Formation of a recirculation zone in low Re-number turbulence

The formation of a recirculation bubble in swirling jets is investigated using direct simulation results and the pole conditions for the azimuthal Fourier modes in cylindrical coordinates to construct interpolation functions in the grid patch containing the emerging bubble. The time before and after the emergence of the bubble for axi-symmetric and 3-d flows show the structural instability of the axi-symmetric bubble. The properties of the 3-d bubble are documented using streamlines, streamsurfaces and associated Poincaré sections and Lyapunov exponents. The streamsurfaces near the unstable manifold of the upstream critical point consist of tightly packed layers and the streamlines inside the bubble are separated into two non-mixing set of lines.