Numerical Simulations of Transitional Axisymmetric Coaxial Jets

Direct numerical simulations of spatially evolving axisymmetric coaxial jets are carried out using nonreflecting radiative boundary conditions at the outflow. The sensitivity of the numerical solution to the domain size is investigated, pointing out the feedback effect of the boundary conditions on the pressure at the inlet. The effects of the Reynolds number on the characteristics of the flow are studied. In the initial phase following an impulse, the evolution of the startup vortex is found to be independent of the Reynolds number, whereas the circulation per unit length at the vortex center increases with the Reynolds number. The Reynolds number is also found to affect the further development of the shear-layers' instabilities. Finally, the effects of the inlet conditions on the dynamics of vortical structures are investigated. Two simulations are carried out, in which the inlet velocity profile is unperturbed and perturbed randomly, and the results are compared with flow visualizations from experiments. In the unperturbed case the rollup of the external shear layer occurs at a much larger distance from the jets exit than in experiments, whereas in the perturbed case a good agreement with the experiments is obtained.