to scale)

Figure 2

Distribution of velocity vectors and axial component of vorticity relative to the rotating asymmetric structure obtained by PIV (Re=6.4×104, H∕R2=0.061; disk rotation is counter-clockwise)

Figure 3

Rotational speed ω of asymmetric structure as a function of disk Reynolds number (Vertical axis is normalized by disk speed Ω)

Figure 4

Schematic of air test rig: (1) axisymmetric model of 3.5in. five-decker HDD, (2) single-sensor hot-wire probe, and (3) capacitance-type displacement probe, R1=12.35mm, R2=47.5mm, H=2.45mm, t=0.8mm, and a=0.5mm

Figure 5

Probe geometry: (a) Capacitance-type displacement meter probe for measuring disk vibration and (b) single hot-wire probe for measuring velocity fluctuation between disks

Figure 6

NRRO of disk vibration versus speed ratio for different disk materials

Figure 7

PSD of displacement sensor signal for aluminum and acrylic disks (N=10,000rpm)

Figure 8

Waterfall plots of disk vibration for aluminum and acrylic disks. Dominant PSD peaks of NRRO are plotted versus rotating speed (upper: aluminum; lower: acrylic; solid lines are from FEM analysis).

Figure 9

Velocity fluctuation between disks near the outer periphery (r=0.96R2)

Figure 10

PSD of velocity fluctuation on the interdisk midplane (r=0.96R2, N=10,000rpm, fn denotes frequency of vortical flow mode)

Figure 11

Comparison of PSD of disk vibration at the periphery and velocity fluctuation at the middle of the vortical flow region (aluminum, N=10,000rpm, N∕Nc=0.33, a=0.5mm, b=3.5mm, hot-wire is located at r=0.8R2; f1–f8 denote vortical flow mode, (m,n) denote vibration modes, and nf0 denotes disk RRO)

## Errata

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