Structure of Vortex Breakdown in a Torsionally Driven Cylinder at Low Reynolds Number

Flow visualisation using tracer and particle techniques is well known to be susceptible to fallacious interpretation for unsteady shear flows. Recent experimental observations using these types of techniques have led to the conclusion that even for low Reynolds number flows, the vortex breakdown bubble in a torsionally driven cylinder is not closed and has an inflow/outflow asymmetry at its tail. These results would suggest that this type of vortex structure is more similar to that observed in the open vortex tube than previously thought and is therefore a "true" vortex breakdown. In this article, an alternative explanation of the observed asymmetry is offered. It is demonstrated that the dye visualisation technique can lead to spurious interpretations even for a steady axisymmetric flow field, due to tracer diffusion and minute errors in tracer injection location. It is concluded that there is no evidence that bubble structure in a confined cylinder flow is asymmetric at these lower Reynolds number.