Three-Dimensional Evolution of Axisymmetric Jets: a Comparison Between Computations and Experiments

We analyze the three-dimensional evolution of a transitional axisymmetric jet for the two cases of an axisymmetric perturbation in the streamwise direction combined with an azimuthal corrugation and a helical perturbation, respectively. The combined analysis of flow visualization experiments and inviscid vortex dynamics simulations elucidates the processes leading to the concentration, reorientation, and stretching of the vorticity. In the first case, counterrotating pairs of streamwise vortex tubes form in the braid regions, and the cores of the vortex rings develop a wavy dislocation. The mechanism for the collapse of the streamwise braid vorticity is the same as in a plane mixing layer. For the helical perturbation, only one sign of streamwise vorticity forms in the braid, while the vortex helix represents streamwise vorticity of the opposite sign. The early stages of the flow visualization experiments show good agreement with the numerical observations, thus confirming that even at moderate values of the Reynolds number the evolution is dominated by inviscid mechanisms.