Hydrodynamic Forces

IAHR Hydraulic Structures Design Manuals 3

Eduard Naudascher

• Publisher: CRC Press
• Year: 2017
• eBook ISBN: 9781351439909
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Produced for the International Association for Hydraulic Research, this monograph covers fluctuating and mean hydrodynamic forces, hydrodynamic forces on high-head gates, and hydrodynamic forces on low-head gates i.e. only the forces induced by flow incident or past the structure.

CHAPTER 1

Introduction

Among the many forces to which hydraulic structures are exposed, only the forces induced by flow incident or past the structure will be treated in this monograph. The inclusion of such related subjects as forces due to waves or forces due to earthquakes or explosions would have required a separate monograph. The same is true for forces due to fluid-transients such as water hammer. As books are available on these subjects, they were excluded from the present monograph series. A similar restriction was necessary also regarding the types of hydraulic structures. Since the IAHR Hydraulic Structures Design Manual aims at giving information primarily to the civil-engineering designer, such mechanical-engineering structures as hydraulic machinery and equipment are covered only in passing.

Hydrodynamic forces are commonly subdivided into a time-averaged mean part and a fluctuating part. Fluctuating forces can be induced by a variety of basically different excitation mechanisms. As it is impossible to treat these thoroughly without due regard to the structural vibrations which they may induce, and since the vibration or structural response is important to the designer anyhow, a detailed treatment of the subject is left to the IAHR Monograph on flow-induced vibrations. In the following, the designer is merely shown how to assess an engineering system to insure that a possible source for load fluctuation is not overlooked. For this purpose, a survey is given mainly on the following excitation mechanisms (Section 2.2): (1) extraneous excitation due to turbulent flow, two-phase flow, and oscillating flow; (2) excitation due to flow instabilities; (3) self-excitation due to structural movements; and (4) excitation due to resonating fluid oscillators.

Regarding the mean forces, even the modern methods of prediction require supporting data from laboratory experiments or model tests, and these are prone to ‘scale effects’. Although such effects may have little consequence for preliminary hydraulic considerations, they can well be essential in the final stage of design. Data on mean hydrodynamic forces are therefore presented so as to yield guidance with respect to the following major influencing factors or scale effects: (1) geometry and viscosity; (2) confining boundaries and roughness; (3) approach-flow conditions including turbulence; (4) gravity, surface tension, and compressibility; (5) cavitation and aeration; (6) unsteadiness and structural vibration.

Special attention is given to hydrodynamic forces on high-head and low-head gates (Chapters 3 and 4). Again, however, the subject is presented in terms of the basic processes producing these forces (i.e. underflow, overflow, and simultaneous underflow and overflow) rather than in terms of the types of hydraulic structures affected. Although the latter would have been more convenient for the design engineer, the former offers the advantage of wider and deeper coverage as it leads naturally to generalizations and the transfer of information from one field of application to another.

The material presented in each section is amply illustrated with practical examples. These illustrations include the following hydraulic structures:

– Spillways and flip buckets (Figures 2.11, 2.12, 2.13, 2.56, 2.68, 2.70);

– Stilling basins (Figures 2.21, 2.22, 2.23, 2.92);

– Piers and submerged bridges (Figures 2.41, 2.60, 2.61, 2.82, 2.91, 2.93);

– Pipelines and pipe bends (Figures 2.28, 2.64, 2.78);

– Hydraulic machines (Figure 2.10);

– Valves (Figures 2.66, 2.67, 2.99, 2.100, 2.101);

– High-head leaf gates (Figures 3.33, 3.34, 3.38, 3.44, 3.53, 3.56);

– Low-head leaf gates (Figures 2.24, 3.34, 4.49, 4.57, 4.63, 4.69, 4.72, 4.75);

– Cylinder gates (Figures 3.1a, 3.57);

– Tainter gates (Figures 2.25, 2.26, 2.27, 4.70, 4.78);

– Flap and drum gates (Figures 4.46, 4.48, 4.56);

– Miter gates (Figures 2.57, 2.58).

For a more detailed compilation of the hydraulic structures treated, the reader is referred to the subject index at the end of this book. Moreover, the IAHR Editorial Committee of this design manual intends to bring out another series of monographs in the near future which will serve the practising engineer as an application index. In this series, primary emphasis will be given to various kinds of hydraulic structures, and the reader will be aided in the selection of the relevant sections of the present manual to be consulted to insure hydraulically safe designs.

On all subjects of the design manual, including hydrodynamic forces, research is being performed so intensively and new practical experiences are accumulating so rapidly today that it is impossible to present material in a final form. All coleagues working in this field are therefore invited to report new findings from both research and practical experience to the IAHR Editorial Committee (c/o IAHR Secretariat, Delft Hydraulics, Delft, Netherlands), in order that these findings may be incorporated in the revised new editions which are planned to appear in five to ten year intervals.

Publication of the IAHR Hydraulic Structures Design Manual was undertaken with the intent to consolidate hydraulic-design information which is normally scattered among a vast number of journals and books. In spite of such consolidation, the manual will probably be regarded as too extensive. The inevitable, and unfortunate, course of events will be that only a very small part of the information will be extracted, simplified, and reduced to straight-forward ‘design criteria’.

But the extent of the problem is far greater than this. In every one of the monographs of the present series, the authors are cautioning the reader that, despite the narrow definition of the monograph subjects, only a small selection of relevant information can be included, and even that information must be confined within the limits of the present state-of-the-art. Thus, even if a designer were to observe every item presented in the design manual, he could still not be sure that its structure would be completely safe from the hydraulic point of view. And, of course, Hydraulics is by no means the only thing that needs to be considered in the design! Where would we end up if monograph series were prepared for every other technical field involved in the design of hydraulic structures? Are engineers not overburdened with the know-how they need to acquire as matters stand? And then: not a single word has been mentioned about the environmental and social impacts of the engineer’s work. How could we do justice to these important subjects? By means of the computer? I doubt it. If the computer fails to help in relatively simple hydraulic problems, as exemplified in this monograph, how should it ever be able to help us avoid the many unwanted side effects on the environment and on society?

The only possible conclusion appears to the author to be greater self-restraint and modesty in our technical aspirations….

CHAPTER 2

Fluctuating and mean hydrodynamic forces

2.1 INTRODUCTORY REMARKS

2.1.1 Fluctuating and mean components of flow and forces

The hydrodynamic force on a structure immersed in a uniform flow, F, is commonly split into a component acting in the direction of the approach flow and two components acting perpendicular to it. The former is called the drag, F_D, and the one acting perpendicular to the axis of an elongated structure is called lift or side thrust, F_L. Figure 2.1 depicts an example of a cylindrical structure in a uniform cross flow for which the velocity, v, fluctuates due to turbulence. If the flow is steady in the mean $\left(\overline{v}=\text{const}\right),$ one may write

$v_x=\overline{v}+v{\prime }_x,v_y=v{\prime }_y,v_z=v{\prime }_z$

(2.1)

In general, the bar over a symbol denotes the time mean of the respective quantity, i.e.

$\overline{v}=\underset{\text{T}\to \infty }{\lim }\frac{1}{T}{\displaystyle {\int }_0^{T}v\left(t\right)\text{d}t}$

(2.2)

and the prime denotes its fluctuating part. For the turbulent flow shown in Figure 2.1, the velocity fluctu...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Reviewers
• List of Symbols
• 1. Introduction
• 2. Fluctuating and Mean Hydrodynamic Forces
• 3. Hydrodynamic Forces on High-Head Gates
• 4. Hydrodynamic Forces on Low-Head Gates
• References
• Name Index
• Subject Index