Investigation of the dynamics of a viscosity-stratified temporal mixing layer using conditional statistics

This study examines the effects of viscosity stratification on the transition dynamics of an incompressible temporal mixing layer. Large-eddy simulations are performed for mixing layers with systematically varied viscosity ratios at identical Reynolds numbers. Transition processes are analyzed using turbulence statistics and conditional averaging based on stream origin, enabling separation of Reynolds-stress contributions from the low- and high-viscosity streams. Increasing viscosity contrast accelerates the onset of nonlinear dynamics and leads to earlier growth of turbulent kinetic energy and dissipation. While mean velocity profiles remain similar across cases, Reynolds stresses exhibit pronounced viscosity-dependent asymmetry. Conditional analyses show that turbulent fluctuation energy is preferentially generated and sustained on the low-viscosity side due to reduced viscous damping. As transition proceeds, localized energy is redistributed across the shear layer, yielding more balanced contributions from the two streams. These results demonstrate how viscosity stratification modifies the generation and redistribution of turbulent fluctuations during shear-layer transition.