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

Abstract

Abstract

Hydraulic jumps with low inflow Froude numbers: air–water surface patterns and transverse distributions of two-phase flow properties

“…These features were the result of a mostly upward ejection of an air–water volume, occurring predominantly in the first half of the roller, close to roller toe. The existence of these features was previously reported by Murzyn and Chanson [ 56 ], Chanson [ 43 ], Chachereau and Chanson [ 28 ] for hydraulic jumps with higher Froude numbers and Wüthrich et al [ 39 ] for breaking bores. Whilst, in breaking bores, the direction of the fingers was mostly pointing against the propagation of the bore roller, both directions were commonly observed in hydraulic jumps (Fig.…”

“…During their lifespan, fingers showed the appearance of Plateau-Rayleigh instabilities, partially responsible for their breakage into smaller droplets of smaller total surface area [ 44 ]. These water droplets were often projected upstream of the roller toe, as reported by Leng and Chanson [ 55 ] and Wüthrich et al [ 39 ].…”

“…The flow motion at the roller toe (top view) was investigated for a hydraulic jump with Re = 1.86 × 10 5 and Fr 1 = 2.4 by means of the ultra-high-speed video camera installed on top of the channel. The video movies captured a number of key air–water surface features, which complement the observations of Wüthrich et al [ 39 ] for non-stationary breaking bores. Examples of these air–water flow surface motions are presented in Fig.…”

“…Both flow conditions revealed the presence of a breaking roller with a fluctuating behaviour and relatively intense air entrainment, considering the low inflow Froude numbers. Several studies suggested that the turbulent fluid motion could result in a deformation of the free surface, leading to enhanced surface roughness, breakup, and disintegration [ 36 – 39 ]. Figure 4 d, e show the sudden increase in water depth at the roller toe, with the formation of three-dimensional " foamy " structures, characterised by substantial air entrainment and entrapment.…”

“…In hydraulic jumps, such air–water surface features included fingers, water droplets, crowns, slugs, spider webs, mushrooms, boils and holes. It was noted that most features were also previously observed in a breaking bore, which was considered a hydraulic jump in transition [ 39 ]. It is important to point out that the focus of the high-speed video camera only captured the first half of the roller, thus explaining why crowns and boils were not observed for hydraulic jumps, as these features were more common in the downstream part of the bore’s breaking roller.…”