Numerical Simulation of Round Jet.

Abstract

A direct numerical simulation was conducted for temporally developing three-dimensional round jets, assuming periodicity in all directions of rectilinear coordinates. Two cases of nearly axi-symmetric and helical perturbations superposed on a thin round mixing layer were taken as the initial flow conditions. Subsequent evolutions of jets depending on the two initial perturbations are described, especially for coherent structures. In the case of the nearly axi-symmetric perturbation, discrete ring like rolls and streamwise ribs connecting the rolls are generated, and increases in enstrophy and helicity are primarily due to the streamwise vortex in the roll. In the case of the helical perturbation, the generation of structures is earlier than in the nearly axi-symmetric case, and rates of increase in enstrophy and helicity are lower because of the long-lived coherent structure of elongated vortices between rolls.