Mixing enhancement in coaxial jets through inflow forcing : a numerical study

Guillaume Balarac (1) , Olivier Métais (1) , Marcel Lesieur (1)

1 LEGI - Laboratoire des Écoulements Géophysiques et Industriels [Grenoble]

Direct numerical simulations are performed to analyze the flow dynamics and the mixing properties of natural unforced and excited coaxial jets at moderate Reynolds number. First, the study of the natural coaxial jet, with species injected in the outer jet alone, allows us to understand the role of the coherent vortices on the mixing process during the transition stage. It is observed that the global flow behavior is controlled by the dynamics of the outer shear layer during the transition. The streamwise vortices are shown to play a significant role in the mixing process since they initiate intense ejections from the seeding regions. Spots of pure unmixed species from the outer jet are seen to persist far downstream. Two different types of inflow forcing are then considered based on the information provided by the natural coaxial jet: first, a purely axisymmetric excitation and second, combined axisymmetric and azimuthal excitations all of moderate amplitude. These excitations are applied to the outer shear layer with a frequency corresponding to the periodic passage of the outer vortical structures. The goal of these excitations is to trigger the vortices formation and to control their dynamics to improve the mixing properties of the jet. With the purely axisymmetric excitation, the outer and inner Kelvin-Helmholtz vortices appear earlier than for the natural jet and the transition process is faster. This early three-dimensionality growth is due to a rapid appearance of streamwise vortices, stretched between consecutive vortex rings, which lead to enhanced mixing. For the combined axisymmetric and azimuthal excitations, the outer Kelvin-Helmholtz rings appear moreover with an azimuthal deformation from the beginning of the jet. This allows for the early generation of streamwise vortices. The vortex stretching phenomenon takes place near the jet inlet with a growth of the axial component of the vorticity fluctuations. The ejections of species from the outer jet thus appear sooner and with a larger intensity. Finally, the mixing efficiency is studied through the global mixedness and the intensity of segregation.

Domaines: Mécanique des fluides [physics.class-ph]; Mécanique des fluides [physics.class-ph]

Guillaume Balarac, Olivier Métais, Marcel Lesieur. Mixing enhancement in coaxial jets through inflow forcing : a numerical study. Physics of Fluids, 2007, 19 (7), pp.075102. ⟨10.1063/1.2747680⟩. ⟨hal-00261425⟩