Microscale wave breaking in stratified air-water pipe flow

Abstract

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 (PIV) 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 nonbreaking 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.

Experimental investigation of intermittent airflow separation and microscale wave breaking in wavy two-phase pipe flow

“…This implies that an equilibrium between increased wind input and dissipation is reached, which is reminiscent of the equilibrium range of the ocean wave spectrum (Phillips, 1958(Phillips, , 1985. Vollestad et al (2019b) found that the first evidence of wave breaking was observed at U sg = 1.8 m/s. Increasing the gas velocity into the amplitude saturation regime was observed to increase both the frequency and intensity of the wave breaking events.…”

“…This estimate was used to evaluate a trigger delay to the PIV system, allowing the leeward side of the wave to be repeatedly present within the liquid phase PIV FOV as the PIV images were acquired (necessary for the analysis performed in this work). The pressure probe trigger system is described in more detail in Vollestad et al (2019b). The two laser pulses were triggered with a ∆t of 100-120 µs (depending on the flow rate combinations applied).…”

“…Babanin (2011) divides wave breaking into plunging, spilling and microbreaking. Microbreaking, or microscale breaking is a weak form of wave breaking, where surface tension prevents the jet formation and air entrainment into the liquid phase as the wave breaks (Tulin & Landrini, 2001;Jessup et al, 1997;Siddiqui & Loewen, 2006;Vollestad et al, 2019b). Microscale breaking waves are short (wavelength in the order of a few decimeters), wind generated waves with amplitudes up to a few centimeters.…”

“…The novelty of this work lies in the simultaneous assessment of active wave breaking and evaluation of the airflow above individual waves. Wave breaking is assessed by the method of Vollestad et al (2019b), evaluating the rms vorticity in the liquid phase on the leeward side of the crest. Statistics of the flow field over breaking and non-breaking waves at two different gas flow rates are examined.…”

Momentum flux measurements in the airflow over wind-generated surface waves

“…In these cases, since the surface displacement is not strictly monochromatic, the wave phase is generally obtained from geometrical properties of the surface profiles such as zero-crossing, minima (trough) and maxima (crest) of the elevation (e.g. Grare 2009;Siddiqui & Loewen 2010;Birvalski et al 2014;Ayati, Vollestad & Jensen 2018;Vollestad, Ayati & Jensen 2019) and Hilbert transform techniques (e.g. Melville 1983;Hristov et al 1998;Bopp 2018).…”