A numerical study of dynamics of a temporally evolving swirling jet

Direct numerical simulation (DNS) of a swirling jet near the outlet of a no zzle with axisymmetric and non-axisymmetric disturbances is performed to in vestigate the dynamic characteristics of the flow. The early (linear) stage of the jet evolution agrees well with the predictions of linear stability theory. In the nonlinear stage, the axisymmetric DNS shows that the interac tion between the primary vortex ring and the streamwise columnar vortex cre ates a secondary vortex structure with opposite azimuthal vorticity near th e columnar vortex. Then a vortex pair consisting of the primary and seconda ry vortices forms and travels radially away from the symmetry axis, causing a rapid increase of the thickness of mixing layer. The non-axisymmetric DN S shows that the streamwise vortex layer developed in the early stage of ev olution due to azimuthal instability breakdowns into small eddies under the joint stretch of the axial and azimuthal shear. The results reveal several mechanisms of mixing enhancement by swirl, i.e., the radial motion of vort ex ring pairs, the rapid growth of streamwise vorticity, and the creation o f three-dimensional small eddies. They are all favorable for fluid entrainm ent in swirling jets. (C) 2001 American Institute of Physics.