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Previous studies have shown that intermediate surface tension has acounterintuitive destabilizing effect on 2-phase planar jets. Here, thetransition process in confined 2D jets of two fluids with varying viscosityratio is investigated using DNS. Neutral curves for persistent oscillations arefound by recording the norm of the velocity residuals in DNS for over 1000nondimensional time units, or until the signal has reached a constant level ina logarithmic scale - either a converged steady state, or a "statisticallysteady" oscillatory state. Oscillatory final states are found for all viscosityratios (0.1-10). For uniform viscosity (m=1), the first bifurcation is througha surface tension-driven global instability. For low viscosity of the outerfluid, there is a mode competition between a steady asymmetric Coanda-typeattachment mode and the surface tension-induced mode. At moderate surfacetension, the Coanda-type attachment dominates and eventually triggerstime-dependent convective bursts. At high surface tension, the surfacetension-dominated mode dominates. For high viscosity of the outer fluid,persistent oscillations appear due to a strong convective instability. Finally,the m=1 jet remains unstable far from the inlet when the shear profile isnearly constant. Comparing this to a parallel Couette flow (without inflectionpoints), we show that in both flows, a hidden interfacial mode brought out bysurface tension becomes temporally and absolutely unstable in an intermediateWeber and Reynolds regime. An energy analysis of the Couette setup shows thatsurface tension, although dissipative, induces a velocity field near theinterface which extracts energy from the flow through a viscous mechanism. Thisstudy highlights the rich dynamics of immiscible planar uniform-density jets,where several self-sustained and convective mechanisms compete depending on theexact parameters.

Effect of viscosity ratio on the self-sustained instabilities in planar immiscible jets