Stratified air-water flow in a horizontal pipe is investigated experimentally using particle image velocimetry and conductance probes. This flow regime is characterized by a complex interplay between a turbulent airflow and propagating waves at the interface. The waves are generated by interfacial shear and pressure forces exerted by the faster flowing airflow. The goal of this study is to characterize the waves by means of statistical and spectral methods, and to explore the influence of different wave regimes on the airflow.
Two cases in which the air bulk velocity increases from 2.4 m/s (case A) to 3.5 m/s (case B), while the liquid velocity remains constant at 0.26 m/s, are assessed in detail. Case A belongs to a region of flow conditions in which wave amplitudes grow as a consequence of increasing gas flow rates, i.e., wave growth regime. Meanwhile, case B is in a regime of saturated wave amplitudes. In the first case, the interface was populated by small amplitude 2D waves of relatively small steepness (ak ≈ 0.07). These waves obey Gaussian statistics and are thus considered to be linear. In the second case, the waves are larger, steeper (ak ≈ 0.13) and considerably more irregular. They display non-linear behaviour (steep crests and long troughs) and their exceedance probability distribution deviates significantly from Gaussian statistics. Bicoherence maps show evidence of both overtone and sub-harmonic interactions.
Airflow velocity fields acquired by PIV were subjected to a conditional phase-averaging method based on a steepness criterion. The phase-averaged vorticity field shows evidence of shear-layer separation above the steeper waves of case B. Hence, in addition to non-linear mode interactions and micro-breaking, shear-layer separation may contribute to the transition from the growth regime to the saturation regime.
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