Phase-locked measurements of linear and weakly nonlinear interfacial waves in a stratified turbulent gas–liquid pipe flow

Abstract

The present work reports an experimental characterization of linear and weakly nonlinear interfacial waves in a stratified air–water horizontal pipe flow. An oscillating paddle was employed to generate controlled waves at the liquid interface. The driving signal of the oscillating paddle was controlled and synchronized with image acquisitions, enabling phase-locked measurements and the application of ensemble averaging techniques. Velocity field measurements in the liquid and gas phases were performed simultaneously using an off-axis particle image velocimetry setup and shadowgraphy. The combined techniques allowed us to extract the coherent part of flow fluctuations related to the excited waves. This was done for a range of flow rates and wave frequencies. The selected conditions are close to the transition from stratified to slug/plug flow regimes. In the presence of linear waves, the coherent disturbances in both phases were weakly dependent on near-wall disturbances. Flow changes in the presence of weakly nonlinear waves were also investigated. In these cases, noticeable modifications in the mean flow and in turbulence distribution were observed near the interface, whereas close to the wall, the flow was weakly affected. This investigation follows the work of Farias et al. [“Characterization of interfacial waves in stratified turbulent gas-liquid pipe flow using Particle Image Velocimetry and controlled disturbances,” Int. J. Multiphhase Flow 161, 104381 (2023)], where the threshold for linear and weakly nonlinear waves was studied. Here, a clear comparison between wave-induced disturbances in linear and weakly nonlinear regimes is reported in the literature for the first time for stratified turbulent gas–liquid pipe flows. The methodology proposed is relatively simple and can contribute to describe wave-related phenomena in stratified pipe flows.