Microscale wave breaking in stratified air-water pipe flow

We perform an experimental analysis of two-phase stratified wavy pipe flow, with the aim to detect and quantify the effect of small scale wave breaking. Particle image velocimetry is employed to analyze the velocity fields below individual waves, and a threshold for the vorticity on the leeward side of the crest is used to assess active wave breaking. Keeping the liquid flow rate constant, we analyze five experimental cases with increasing gas flow rates. The cases span the flow map from when first interfacial waves are observed, to the amplitude saturation regime, where the rms interface elevation is independent of the gas flow rate. While some wave breaking events are observed also in the wave growth regime, wave breaking is found to be much more frequent when the gas flow rate is increased into the amplitude saturation regime, and 35%-40% of the waves passing the measurement section are assessed to be in a state of active breaking in this regime. A conditional averaging of the flow field is performed, and the turbulent dissipation rate below breaking and non-breaking waves is estimated. The effect of microscale breaking is observed down to a depth of 10 mm below the water surface. Below the crest of microscale breaking waves, the turbulent dissipation rate is increased by a factor 2.5 to 4 compared with non -breaking waves. This fraction increases with U-sg, implying that the breaking events become more energetic as the gas flow rate is increased.