The physics of mixing and relaminarization characteristics of a co-axial jet with disparate viscosity

The mixing of fluids in a coaxial jet is studied under various viscosity ratios using Large-Eddy simulations (LES), Reynolds-Averaged Navier Stokes (RANS), particle image velocimetry (PIV), and planar laser-induced fluorescence (PLIF). In the simulations, the state-of-the-art RANS and LES models are employed, and the accuracy of predictions is tested against data obtained by the simultaneous experimental measurements of velocity and concentration fields. We show that the standard RANS approach without including models for viscosity variations is not applicable whereas dynamic LES models provide high-quality agreement with the measurements. To identify the underlying sources of discrepancy in RANS predictions, two distinct mixing modes are defined based on the viscosity ratio. Then, for each mode, the evolution of mixing structures, analysis of the turbulent activity, and decay of turbulence are investigated using LES results. Overall, the interfacial dynamics and flow relaminarization characteristics are found to be quite distinct in each mixing mode. The scaling of the energy spectrum cascade suggests that, rather than the turbulence, the unsteady laminar shedding is responsible for the eddies observed that explain the reduced mixing in the pipe downstream.