Two-phase flow measurements of an unsteady breaking bore

Abstract

A key feature of breaking bores, jumps and spilling breakers is the roller region, characterised by intense shear and recirculation, associated with air bubble entrainment and splashing. Detailed unsteady air-water flow measurements were conducted in a breaking bore propagating in a largesize channel, using an array of dual-tip phase detection probes and an ultra-high-speed video camera. The results showed a steep roller front, with a very-dynamic air-water bubbly region, albeit with a relatively limited air-water roller region. In this study, a major challenge was the inconsistency in light intensity linked to the travelling nature of the bore. A simple flow visualisation technique was applied to retrieve the two-dimensional side-looking profile of the roller edge and average void fraction. The results were validated independently with a phase-detection probe. While the probe data lacked spatial variability, the study reinforces the needs of high quality validation dataset, including in unsteady transient flows.

Sensitivity analysis in air-water measurements under highly unsteady flow conditions: the breaking bore

“…This leads to an instantaneous void fraction signal of either 0 or 1, represented by the black line in Figure 2. All repetitions were synchronised using the first air-towater interface detected by the left tip of the probe, in line with Chanson (2004) and Leng and Chanson (2019), and the analysis conducted on data recorded by the right tip. The same test was repeated n = 2,000 times.…”

“…For both tips the signal was acquired with a frequency of 100 kHz for 2,000 repetitions at the same elevation z = 0.089 m (z/d1 = 1.06). All measurements were taken at a distance x = 8.5 m from the channel inlet, in line with Leng and Chanson (2019). All tests were conducted with similar atmospheric conditions.…”

“…For this, a quantification of their air-water flow properties is essential. While there is a long history or air-water flow measurements in steady flows, little attention was given to unsteady flows, for which long duration measurements are replaced by a large number of repetitions to reduce the bias associated with the randomness of the process (Bradshaw 1971;Chanson 2004; Leng and Chanson 2019). The total number of repetitions plays a key role in the ensemble-statistics analysis.…”

Strong free-surface turbulence in breaking bores: a physical study on the free-surface dynamics and air–water interfacial features

“…This region marking the sudden change between the initially steady flow and the propagating bore is called the ‘roller toe’, and is shown in figure 2. The rolling motion entrains air into the bore, forming large air pockets immediately downstream of the roller toe, which are further broken up into finer air bubbles in the developing shear layer (Leng & Chanson 2019 a ). The velocity shear also triggers Kelvin–Helmholtz instabilities within the roller toe region, providing the primary source of vorticity in the breaking bore (Hornung, Willert & Turner 1977).…”

“…The interaction between this convoluted motion within the roller and the free surface generates substantial SFST, resulting in a number of recurring and rapidly evolving foamy structures (Wüthrich, Shi & Chanson 2020 b ). Bubbles in the aerated region range in size from the submillimetric scale to tens of centimetres (Leng & Chanson 2019 a ), and they are known to play crucial roles in several phenomena, including an increase in the mixture flow bulk and an enhancement in the air–water mass transfer (Ervine & Falvey 1987; Wood 1991; Chanson 1997). The reproduction of bores in the laboratory (figure 2, inset) pointed out similarities with field observations, revealing the presence of a number of complex air–water surface features whose behaviour remains widely unexplored.…”