Instabilities of vortex rings generated by surface-tension gradients between co-axial disks

Instabilities of vortex rings generated by surface-tension gradients between co-axial disks with Prandtl numbers of 0.001 and 0.01 are investigated by using the linear stability analysis (LSA) method. The continuity-vortex-energy equations are used as the perturbation equations for stability analysis and discretized using a Chebyshev-collocation method. The critical Reynolds numbers and the angular wavenumbers of the unstable mode are obtained for vortex rings with a range of aspect ratios between 0.05 and 1.2. From stability analyses, it is found that the product of the critical model and the aspect ratio approaches a constant when the aspect ratio decreases. The critical mode is m = 16 when the aspect ratio is 0.05. Analyses also indicate that the vortex rings must be of certain energy for the perturbations of flow to grow in amplitude. The viscosity of fluid can dampen perturbations if the magnitude of the stream function of the basic flow is below the critical value. The vortex rings generated by surface-tension gradients become unstable when the magnitude of the stream function is above the critical value.