The objective of this study is to understand the effects of non-linearity on the interfacial stability of a two fluid stratified flow through a horizontal channel. An efficient perturbation expansion based high-order spectral method is developed, for the simulation of the generation and nonlinear evolution of interfacial waves. The method is capable of accounting for the nonlinear interactions of a large number of wave components in a broadband spectrum, and obtains an exponential convergence of the solution with grid refinement and interaction order. The method is applied to investigate the role that nonlinear effects have on the initial development of Kelvin-Helmholtz (KH) instabilities and nonlinear wave-wave interactions with the purpose of gaining insight into the physical mechanisms which cause slug flow. It will also be demonstrated that nonlinearity can introduce unstable inter-facial wave growth in regions predicted to be stable by linear analysis. In addition, it is shown that energy transfer from short (KH unstable) waves to long (KH stable) waves due to nonlinear resonant wave-wave interactions is an effective mechanism for the development of slug flow.

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