Large eddy simulation of the near field of round and coaxial jets with mild swirl.

摘要

The near fields of round and coaxial turbulent free jets, both with and without a mild level of swirl, were examined using large eddy simulation (LES). The LES was formulated with the Kim and Moin fractional step time advancement scheme and the dynamic Smagorinsky subgrid scale model. The coaxial jet nozzle was configured with equal cross sectional areas for the inner and outer flow streams; the ratio of freestream velocity in the two flows was 1.0. Time-averaged velocity statistics were in accordance with other numerical and experimental results. The effects of swirl on the evolution of and interaction between turbulence structures were visualized using plots of the vorticity and discriminant, generated from the instantaneous velocity fields.; Azimuthal-oriented rings of vorticity formed slightly downstream from the nozzle. Streamwise-oriented braid structures formed in their wake, and were stretched downstream by the flow. The rings collided with the upstream ends of the braids; the interaction between the two components of vorticity eventually caused the rings to break apart into smaller, less organized turbulence structures. The addition of swirl to the inner portion of the flow at the nozzle strengthened the braid structures, which enhanced flow entrainment and the eventual disintegration of the rings. The presence of swirl in the outer flow stream of the coaxial jet strengthened the wake behind the rings, but few organized turbulence structures formed as a result. Therefore, a mild level of swirl in the outer stream of the coaxial jet was found to have little effect on entrainment over the first two jet diameters of flow.