Bamin Khomami

In this study we have investigated the effect of fluid elasticity and dynamic modulation on the stability of single and multilayer pressure driven channel flows as well as single and multilayer flows down an inclined plane. In our experiments, the instability is observed and measured with the aid of digital image processing techniques. Based on these experiments, we have prepared a number of stability contours and growth/decay rate plots that clearly demonstrate the effect of various important parameters (e.g. number of layers and their arrangement, viscosity ratio and elasticity ratio) on the stability of the this class of flows. In addition, we have theoretically examined the linear stability of systems under consideration by utilizing asymptotic techniques and spectrally based numerical methods with various constitutive equations. Upon comparison of theoretical and experimental results, it is shown that the linear and weakly nonlinear stability of this class flows can be accurately predicted provided a constitutive model that can quantitatively capture the steady and dynamic rheological properties of the test fluids is used in the analysis. We have also shown that dynamic modulation can significantly affect the stability of single and single and multilayer channel and inclined plane flows. Specifically, in presence of dynamic modulation flows that are otherwise unstable for extensive range of viscosity and elasticity ratios, can be stabilized or visa-versa. In particular, in a certain parameter range resonance between a free surface and an interface or between various interfaces gives rise to an extremely rich dynamics. Finally, the mechanisms of interfacial and free surface instabilities in this class of flows in presence or absence of dynamic modulation has been determined utilizing a rigorous energy analysis.