1. INTRODUCTION 1.1 Presentation

1. INTRODUCTION

1.1 Presentation

In large dams, channels and reservoirs flood release facilities and safe dissipation of the kinetic energy of the flow are necessary. A safe dissipation of the kinetic energy can be achieved by a spillway which is a structure designed to “spill” flood water under controlled conditions. The construction of steps on the spillway is to increase the energy dissipation and to reduce the size of the dissipation system at the downstream end.

Stepped spillways have been used for about 3500 years. Ancient stepped structures are spread around the Mediterranean where the ancient civilisations settled down. The world’s oldest stepped spillway is probably the overflow stepped weir in Akarnanian, Greece, built around 1300 B.C (Figure 1.1).

Figure 1.1- Arkananian stepped weir (Greece 1300 B.C) courtesy of Prof. Knauss

From 16th to 18th century, large stepped cascades comparable to stepped spillways were also built for aesthetic reasons. Figure 1.2 and Figure 1.3 show some examples.

In the first half of the 20th century, stepped spillways were abandoned because of the introduction of concrete as a construction material and the development of hydraulic jump energy dissipator. In the last decades new materials such as reinforced gabions and roller compacted concrete (RCC) were introduced. The introduction of these new materials and the active researches increased the interest in the stepped spillways. Nowadays, the stepped spillways are built for several reasons and purposes: from aesthetic uses (Figure1.4) to civil and environmental engineering applications. Civil engineering applications include dam stepped spillways and stepped gutter along the roads (Figure 1.5) to handle the flood releases and to reduce the impact of the debris flows. Figure 1.6 and Figure 1.7 show the Pedrogao dam which is a RCC gravity dam (H = 43 m, L = 473 m) with an uncontrolled overflow

stepped spillway (h=0.6 m, 1V:0.75H). The dam is equipped also with a fish lock/lift. The reservoir is located immediately downstream of the Alqueva dam which is multipurpose reservoir for irrigation (326 km of open channels, 9 main pump stations).

Environmental engineering applications include stepped cascades utilised in water treatment plants and stepped structures besides rivers for re-aeration.

Figure 1.2- Detailed of water staircases of Touvet Castle (Isère, France 1770) courtesy of Mr F Botton

Figure 1.4- Stepped fountain (Hong Kong) Figure 1.5- Stepped road gutter courtesy of Prof. Chanson courtesy of Prof. Chanson

Figure 1.6B- Pedrogao dam, Moura (Portugal, 2006) courtesy of Prof. Chanson

1.2 Definitions

A stepped chute is an open channel characterised by a series of steps. Different geometry can be adopted and they are presented in Figure 1.7.

Stepped chute flows are characterised by strong air-water mixing. Air bubble entrainment within the flow gives a white appearance to the air-water flow defined as “white waters” and as it is shown in Figure 1.8.

The air entrainment on a stepped spillway is pictured in Figure 1.9.

Figure 1.8- Experimental test section, white waters, looking downstream dc/h=1.45, step edge No.10, single tip probe (Ø=0.35 mm, ∆z=3.6 mm)

Figure 1.9- Flow aeration in skimming flow regime.

Highly aerated flows are characterised by a very complex structure. Traditional instrumentations like point gauge, Pitot tube, LDA/LDA, ADV and PIV cannot be used. Instead the most reliable

paragraph 2.2.2). This type of probe can only detect air-water interfaces along the streamline where they are located but they can not distinguish the difference between air bubbles, free surface water oscillations, droplets or packets. Figure 1.10 presents a sketch of a phase-detection conductivity probe. The term “air bubble” is used to describe air one-dimensional entity surrounded by two air-water interfaces whereas a “water droplet” is water entity bounded by two air-water interfaces. For values of void fraction less than 0.3, the concentration of air is so low that it is reasonable thinking of an air-bubble impacting on the probe. Similarly, for high values of void fraction (C>0.7) it is likely to be a droplet. For 0.3 ≤ C ≤ 0.7 it is hard to define the structure of the flow and any consecutive air-water interfaces detected by the conductivity probe are defined as particles. In Figure 1.11 the void fraction and the velocity distributions are presented.

Bubble chord length and water droplet chord length are defined with cha and chw, respectively.

CHANSON defined bubble chord length as the length of the straight line connecting the two intersection of the air-bubble free-surface with the leading tip of the measurement probe as the bubble is transfixed by the probe tip. A similar definition is used for the droplet chord length.

Figure 1.11- Typical void fraction and velocity distributions at a step edge

Some parameters are commonly introduced to describe the aerated flow properties (WOOD 1991, CHANSON 1997, 2001). The characteristic clear-water flow depth d is defined as:

90

Y

0

d=

_∫

y is the distance measured normal to the bottom; C is the void fraction;

Y90 is the distance where C=90%.Y90 characterises the air-water flow thickness.

The depth flow mean air concentration Cmean is defined as:

Cmean=1-d/Y90 (1-2)

The mean flow velocity equals:

w w

U

=

q / d

qw is the water discharge per unit width;

d is the clear-water flow depth (Eq.1-1).

Another important parameter is d·sinθ/h where d is the equivalent clear-water depth, h the vertical step height and θ is the slope. d·sinθ/h represents the ratio of the boundary layer thickness to cavity length.

1.3 Flow regimes

On a stepped spillway three different flow regimes can be observed: nappe, transition and skimming flow regime. For a given geometry, flow regimes depend on the flow rate. At small discharges the waters flow as a succession of free-falling nappes as it is shown in Figure 1.12 (nappe flow). CHAMANI and RAJARATNAM (1994), CHANSON (1994), TOOMBES (2002) are some of the studies concerning the nappe flow regime. CHANSON (1994) suggests that three types of nappe flow can be distinguished: nappe flow with fully developed hydraulic jump at low flow rates (sub-regime NA1), nappe flow with partially-developed hydraulic jump (sub-regime NA2) and nappe flow without hydraulic jump (sub-regime NA3).

Figure 1.12- Nappe flow regime [dc/h=0.26, qw=0.013 m2/s]

For a range of intermediate flow rates, a transition flow regime is observed as it is shown in Figure 1.13. CHANSON and TOOMBES (2004) noted that the flow had a chaotic appearance: it exhibited strong splashing and droplet ejections at any position downstream of the inception point. Small to medium air cavities were observed irregularly: some aeration in the step corners was observed immediately upstream of the inception point of air entrainment and small air cavity could be observed followed by a larger one.

Figure 1.13- Transition flow regime [dc/h=0.70, qw=0.057 m2/s]

Modern stepped spillways are typically designed for large discharges corresponding to a skimming flow regime (RAJARATNAM 1990, CHANSON 1994, CHAMANI and RAJARATNAM 1999). For skimming flows down a stepped chute, the water flows as a coherent stream, skimming over the steps. The main flow passes over the pseudo-bottom formed by the step edges. The flow is non-aerated, smooth and glassy at the upstream end of the chute. Large amount of air and a very strong air-water mixing can be observed downstream the inception point of free-surface aeration. Free-surface aeration occurs when the turbulent shear next to the free-surface becomes larger than the bubble resistance offered by surface tension (e.g CHANSON and TOOMBES 2003, GONZALEZ 2005, GONZALEZ et al. 2005). Recirculating vortices and small-scale vorticity can be observed beneath the pseudo-bottom and at the corner of the steps, respectively. The vortices are maintained by the transmission of the shear stress from the main stream and they represent the main cause of the flow energy dissipation. In fact, the interactions between mixing layer and horizontal step face, the skin friction at the step faces contribute to the energy dissipation. The energy dissipation mechanisms include cavity recirculation, momentum exchange with the stream, interactions between free-surface and mainstream turbulence.

CHANSON (1994) suggested that three types of skimming flow sub-regimes may be found depending on the behaviour of the vortices: unstable recirculation with wake-step interference (SK1), unstable recirculation with wake-wake interaction (SK2) and stable recirculation (SK3).

1.4 Aim and structure of the thesis

The flow on a stepped spillway is characterised by air entrainment and high levels of turbulence. Thus, it is not possible to rely on analytical nor numericalmodels to investigate the flow. An experimental study is necessary to understand the behaviour of the flow and its properties. It is the purpose of this study to investigate the properties of the skimming flows with a focus on the spray region and its features. New measurements were conducted in a large-size facility (θ=22°, h=0.1 m) with several phase-detection intrusive probes. Detailed air-water flow properties were recorded systematically for several flow rates. Table 1.1 summarises the structure of the present study.

Chapter Contents

1 Introduction

Overview of stepped spillways and practical applications, basic terms used in the hydraulics of stepped spillways , flow regimes and summary of the structure of the thesis. 2 Experimental apparatus and instrumentation

Detailed description of the instrumentation: point gauges, single and double conductivity probes, air-bubble detector. Data processing and experimental procedure.

3 Experimental results and macroscopic structure of the air-water flow

Observation classification and investigation of the flow. Results and analyses of the macroscopic properties: void fraction, bubble count rate, interfacial velocity, turbulence intensity and flow resistance.

4 Microscopic structure of the air-water flow

Investigation and analysis of the flow microscopic properties: air bubble/droplets chord length distributions, auto and cross-correlation analyses.

5 Conclusions

Summary of the conclusions achieved with this experimental study

6 Appendices

A. Summary of the air-water flow properties

Complete experimental results of the investigated flows: void fraction and bubble count rate distributions.

B. Comparative performances of two conductivity systems

Systematic comparisons of the results obtained with for dc/h=1.15 a single tip probe

(Ø=0.35 mm) and two double tip probes (Ø=0.25 mm, ∆x=7 mm and ∆x= 9.6 mm). Additional comparison of present results with the data of GONZALEZ et al. (2005) collected with a double-tip probe (Ø=0.025 mm, ∆x=7.74 mm) for dc/h=1.18. Same

patterns in the flow properties distributions are highlighted by the comparisons.

C. Chord size data with single and double-tip conductivity probes

Comparisons between true chord size and pseudo chord size of bubbles and droplets using present data and the data of GONZALEZ et al.(2005). The purpose of the comparisons is to asses the validity of pseudo-chord size and to highlight the effect of probe sensors.

D. Probability distribution functions of true chord sizes

Probability distribution functions of true chord sizes for all flow rates (dc/h=1.15,

dc/h=1.33, dc/h=1.45) investigated with double-tip probes (Ø=0.25 mm, ∆x=7 mm and

∆x= 9.6 mm).

E. Basic results of auto and cross-correlation analyses