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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