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

Abstract

We perform an experimental analysis of co-current, stratified wavy pipe flow, with the aim of investigating the effect of small scale wave breaking (microscale breaking) on the airflow. Particle image velocimetry is applied simultaneously in the gas and liquid phases. Active wave breaking is identified by high levels of vorticity on the leeward side of individual waves, and the statistics of the airflow above breaking and non-breaking waves are extracted from the gas-phase velocity fields. Keeping the liquid superficial velocity constant ($U_{sl}=0.1~\text{m}~\text{s}^{-1}$), we consider two experimental cases of different gas flow rates. The lowest flow rate ($U_{sg}=1.85~\text{m}~\text{s}^{-1}$) is slightly higher than the onset of microscale breaking, while the higher flow rate ($U_{sg}=2.20~\text{m}~\text{s}^{-1}$) is within the regime where wave breaking is observed to be frequent, and the root-mean-square interface elevation $\unicode[STIX]{x1D702}_{rms}$ is independent of gas flow rate. Results show that for the lowest gas flow rate considered, active wave breaking has a stabilizing effect on the airflow above the waves, reducing the sheltered region on the leeward side of the wave and the turbulence above the wave crest compared with non-breaking waves at similar steepness. At the higher gas flow rate the effect of active wave breaking is found to be small, and the main geometrical properties of the waves are found to dominate the evolution of the separated flow region.

Turbulent and wave kinetic energy budgets in the airflow over wind-generated surface waves

“…Ensemble averaging the velocity fields for a given instantaneous wave phase yields the corresponding wave-phase average (e.g. Melville 1983; Siddiqui & Loewen 2010; Vollestad, Ayati & Jensen 2019). Hence, an instantaneous velocity field can be decomposed into wave-phase-coherent and turbulent components By further separating the wave-phase-coherent velocity into the sum of the mean and wave-induced components, i.e.…”

Modification of Airflow Structure Due to Wave Breaking on a Submerged Topography

“…We perform a similar analysis, but additionally, waves are separated into breaking or non-breaking to investigate if the state of breaking also influences the extent of the sheltered region. Vollestad et al (2019) considered the airflow above waves in stratified two-phase pipe flow. The area on the leeward side of the crest where u < C p , divided by the length of the leeward side region of interest was evaluated as a measure of the mean height of the sheltered…”

“…Similar to the results by Reul et al (2008), this was seen to be well correlated with the front-face steepness. Vollestad et al (2019) separated the waves into breaking or nonbreaking, employing a threshold for the r.m.s. vorticity in the liquid phase to distinguish actively breaking and non-breaking waves.…”

“…Intermittent airflow separation over water waves is characterized by a detachment of the shear layer at the crest, enhanced dissipation downstream of the separation point, and a region of low wind speed below the detached shear layer (Buckley and Veron 2016;Husain et al 2019). As airflow separation is an intermittent process, phase-averaged flow fields do not show the separated flow structures directly, but increased frequency and intensity of the airflow separation events are observed to enhance the size of the recirculating structure observed on the leeward side of the wave, when the phase-averaged velocity field is seen in a frame of reference moving with the wave speed (Husain et al 2019;Vollestad et al 2019). The separated region is associated with a significant pressure drop (Banner 1990;Reul et al 2008;Sullivan et al 2018), resulting in a phase shift of the wave-coherent pressure field relative to the wave slope (Donelan et al 2006;Husain et al 2019), and enhanced form drag relative to the non-separated case.…”

“…Analyzing microscale breaking waves in a stratified pipe flow set-up, Vollestad et al (2019) found that for a moderate wind speed, active wave breaking had the effect of reducing the sheltered region, compared with waves at similar crest front-face steepness. This was attributed to a reduction in the extent and/or frequency of airflow separation.…”