Accurate numerical simulations of vortex flows. I: Vortex breakdown

Vortex breakdown in uniform axial flow is investigated numerically by solving the Navier–Stokes equations using a hybrid spectral – finite difference method. Two Reynolds numbers and a high swirl number are considered with the lower Re-number resulting in axi-symmetric flow, that possesses an asymptotic state, and a higher Re-number, that leads to unsteady flow and a moderate spectral range. The structural properties of the vorticity field are analyzed in terms of vorticity lines and surfaces. A recirculation bubble and a wake emerge that undergo significant fluctuations.