A Numerical and Experimental Study of the Bistable-Unstable Transition in Pressurized Flexible Disk Files

The bistable operating region of flexible disk files is limited at decreasing air flow rates by a transition at which undesirable gaps appear in the otherwise uniformly spaced disks of the pack. These spontaneous gaps, as opposed to the externally controlled gaps of the bistable range, are a consequence of local air flow fields at the outer edge of the disks. Experiments with external shrouds of various clearance showed axial periodicity in the occurrence of the gaps and provided physical insight which led to quantitative numerical solution of the nonlinear fluid equations. Two scales of the flow, i.e., the flow between individual disks and a peripheral unstable Couette flow, are found to couple in such a manner as to delineate the transition.