Luke Toombes

Air-water flows down stepped chutes: turbulence and flow structure observations

Interactions between turbulent waters and atmosphere may lead to strong air-water mixing. This experimental study is focused on the flow down a staircase channel characterised by very strong flow aeration and turbulence. Interfacial aeration is characterised by strong air-water mixing extending down to the invert. The size of entrained bubbles and droplets extends over several orders of magnitude, and a significant number of bubble/droplet clusters was observed. Velocity and turbulence intensity measurements suggest high levels of turbulence across the entire air-water flow. The increase in turbulence levels, compared to single-phase flow situations, is proportional to the number of entrained particles.

Hydraulics of stepped chutes: The transition flow

Stepped spillway flows may behave as a succession of free-falling nappes at low flows and as a skimming flow at large discharges. However there is a range of intermediate flow rates characterised by a chaotic flow motion associated with intense splashing: i.e. the transition flow regime. Detailed air-water flow properties in transition flows were measured in two large experimental facilities. The results provide a complete characterisation of the air concentration, velocity and bubble count rate distributions. They highlight some difference between the upper and lower ranges of transition flows in terms of longitudinal free-surface profiles and air concentration distributions. Overall a dominant feature is the very-strong free-surface aeration, well in excess of observed data in smooth-invert and skimming flows.

Strong interactions between free-surface aeration and turbulence in an open channel flow

High-velocity free-surface flows may be characterised by strong free-surface aeration. In turn the entrained air bubbles are expected to interact with the flow turbulence. An experimental study was conducted in supercritical open channel flows down a cascade. Measurements included time-averaged air-water flow properties, air and water chord sizes, and interfacial areas. High levels of turbulence were associated consistently with large air-water interfacial areas. Altogether the study contributes to a better understanding of the basic interfacial processes in re-aeration cascades.

Experimental Investigations of Free-Surface Aeration in the Developing Flow of Two-Dimensional Water Jets

Turbulent water jets discharging into the atmosphere are often characterised by a substantial amount of free-surface aeration. The effects can be detrimental or beneficial. In any case, the knowledge of the air entrainment mechanisms is essential for an optimum design. New experimental data are presented in the developing flow region of two-dimensional water jets discharging into air. The results indicate that the air diffusion takes place rapidly downstream of the nozzle and it is nearly independent of the momentum transfer process. Further the distribution of air bubble frequency may be related to the air content distribution by a parabolic relationship

Flow patterns in nappe flow regime down low gradient stepped chutes

L. Toombes And H. Chanson, Journal of Hydraulic Research, IAHR, 2008, 46(1), 4–14

Free-surface aeration and momentum exchange at a bottom outlet

This study aims to provide some new understanding of the air–water flow properties in highvelocity water jets discharging past an abrupt drop. Such a setup has been little studied to date despite the relevance to bottom outlets. Downstream of the step brink, the free-jet entrains air at both upper and lower air–water interfaces, as well as along the sides. An air–water shear layer develops at the lower nappe interface. At the lower nappe, the velocity redistribution was successfully modelled and the velocity field was found to be similar to that in two-dimensional wake flow. The results highlighted further two distinct flow regions. Close to the brink (Wex < 5000), the flow was dominated by momentum transfer. Further downstream (Wex > 5000), a strong competition between air bubble diffusion and momentum exchanges took place

Hydraulic structures and society - Engineering challenges and extremes in perspective

A hydraulic structure is an artificial system which interacts with the flow of water. A number of engineering challenges are closely linked to the hydrodynamics and fluid flow motion. Two key issues during the design and operation of hydraulic structures are conveyance and energy dissipation. The energy dissipation at a hydraulic structure can be enormous and its estimate is far from trivial. The re-evaluation of spillway discharge capacity, including the spillway re-design, is a further challenge, especially in the regions with extreme hydrology and limited records. Our community needs to broaden the knowledge base in hydraulic structures, through the development of independent learning skills, further education in hydraulic engineering and innovative research and development (R&D). It is believed that the proceedings of the 5th IAHR International Symposium on Hydraulic Structures (ISHS2014) provide the engineering profession with real-world state-of-the-art expertise in hydraulic structure design.

Hydraulics of stepped chutes: The transition flow. By H. Chanson and Luke Toombes:Reply by the Authors

The Authors are thanked for their thoughtful work that has elucidated the intriguing boundary between free-falling napped flow and skimming flow in stepped chute (or spillway). Their work gives an excellent account of this difficult transition, which has been troubling the Discussers and others with whom they have spoken on the matter. Here the Discussers wish to share some complementary work, which they believe constructively expands on the excellent work of the Authors. An investigation of the nature of the transition from napped flow to skimming flow was undertaken by the Discussers using a simple analytical model. The geometry of the model, which is the same as that used by the Authors, is shown in Fig. 1. The appropriate equations of motion for the flow passed the Nth step based on the control volume of Fig. 1 are the

Characteristics of Free Overfall for Supercritical Flows. Discussion

of the flow patterns were thoroughly documented. The location of the nappe impact was reasonably predicted by a simple trajectory equation (Chanson 1995; Toombes 2002). Downstream of the nappe impact, flow was characterised by a highly fragmented spray. The spray and splashing appeared to be concentrated towards the centreline of the channel. Sidewall standing waves, similar to ship bow waves, formed on both sidewalls downstream of the nappe impact. Properties of these standing waves compared reasonably with the properties of waves observed on the opposite mitre bends and channel junctions, and at abrupt channel expansions (Chanson and Toombes 1997, 1998). The flow was supercritical in the downstream channel. No hydraulic jump was oberved. Shock waves were observed in the downstream channel, originating from the sidewalls at or close to the nappe impact. The angle of the crosswaves was inversely proportional to the inflow Froude number. The shock waves intersected on the channel centreline and continued to propagate towards the opposite wall (Fig. 1). Energy dissipation at the overfall was the result of friction losses, jet disintegration, nappe impact on the downstream invert, and recirculation within the pool of water beneath the free-falling jet. The energy dissipation was roughly equal to the drop height within the range of the experimental flow conditions (Table 1). The discussers are concerned by some broad conclusions contained in the original paper, which ignored the three-dimensional nature of the flow, the existence of sidewall standing waves, and strong splashing and spray in the centre of the downstream channel. These three-dimensional flow patterns have direct implications in terms of channel design. The discussers observed a maximum sidewall standing height y M that satisfied