Hubert Chanson

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.

Experimental Study of the Air-Water Shear Flow in a Hydraulic Jump

Although the hydraulic jump has been investigated experimentally for nearly two centuries, little information is known of the air-water flow properties in the shear region. New experiments were performed in a horizontal channel with partially-developed inflow conditions. Distributions of air concentration, mean air-water velocity and bubble frequency were recorded and presented herein. The results indicate an advective diffusion of air in the shear layer. The velocity profiles have a similar shape as wall jet flows but different quantitative parameters must be introduced. The relationship between air content and bubble frequency has a parabolic shape which is not yet understood but was observed previously in open channel flows.

Hydraulics of Skimming Flows over Stepped Channels and Spillways

Stepped chutes have become recently a popular method for discharging flood waters. The steps increase significantly the rate of energy dissipation taking place on the spillway face and reduce the size of the required downstream energy dissipation basin. This paper reviews the hydraulic characteristics of skimming flows. The onset of skimming flows is discussed. New results are presented to estimate the flow resistance along stepped chutes. The study indicates some major difference between the flow patterns on steep and flat stepped chutes. An analogy with flow over large roughness is developed. Then the calculations of the start of air entrainment are detailed. The results indicate that free surface aeration occurs much more upstream than on smooth spillways. The effects of flow aeration are later discussed.

Turbulence measurements in positive surges and bores

A positive surge results from a sudden change in flow that increases the flow depth. New experiments were conducted in a large channel. Most positive surge tests were conducted with a horizontal bed slope, a constant flow rate and uncontrolled flow conditions. The only dependant variable was the downstream gate opening after closure. Detailed turbulence measurements were performed with high-temporal resolution using side-looking acoustic Doppler velocimetry. Two types of positive surge were observed: undular surge for Froude numbers less than 1.7, and weak (breaking) surges above. Instantaneous velocity measurements beneath advancing surges showed a marked effect of the surge passage on the velocity field. Streamwise velocities showed rapid flow deceleration at all vertical elevations. Large fluctuations of longitudinal and transverse velocities were recorded beneath the surges, including some unsteady flow recirculation beneath a weak surge front. Turbulent stresses were deduced from high-pass filtered data. The results showed large normal and tangential Reynolds stresses beneath the surges. A comparison between undular and weak surges suggested some major difference. In weak surge flows, the data showed rapid flow separation beneath the surge front. In undular surges, maximum Reynolds stresses were observed beneath and just before each wave crest behind the leading wave.

Air Entrainment in the Developing Flow Region of Plunging Jets - Part 1: Theoretical Development

Air-water bubbly flows are encountered in many engineering applications. One type of air-water shear flows is the developing flow region of a plunging jet. The mechanisms of air entrainment by plunging liquid jets are discussed in the light of new experimental evidence. Then the air bubble diffusion is analysed analytically in the near-flow field of both circular and two-dimensional plunging jets. The theoretical developments are compared with existing circular plunging jet data and new experiments performed with a two-dimensional vertical supported jet. The study highlights two mechanisms of air entrainment at the plung point depending upon the jet impact velocity and results suggest that the dispersion of air bubbles within the shear layer is primarily an advective diffusion process.

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.

Air Entrainment in the Developing Flow Region of Plunging Jets - Part 2: Experimental

When a water jet impinges a pool of water at rest, air bubbles may be entrained and carried away below the pool free-surface; this process is called plunging jet entrainment. The study presents new experimental data obtained in a vertical supported jet. Distributions of air concentration and mean air-water velocity, and bubble chord length distributions measured in the developing shear layer are presented. The results indicate that the distributions of void fraction follow closely analytical solution of the diffusion equation. Further, the momentum shear layer and the air bubble diffusion layer do not coincide. Chord length data show a wide range of air bubble sizes and overall the experimental results suggest strong interactions between the entrained air bubbles and the momentum transfer mechanisms.

Similitude and scale effects of air entrainment in hydraulic jumps

A hydraulic jump is characterized by some strong turbulence and air entrainment in the roller. New measurements were performed in two channels in which similar experiments with identical inflow Froude numbers and relative channel widths were conducted with a geometric scaling ratio of 2:1. Void fraction distributions showed the presence of an advection/diffusion shear layer in which the data followed an analytical solution of the diffusion equation for air bubbles. The data indicated some scale effects in the small channel in terms of void fraction and bubble count rate. Void fraction distributions implied comparatively greater detrainment at low Reynolds numbers yielding to lesser overall aeration of the jump roller. Dimensionless bubble count rates were significantly lower in the smaller channel especially in the mixing layer. The study is believed to be the first systematic investigation of scale effects affecting air entrainment in hydraulic jumps using an accurate air–water measurement technique.

Air Entrapment and Air Bubble Dispersion at Two-Dimensional Plunging Water Jets

Air-water bubbly flows are encountered in numerous engineering applications. One type of air-water shear flow, the developing flow region of a plunging jet, is discussed in the light of new experimental evidence. Distributions of air concentration and mean air-water velocity, and bubble chord length distributions are presented for inflow velocities ranging from 2 to 8 m/s. The results indicate that the distributions of void fraction follow closely analytical solutions of the diffusion equation, as developed by CHANSON (1995a,1997). In air-water shear layers, the velocity distributions have the same shape as in monophase flows but the characteristic parameters of the shear layer differ from monophase flow results, because of the interactions between the entrained air bubbles and the turbulence.

Air entrainment in two-dimensional turbulent shear flows with partially developed inflow conditions

Abstract In plunging jet flows and at hydraulic jumps, large quantities of air are entrained at the intersection of the impinging flow and the receiving body of water. The air bubbles are entrained into a turbulent shear layer and strong interactions take place between the air bubble advection/diffusion process and the momentum shear region. New air-water flow experiments were conducted with two free shear layer flows: a vertical supported jet and a horizontal hydraulic jump. The inflows were partially developed boundary layers, characterized by the presence of a velocity potential core next to the entrapment point. In both cases, the distributions of air concentration exhibit a Gaussian distribution profile with an exponential longitudinal decay of the maximum air content. Interestingly, the location of the maximum air content and the half-value band width are identical for both flow situations, i.e. independent of buoyancy effects.