Effect of Interfacial Waves on Turbulence Structure in Stratified Duct Flows

Stratified flows are encountered in many industrial applications. The determination of the flow characteristics is essential for the prediction of pressure drop and holdup in the system. The aim of this study is to gain insight into the interaction of a gas and a liquid phase flowing in a stratified regime, with especial focus on the effect of interfacial waves on the turbulence structure of the liquid phase. Measurements of mean velocities and turbulent intensities in the liquid phase of a stratified air-water duct flow are performed. Mean velocity profiles and turbulence structure are affected differently for different wave amplitudes. The effect of small amplitude waves is restricted to the near-interface region, resembling the effect of increasing shear rate on a flat interface. On the other hand, large amplitude waves modify the flow structure throughout the whole liquid depth. The mean velocity is greatly enhanced, resulting in a higher bulk velocity. Turbulent intensities are also significantly enhanced especially in the interface region. This big difference in flow structure is not observed after the appearance of the first waves but rather when a certain critical wave amplitude is triggered, indicating that the prediction of this critical wave type turns out to be more important than the determination of the transition from a smooth to a stratified wavy regime.