| 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 made. The disks were centrally clamped to a common hub, and rotated within a cylindrical enclosure. Time-resolved measurements of the circumferential velocity component were made {dollar}Omega{dollar} = 60, 64, 300, 1,200 and 3,600 rpm using a laser-Doppler velocimeter. Mean and rms circumferential velocity profiles are presented and discussed.; At 60 rpm the flow is steady. Above this rotation rate, the rms circumferential velocity shows a characteristic pattern when plotted as a function of the radius. The flow is unsteady and periodic in time in the regions where the rms peaks. Additional velocity measurements were performed at a fixed radial location varying the rotation rate from 60 to 3,800 rpm. The flow exhibits a dominant frequency that is always close to an integer multiple of the disk rotational frequency. Transitions between dominant oscillation modes are sharp and form a descending staircase-like pattern with increasing {dollar}Omega{dollar}. The transitions shift by about 20% depending on whether the rotation rate is increased or decreased.; An analysis based on force balance considerations leads to improved understanding of the flow, by subdividing the flow into five regions, each described by a different force-momentum balance. As a result, characteristic shear layer thickness, time, pressure and velocity scales are obtained. The analysis also shows that detached, axially aligned, shear layers develop. This finding is confirmed by the numerical calculations.; A version of the REBUFFS finite difference code was modified to make it second order accurate and explicit in time, and second order accurate in space. Axisymmetric and fully three dimensional simulations were performed using the resulting program, named ROTDISK. The axisymmetric results show that the cross-stream flow is indeed unsteady, but the predicted oscillation frequencies do not match the experimental values. Fully three-dimensional simulations reveal a transition in which cross-stream flow oscillations perturb the circumferential velocity component. As a result, a discrete number of regions with high axial vorticity concentrations develop along the circumferential direction. |
