Primary oscillatory instability in a rotating disk-cylinder system with aspect (height/radius) ratio varying from 0.1 to 1

Three-dimensional instability of axisymmetric flow in a rotating disk-cylinder configuration is studied numerically for the case of low cylinders with the height/radius aspect ratio varying between 1 and 0.1. A complete stability diagram for the transition from steady axisymmetric to oscillatory three-dimensional flow regime is reported. A good agreement with the experimental results is obtained. It is shown that the critical azimuthal wavenumber grows with the decrease of the aspect ratio, reaching the value of 19 at the aspect ratio 0.1. It is argued that the observed instability cannot be described as resulting from a boundary layer only. Other reasons that can destabilize the flow are discussed.