Flow Transition Design in Hydraulic Structures

S.K. Mazumder

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9780367236380
• eBook ISBN: 9781000768558
• Topic: Biological Sciences
• Subtopic: Environmental Science
• Index: Biological Sciences

Transitions are provided in hydraulic structures for economy and efficiency. This book covers all types of flow transitions: sub-critical to sub-critical, sub-critical to super critical, super-critical to sub-critical with hydraulic jump, and super-critical to super-critical transitions. It begins with an introduction followed by characteristics of flow in different types of transitions and procedures for hydraulic design of transitions in different structures. Different types of appurtenances used to control flow separation and ensure uniform flow at exit of transition and diffusers are included. Examples of hydraulic design of a few typical hydraulic structures are given as well.

1

Introduction

1.1 Definition

Transitions may be broadly defined as that portion of a nonuniform channel undergoing a change in the normal prismatic section. In open channel and closed conduit flows, there are situations when the normal section of flow is to be restricted. In falls, aqueducts, siphons, super-passages, bridges, and flumes, and in many other similar hydraulic structures, the original section of flow is often reduced in order to economize the construction cost. In another example, fluming of normal flow section offers an expedient device for measurement of discharge in e.g. Parshall (1926, 1950) flumes, critical flow meters (Mazumder & Deb Roy, 1999), and Venturi meters (Mazumder, 1966a). All flow transitions bring about a change in depth and mean velocity of flow. To smoothly guide the flow from the normal wider section to the contracted narrow section (also called flumed section or simply flume), it is customary to provide a pair of transitions known as inlet or contracting transitions. Similarly, a pair of outlet or expanding transitions is provided to connect the flumed contracted section to the normal wider section to ensure smooth flow conditions. Figure 1.1 illustrates schematically the inlet and outlet transitions and flumed section in a weir with mean width (B) at entry, flumed width (b), depth of flow (y₁) at entry, critical depth (y_c) at flumed section, upstream specific energy of flow (E_f1), downstream flow depth (y₂), downstream specific energy (E_f2), and different head losses (H_L).

FIGURE 1.1
Inlet and outlet transitions and flumed section in a typical weir with hydraulic jump indicating head loss at entry (H_Li), head loss at exit (H_Lo), head loss in jump (H_Lj), and total head loss (∑H_L).

1.2 Necessity

Transitions are useful for reducing loss in head and to ensure that the flow in the flumed section is smooth without any undue disturbances in flow upstream and downstream of transitions. In unlined canals, if suitable transitions are not provided, the vortices created at the entry and exit of the flumed section create not only head losses but also lead to flow nonuniformity resulting in nonuniform distribution of velocity and scouring, which requires costly protective measures. For example, when the normal waterway is restricted under a bridge (Mazumder et al., 2002) or in an aqueduct to economize the cost of construction of the bridge or the aqueduct, it is obligatory to provide suitable transitions at the entry and exit of the structure to reduce head losses and scour. The more the loss in head, the higher the afflux, which creates a lot of problems such as sediment deposition, nonuniformity of flow, and flow instability, thus requiring additional protective measures such as guide bunds, embankments, spurs, and pitching, which are expensive. Proper transitions ensure that the flow is normal within the flumed section, inside and outside the transitions. In an intake structure of hydropower plants, if suitable transitions are not designed, there will be a lot of problems such as vibration and cavitations, which will affect the performance of the power plant including its ultimate failure or reduction in life and high maintenance cost. In the desilting devices for hydroelectric plants, suitable transitions at entry and exit improve the performance of the structure ensuring greater efficiency and elimination of objectionable sediments causing damage to the water conductor system, turbines, and the diffusers.

1.3 Classification

Owing to the diversity of functions which hydraulic transitions are made to serve, a satisfactory scheme of classification is beset with many difficulties. In order to arrive at a general grouping of the various types of transitions, it is better to first discuss a few of their functions:

• Metering of flow e.g. in weirs, gates, venture-type flumes, and standing wave flumes
• Energy dissipation e.g. in drop structures
• Reduction of flow section e.g. in aqueducts and siphons
• Reduction of flow velocity to prevent scouring e.g. in flow diffusers
• Increase of flow velocity to prevent shoaling e.g. in river training devices
• Minimization of head loss in order to reduce afflux e.g. in bridges and culverts
• Minimization of head loss e.g. in power canals and conduits so that the power plant has more output of energy.

Generally speaking, flumes may be divided into two main classes, which are described as follows:

• Class I: Flumes with free water surface open to atmosphere with open-channel transitions.
  Pressure on the surface is constant and more or less hydrostatically distributed varying along the depth with maximum pressure near bed.
• Class II: Flumes with sealed water surface where roof is under pressure with closed conduit transitions such as confusers and diffusers.

Class I open-channel transitions can be further subdivided into the following three groups as per geometry of transition:

1. By changing width without changing bed level i.e. change in horizontal planes only. Venturi-type flumes, aqueduct transitions, etc. fall under this category.
2. By varying bed level without changing width i.e. change in vertical planes only as in case of weirs, spillways, etc.
3. By simultaneously varying both bed width and bed level i.e. change in both horizontal and vertical planes (Figure 1.1). Transitions in case of standing wave flumes, siphons, siphon aqueducts, etc. belong to this category.

Class I transitions may be further subdivided according to flow regimes in open channel, namely, subcritical and supercritical flow, as follows:

• Transition from subcritical to another subcritical flow as in aqueduct and siphon
• Transit...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Author
• 1 Introduction
• 2 Transition Flow Characteristics
• 3 Different Methods of Hydraulic Design of Flow Transitions
• 4 Appurtenances for Economic and Efficient Design of Transition Structures
• 5 Illustrative Designs of Flow Transitions in Hydraulic Structures
• Index