NONLINEAR DYNAMICS AT A HOPF-BIFURCATION WITH AXISYMMETRY BREAKING INA JET

A complex dynamics with several bifurcations within a 4% variation of the Reynolds number has been found to accompany the transition from a steady to a chaotic flow in a homogeneous round fluid jet. The results of direct numerical simulations are explained using a fifth order wea kly nonlinear theory describing the interaction of two counter-rotatin g helical modes, arising as a consequence of degeneracy of the lineari zed Navier-Stokes operator spectrum. Two secondary bifurcations and th ree different asymptotic states are shown to be correctly accounted fo r by the theory. The validity of the fifth order theory ceases shortly before the onset of chaos.