Comparisons of experimental and numerical results for axisymmetric vortex breakdown in pipes

Experimental data for the bubble mode of breakdown in pipe flows is compared with calculations of the axisymmetric form of the Navier-Stokes equations. The location of breakdown in the calculations is found to be extremely sensitive to the inlet conditions; however, generally good agreement between the experimental and numerical breakdown locations is obtained. Detailed comparisons of the velocity distributions show that the bubble form of breakdown is well approximated by the axisymmetric Navier-Stokes equations for the flow upstream of the middle of the recirculation region. In the bubble tail, larger discrepancies exist between the experiment and calculation suggesting that the bubble form of breakdown is unstable to non-axisymmetric disturbances in the tail. Also, high Reynolds number bubble structures are found to exhibit periodic vortex ring shedding from the tail of the breakdown bubble.