Convective instabilities in the coextrusion process

This paper is devoted to the numerical study of the stability of the plane flow for two immiscible fluids obeying Maxwell constitutive equation in a c hannel of aspect ratio 8. For fixed viscosity and flow rate ratios, the sha pe and non-linear behavior of interfacial waves induced by small perturbati ons of the input flow rate are studied using finite element simulations for various elasticity stratifications. The stationary flow, close to a Poiseu ille flow and presenting a flat interface is first computed. Then the inter facial wave generated by a pulse-like disturbance on entry flow rates is co mputed by using time dependent computations. It takes the shape of a wave p acket which is amplified or damped while moving towards the die exit. In un stable cases, the maximum deviation increases with elasticity (the Weissenb erg number). Interestingly, the elastic stratification between the two laye rs can counteract unstable waves due to viscous stratification.