The Influence of the Swirl Velocity Ratio on the Nature of the Dominant Helical Structures in a Swirling Jet Model

Abstract

The nonlinear evolution of helical perturbations is investigated in a swirling jet model, consisting of a line vortex along the jet centerline surrounded by a jet shear layer with both azimuthal and streamwise vorticity. Inviscid Lagrangian vortex dynamics simulations demonstrate the nonlinear competition between a centrifugal Rayleigh instability and a Kelvin-Helmholtz instability feeding on both components of the base flow vorticity. The interaction of these two instabilities allows for very different flow behaviors to emerge, depending on (1) the swirl velocity ratio and (2) whether the helical perturbation wave and the vortex lines of the jet shear layer wind around the jet axis in the same or in opposite directions. For very high levels of swirl, large scale vortex helices evolve that can contain azimuthal vorticity either of the same or of opposite sign to that initially present in the jet shear layer. These different evolutions are triggered by the differences in the direction of the strain field set up by the dominant vortex helix. The effect of lowering the level of swirl is most prominent when perturbation occurs in a direction opposite to that of the vortex lines. In this case, both Kelvin-Helmholtz and centrifugal instabilities grow at similar rates leading to pairs of counterrotating helical vortices.KeywordsVortex RingVortex LineVortex SheetVortex FilamentStreamwise VorticityThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.