Interfacial instability in vertical counter-current gas-liquid film flow: theory, direct numerical simulation and experiment

The dynamics of vertical counter-current gas-liquid flows are largelydetermined by interfacial instability, which gives rise to a multitudeof complex wave patterns and internal flows. To study the genesis andevolution of the instability in detail, we employ theoretical stabilityanalysis, experiment and a newly developed level set method basedin-house solver to carry out direct numerical simulations. Crucial results of these simulations, such as growth rate and phase velocity ofinterfacial waves, are rigorously compared against linear and weaklynonlinear theory; thereby showing remarkable agreement. The analysisalso reveals the spatio-temporal character of the waves, depictingregimes of absolute and convective instability. Complementing thebenchmark set by (non-)linear theory, we perform film thicknessmeasurements of a real gas-liquid system (air-silicone oil) by means ofa non-intrusive light-induced fluorescence technique to further validatethe solver regarding its capability of capturing interfacial dynamicsaccurately. These measurements are in good agreement with the results ofthe nonlinear direct numerical simulations with respect to wavelengthand wave shape of the most unstable mode.