Large-Eddy Simulation of coaxial jets:: Coherent structures and mixing properties

Large-Eddy Simulations are performed to investigate the Reynolds number influence on coaxial jets. These simulations are validated by comparison against laboratory experiments. For high enough Reynolds numbers, the jet develops streamwise vortices from the beginning of the transition corresponding to the so-called "mixing transition". This implies a non-stationary behaviour of the recirculation bubble present for a very high velocity ratio between the outer and the inner jets. We investigate the Reynolds number influence on mixing by seeding a passive tracer in the outer stream. The tracer exhibits mushroom shape structures associated with counter-rotating streamwise vortices from the beginning of the jet development in the "mixing transition" state. We observe that the turbulent activity induced by the bubble allows for an efficient turbulent mixing activity.