VORTEX BREAKDOWN IN A ROTATING CYLINDRICAL CAVITY

Flow visualization, laser-Doppler anemometry and pressure measurements have been used to identify and delineate the regimes of vortex breakdown in a rotating cavity with a central axial flow of air. For the particular cavity tested (where the ratio of the outer to the inner radius was ten and the ratio of the axial width to the inner radius was approximately five), spiral breakdown and axisymmetric breakdown occur in both laminar and turbulent flow. Rossby numbers ε characterizing the boundaries between the breakdown modes were established from visual observations of flow behaviour, from discontinuities in velocity components and in the pressure drop across the cavity, and from changes in the velocity spectra. In laminar flow, spiral breakdown occurs for 1·6 ≲ ε ≲ 3·2 and axisymmetric breakdown occurs for 0·8 ≲ ε ≲ 1·5. In turbulent flow, spiral breakdown occurs for 21 ≲ ε ≲ 100 and 1·5 ≲ ε ≲ 2·6, and axisymmetric breakdown occurs for 2·6 ≲ ε ≲ 21 and 0·8 ≲ ε ≲ 1·5. At the higher Rossby numbers, the flow under laminar conditions is significantly different to that under turbulent conditions; at the lower Rossby numbers, it was found to be impossible to distinguish between laminar and turbulent flow. © 1979, Cambridge University Press. All rights reserved.