Mixing and laminarization characteristics of coaxial jets with disparate viscosity

Understanding the complex mixing phenomena in co-axial turbulent jets with fluids of disparate viscosity is crucial for numerous industrial applications, like reactive mixing. In this study, we perform highly-resolved three-dimensional Large-eddy simulations (LES) to investigate the mixing characteristics. Velocity and scalar fields are validated against in-house PIV and PLIF measurements, respectively. Since viscosity has a direct effect on flow regime, two cases, TTT (turbulent-turbulent-turbulent) and TLL (turbulent-laminar-laminar), are considered to represent different flow conditions. We investigate evolving interfacial dynamics, turbulent-non-turbulent interface phenomena, momentum equation budget, mixing behavior, and laminarization with increasing viscosity. The interfacial wave evolution shows distinct behavior: m1 (low viscosity) experiences rapid homogenization, while m40 (high viscosity) exhibits "mushroom"-like structures transitioning to folded-segregated patterns. The magnitude and orientation of variable viscosity terms in RANS equations are quantified using LES data, revealing their significance and gradient diffusion hypothesis failure. This research provides valuable insights into mixing physics and RANS limitations in hybrid flow conditions.