Experimental investigation of airflow above waves in a horizontal pipe

Abstract

We investigate the effect of waves on the airflow in horizontal two-phase pipe flow. Velocity fields in the gaseous phase were acquired by particle image velocimetry (PIV), while interfacial elevation was measured with conductance wave probes. The velocity fields were sampled on a wave-following coordinate system which allows for a decomposition of the velocity field into a mean, wave-coherent and fluctuating component by means of a three-component Reynolds decomposition. Additionally, coherent vortical structures were identified by the swirling strength criterion, and their distribution along the waves is investigated. Results suggest that the interaction between turbulent airflow and propagating waves in a pipe has a number of features reminiscent of wind-wave interaction in open systems. Above waves generated by sufficiently high gas flow rates, there is a distinct region of sheltered airflow, and a lifting of the critical layer on the leeward side of the crest. Streamlines of the phase-averaged flow field show a cat's eye structure located close to the crest in this region. Above waves of moderate steepness, we observe a shear layer that remains adjacent to the wave surface. Above steeper waves and higher gas flow rate, this layer detaches from the surface just downstream of the crest. Shear layer separation above waves is traditionally linked to the onset of wave breaking, and it is interesting to note that the case where we observe a separated shear layer in the phase-averaged vorticity field is in a regime of amplitude saturation. The swirling strength criterion reveals that vortical structures are shed from the interface and populate the detached shear layer above the trough. Below the detached shear layer, there is a region populated by counter-rotating vortices. The critical height coincides with the border between these two regions.

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

“…Previous analysis of the experimental set-up considered in this work (Vollestad et al. 2019 a ) has shown that intermittent airflow separation is a frequent event as the gas flow rate in increased into the amplitude saturation regime described by Ayati et al. (2015), and microscale wave breaking has been observed when the superficial gas velocity , for a superficial liquid velocity (Vollestad et al.…”

“…(2019 b ) to evaluate phase-averaged statistics of the liquid phase of microscale breaking waves, while the wave-following coordinate system in the gas phase is similar to the coordinate system used by Vollestad et al. (2019 a ), analysing airflow over waves.…”

“…This ensures that points in the trough-to-crest region are sampled at the same relative height above the interface, ensuring a more physical phase-averaged value in this region. This coordinate system is similar to the coordinate system applied by Vollestad et al (2019a), evaluating the phase-averaged properties of airflow above waves. The horizontal and vertical velocity components of breaking and non-breaking waves are sampled individually, and averaged at constant (θ, ζ)-coordinates to obtain the phase-averaged velocities u and v .…”

“…These studies have provided spatio-temporally and phase-averaged flow fields, and hence valuable experimental data for model validation of more complex numerical models, and new insight into the dynamics of the two phases. Previous analysis of the experimental setup considered in this work (Vollestad et al, 2019a) has shown that intermittent airflow separation is a frequent event as the gas flow rate in increased into the amplitude saturation regime described by Ayati et al (2015), and microscale wave breaking has been observed when the superficial gas velocity U sg ≥ 1.8 m/s, for a superficial liquid velocity U sl = 0.1 m/s (Vollestad et al, 2019b) (the superficial velocity of phase k is U sk =ṁ k /(ρ k A), whereṁ k and ρ k is the mass flow rate and density of phase k, while A is the cross sectional area of the pipe). Microscale wave breaking was found to influence the liquid phase down to a depth of approximately 1 cm below the crest (approximately one significant wave height), increasing the turbulent dissipation rate below the crest by a factor 3-4 compared with non-breaking waves (Vollestad et al, 2019b).…”

Microscale wave breaking in stratified air-water pipe flow

“…Recently, PIV has been applied to study the stratified regime in two-phase pipe flow [28][29][30][31][32] . The main results from these studies are spatio-temporally and phase-averaged flow fields.…”