Self-Aeration and Turbulence in a Stepped Channel: Influence of Cavity Surface Roughness

The strong interactions between free-surface flows and atmospheric surroundings may lead to substantial air-water mixing with void fractions ranging from zero in clear-water to 100%. In this study, the air-water flow properties were studied in a large stepped water channel operating at large Reynolds numbers. Interactions between free-surface and cavity recirculation were systematically investigated in the skimming flow regime. Some surface roughness was introduced on the cavity walls and identical experiments were performed with several configurations. Basic results demonstrated some influence of step surface roughness on the flow properties leading to some counter-intuitive finding. The presence of cavity roughness was associated with higher flow velocities and comparatively lower turbulence levels. Distributions of bubble/droplet chords spanned over several orders of magnitude without significant influence of the cavity roughness. The distributions of turbulence levels and bubble count rates showed some correlation and highlighted strong interactions between entrained particles (bubbles, drops) and the flow turbulence

Surface Waves and Roughness in Self-Aerated Supercritical Flow

In high-velocity open channel flows, free-surface aeration is commonly observed. The effects of surface waves on the air-water flow properties are tested herein. The study simulates the air-water flow past a fixed-location phase-detection probe by introducing random fluctuations of the flow depth. The present model yields results that are close to experimental observations in terms of void fraction, bubble count rate and bubble/droplet chord size distributions. The results show that the surface waves have relatively little impact on the void fraction profiles, but that the bubble count rate profiles and the distributions of bubble and chord sizes are affected by the presence of surface waves

Bubble Entrainment, Spray and Splashing at Hydraulic Jumps

The sudden transition from a high-velocity, supercritical open channel flow into a slow-moving sub-critical flow is a hydraulic jump. Such a flow is characterised by a sudden rise of the free-surface, with some strong energy dissipation and air entrainment, waves and spray. New two-phase flow measurements were performed in the developing flow region using a large-size facility operating at large Reynolds numbers. The experimental results demonstrated the complexity of the flow with a developing mixing layer in which entrained bubbles are advected in a high shear stress flow. The relationship between bubble count rates and void fractions was non-unique in the shear zone, supporting earlier observations of some form of double diffusion process between momentum and air bubbles. In the upper region, the flow consisted primarily of water drops and packets surrounded by air. Visually significant pray and splashing were significant above the jump roller. The present study is the first comprehensive study detailing the two-phase flow properties of both the bubbly and spray regions of hydraulic jumps, a first step towards understanding the interactions between bubble entrainment and droplet ejection processes

Turbulence and aeration in hydraulic jumps: free-surface fluctuation and integral turbulent scale measurements

In an open channel, a change from a supercritical to subcritical flow is a strong dissipative process called a hydraulic jump. Herein some new measurements of free-surface fluctuations of the impingement perimeter and integral turbulent time and length scales in the roller are presented with a focus on turbulence in hydraulic jumps with a marked roller. The observations highlighted the fluctuating nature of the impingement perimeter in terms of both longitudinal and transverse locations. The results showed further the close link between the production and detachment of large eddies in jump shear layer, and the longitudinal fluctuations of the jump toe. They highlighted the importance of the impingement perimeter as the origin of the developing shear layer and a source of vorticity. The air–water flow measurements emphasised the intense flow aeration. The turbulent velocity distributions presented a shape similar to a wall jet solution with a marked shear layer downstream of the impingement point. The integral turbulent length scale distributions exhibited a monotonic increase with increasing vertical elevation within 0.2 < Lz/d1 < 0.8 in the shear layer, where Lz is the integral turbulent length scale and d1 the inflow depth, while the integral turbulent time scales were about two orders of magnitude smaller than the period of impingement position longitudinal oscillations

Free-Surface Instabilities in High-Velocity Air-Water Flows down Stepped Chutes

Recent advances in technology have permitted the construction of large dams and spillways. One type, the stepped spillway, is designed to spill floods over the chute with substantial regular energy losses. This article presents an experimental investigation of free-surface instabilities within the non-aerated and aerated region of a stepped spillway. Intense splashing and ejection of water droplets are characteristics of the transition flow regime and typically follow the primary breakup of the liquid phase. These formation processes might be of particular interest concerning the dimensioning of stepped chute sidewalls. The current study determines velocities of evolving liquid ligaments by means of high-speed video analysis. The video sequences are recorded within the upper transition flow sub-regime. To minimise the influence of sidewall effects, the camera is focussed on a vertical plane inside the channel. Simultaneously, unsteady water surface elevations are measured with a series of acoustic displacement meters mounted alongside the flume, perpendicular to the pseudo bottom formed by the step edges. A correlation analysis is performed on the output of the acoustic sensors in order to determine characteristic time scales of surface fluctuations as well as celerities of liquid structures next to the free-surface. The turbulent fluctuations of the free-surface increase rapidly with further distance from the inception point of self-aeration. This is likely to be associated with enhanced air entrainment and equally increased amount of air-water ejections above the aerated flow region, showing the occurrence of strong hydrodynamic fluctuations within the transition flow regime. The present investigation emphasises the feasibility of using high-speed video analysis to provide relevant flow information next to the sidewall of spillway models

Discussion of "Hydraulics of broad-crested weirs with varying side slopes" by J. E. Sargison and A. Percy

The hydraulics of broad-crested weirs is influenced by the weir inflow design. It is highlighted herein that the inflow geometry including the rounding of the weir upstream edge has a marked effect on the flow pattern and discharge coefficient. In the case of an upstream vertical wall, the optimum design includes a rounded upstream corner (Harrison 1967, Bos 1976, Montes 1998). An upstream side slope may provide an alternative design for embankment structure although with a lower discharge coefficient (Sargison and Percy 2009)

Connexin communication compartments and wound repair in epithelial tissue

Epithelial tissues line the lumen of tracts and ducts connecting to the external environment. They are critical in forming an interface between the internal and external environment and, following assault from environmental factors and pathogens, they must rapidly repair to maintain cellular homeostasis. These tissue networks, that range from a single cell layer, such as in airway epithelium, to highly stratified and differentiated epithelial surfaces, such as the epidermis, are held together by a junctional nexus of proteins including adherens, tight and gap junctions, often forming unique and localised communication compartments activated for localised tissue repair. This review focuses on the dynamic changes that occur in connexins, the constituent proteins of the intercellular gap junction channel, during wound-healing processes and in localised inflammation, with an emphasis on the lung and skin. Current developments in targeting connexins as corrective therapies to improve wound closure and resolve localised inflammation are also discussed. Finally, we consider the emergence of the zebrafish as a concerted whole-animal model to study, visualise and track the events of wound repair and regeneration in real-time living model systems

Minimum Specific Energy and Critical Flow Conditions in Open Channels

In open channels, the relationship between the specific energy and the flow depth exhibits a minimum, and the corresponding flow conditions are called critical flow conditions. Herein they are re-analysed on the basis of the depth-averaged Bernoulli equation. At critical flow, there is only one possible flow depth, and a new analytical expression of that characteristic depth is developed for ideal-fluid flow situations with non-hydrostatic pressure distribution and non-uniform velocity distribution. The results are applied to relevant critical flow conditions : e.g., at the crest of a spillway. The finding may be applied to predict more accurately the discharge on weir and spillway crests

Hydraulic engineering in the 21st century: Where to?

For centuries, hydraulic engineers were at the forefront of science. The last forty years marked a change of perception in our society with a focus on environmental sustainability and management, particularly in developed countries. Herein, the writer illustrates his strong belief that the future of hydraulic engineering lies upon a combination of innovative engineering, research excellence and higher education of quality. This drive continues a long tradition established by eminent scholars like Arthur Thomas IPPEN, John Fisher KENNEDY and Hunter ROUSE

Hydraulic engineering legends Listed on the Eiffel Tower

While the Eiffel Tower has become a landmark of Paris and France, few know about the names of 72 scientists engraved around the first floor. Herein, the names of 14 hydraulic engineers and scholars are reviewed and their selection is discussed. It is shown that most were leading engineers and lecturers during the French Revolution and early 19th century, and Gustave Eiffel's selection highlighted the influence of leading engineers on the French Society