A model of a computer hard disk drive was constructed and measurements of the air flow in the unobstructed space between a pair of disks were obtained. The disks were centrally clamped to a common hub, and rotated within an axisymmetric (cylindrical) enclosure or shroud. Measurements of the circumferential velocity component were made at four radial locations and along the midplane at three rotation rates (0 = 300, 1200, and 3600 rpm) using a laser-Doppler velocimeter. The resulting mean and rms circumferential velocity profiles are presented and discussed. The data show that the circumferential velocity component profiles are fairly uniform in the axial direction in the space between the disks, except near the shroud where the flow is strongly sheared. The circumferential velocity peaks at a critical radius. Between the hub and the critical radius location the flow is in solid body rotation. Between the critical radius and the shroud the circumferential velocity decreases to zero, gradually at first and then very quickly as the shroud is approached. Analysis based on simplified force balance considerations facilitates the interpretation of the experimental observations and leads to improved understanding of the complex flow phenomena. Numerical calculations of the present configuration assuming axisymmetric steady flow were performed by Chang et al.(submitted to Int. J. Heat Mass Transfer). These calculations show reasonable agreement with the averaged velocity data but, for the reasons discussed, fail to reproduce features of the rms distribution associated with non turbulent flow unsteadiness.
Time-resolved measurements of the circumferential velocity component were obtained with a laser-Doppler velocimeter in the space between the center pair of four disks corotating in air in an axisymmetric cylindrical enclosure. The separate influences on the flow of two obstructions of similar shapes but having different lengths were investigated. The results show that both obstructions significantly alter the mean and rms distributions of velocity in quantitatively different but qualitatively similar ways. Both obstructions also alter the characteristic frequencies of flow oscillations associated with large scale motions present in the flow, apparently of the type that arise in unobstructed configurations. The measurements suggest that an obstruction can induce bimodal states of motion over frequency ranges that depend on the obstruction’s length. The presence of an obstruction increases the strength of the cross-stream secondary motion in the inter-disk space by redirecting fluid moving in the circumferential direction towards the radial direction. While this reduced the magnitude of the velocity deficit in the obstruction wake, for the cases investigated the flow did not recover within one revolution from the effects of either obstruction.
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