Axisymmetric propagating vortices in the flow between a stationary and a rotating disk enclosed by a cylinder

The destabilization of the stationary basic flow occurring between two disk s enclosed by a cylinder is studied experimentally when the radius of the d isks is large compared to the spacing. In the explored range of the cell as pect ratio, when one disk only is rotating, circular vortices propagating t o the centre are observed above a critical angular velocity. These structur es occur naturally but can also be forced by small modulations of the angul ar velocity of the disk. For each rotation rate the dispersion relation of the instability is experimentally reconstructed from visualizations and it is shown that this dispersion relation can be scaled by the boundary layer thickness measured over the disk at rest. The bifurcation is found to be of supercritical nature. The effect of the forcing amplitude is in favour of a linear convective nature of this instability of the non-parallel inward f low existing above the stationary disk. The most unstable temporal frequenc y is found to be about four times the frequency of the rotating disk. The e volution of the threshold of this primary instability is described for diff erent aspect ratios of the cell. Finally, two sets of experiments made unde r transient conditions are presented: one in order to investigate further a possible convective/absolute transition for the instability, and the other to compare with the impulsive spin-down-to-rest experiments of Savas (1983 ).