Hydraulics
 

 

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What is hydraulics?

Fire has been said to be a good servant, but a bad master.  The same can be said of water. Floods and droughts are periods of human misery, marked by losses of life, property and opportunity. Those countries which have yet to control their waters are typified by endemic diseases, a lack of development and a limited life expectancy.

The basic aim of hydraulics is to understand, and control for the benefit of society, the occurrence, movement and use of water, whether it is in lakes, rivers, pipes, drains, percolating through soils or pounding the coastline as destructive waves.  To modify the behaviour of water calls inevitably for a large investment of time, resources and effort. Thus hydraulic engineering has only appeared once a society is cen­tralised under an organised government.

Hydraulics is often confused with the allied science of fluid mechanics.  Good reason for this exists, since a considerable overlap occurs between the two studies.  However, fluid mechanics deals with gases, as well as the common liquids, and to most civil engineers a study of gas behaviour is irrelevant to their professional needs.

Thus the modern definition of hydraulics limits the subject to the study of water and those other liquids which a civil engineer is called upon to store, convey or pump.

Why do we study hydraulics?

All organised societies need adequate water supplies, drainage to dispose of waste or excess water, as well as the protection from uncontrolled water.  Thus an obvious necessity for a study of hydraulics exists.

However, this self-evident need does tend to persuade many students that hydraulics is separate from, and often more difficult than, the other major civil engineering studies such as:-

·        structural mechanics,

·        structural design and

·        soil mechanics. 

This is an unfortunate belief, and can lead to the attitude that hydraulics is irrelevant to (for example) structural projects.  Yet there are outstandingly successful Roman projects (some of which still function after 1700 years) which have survived because their designers knew that, if water could penetrate into the structures, failure would be inevitable in a few years.

As your studies progress in this area of study, you will become aware of the inter­action of hydraulic factors with other civil engineering studies.  Thus it is important that you realise that hydraulic problems do not occur in isolation and that in professional life it will be essential to integrate hydraulics with other academic subjects.

The development of hydraulics

Hydraulic engineering has been practised for as long ago as written history.

The ancient Egyptians, Babylonians and Chinese constructed canals, dams and devices for lifting water.  Whilst some of these works were very successful, others are known to have failed.  The lack of any comprehensive theory of hydraulics made many of their major hydraulic projects something of a gamble.

By the time Greek civilisation had become established (between 500 and 100 BC) enough information had been collected to divide Hydraulics into Hydrostatics (the study of motionless or static water) and Hydrodynamics (the science of moving water).  In Hydrostatics, the only force acting is the weight of the body of liquid, and with such a simple situation, the Greeks were able to establish almost every rule and application.  The effect of this was the production of a variety of machines operated by water.

Hydrodynamics, however, is made complicated by other forces, and even today it is recognised as a more complex subject to study.  The Greeks, biased as they were against experimentation, were unable to understand its complexity.

The Roman Empire followed the Greek civilisation and was obviously marked by a vast upsurge in hydraulic engineering.  The empire covered much of Europe, Asia and northern Africa, and was studded with public water supply schemes, drainage works and bridges, many of which still stand today.  There is no doubt that the Romans were competent appliers of hydraulics but, despite this, there is evidence that they lacked any deep understanding of the science.  A good example of this is their method of charging for private water supplies to the wealthier citizens.  Their approach was to believe that the flow rate delivered depended only on the diameter of the pipe used for the supply and thus the water charges were based on the pipe size.   Today we appreciate that a flow rate is expressed in units such as cubic metres per second (m3/s) and that it is affected not only by the area open to flow (square metres: m2) but also by the velocity of flow, in metres per second (m/s).

Without a real understanding of what flow actually was, it is not surprising that the Romans contributed little to our understanding of hydrodynamics.  The amazing thing is, that their works proved as successful as they did.

Until the sixteenth century, little real progress occurred.  Then with the Renaissance, it became, for the first time, common for intelligent men to improve their understanding by conducting experiments with real liquid flows. 

This growth of experimental knowledge, combined some 200 years later with a renewed interest in mathematical analysis by such workers as Newton, Pascal and Descartes, led to the start of a well established hydrodynamic theory that we use today.

Since then, hydraulics has developed and has become a more exact science. Perhaps the major cause of this was the Industrial Revolution with its vast demand for water supplies, drainage and water-powered machines.  The businessmen who controlled the new industrialisation demanded that civil engineers should supply exactly the water or drainage or power that was required, and so forced the development of more precise design methods.

Today, hydraulic engineering has reached the stage of confidence that makes it possible to re-channel major rivers, to develop hydroelectric power adequate to supply a small country's needs and to build ports and breakwaters on coasts where it was formerly impossible to dock more than a small canoe.

The difficulties a student encounters

Hydraulics is still divided into the two categories the Greeks recognised, hydrostatics and hydrodynamics.

Hydrostatics is always the first part studied and usually occupies less than one quarter of the total study time.  This is because it deals only with a single type of force due to the weight of the liquid in the tank or behind the dam wall.  It resembles closely the study of solid body mechanics and utilises much the same methods.  For example, it is often necessary in the hydrostatic design of (say) a lock gate to take the moments of the various liquid/fluid forces about a point, just as one would do for loads on a beam in structural mechanics.  Thus any student having difficulties with hydrostatic problems would be well advised to revise the basics of applied mechanics.

Hydrodynamics is the largest and certainly the most interesting part of hydraulics.  It does, however, create real problems for some students, who do not recognise that the laboratory experiments are as necessary to an overall understanding as is the published theory.

Hydrodynamics is a complex subject, and everyday life offers very little opportunity to become familiar with some of its important effects.  For example, the boundary layers, total energy and hydraulic grade lines in pipe flow, the variable depths of flow that can occur in open channels and the remarkably high, yet short-lived, pressures that occur when a valve on a pipeline is closed rapidly, are all difficult to understand unless one can visualise them. 

Laboratory experiments (Photographs) and demonstrations are the only possible chance for this.  Thus laboratory periods are at least as important as theoretical classes and should always be taken as the opportunity to understand what is happening in the fluid system.

The other problem that students commonly encounter with hydrodynamics is that of integrating the various sub-topics that are covered, such as the Continuity Equation with the Bernoulli Equation.

The only reason for studying hydraulics is surely to be able to apply the subject matter to real life problems and that requires, above all else, an overall grasp of the material.  Details of the specific formula for (say) the flow rate over a particular type of weir can always be found in any standard text book.  What no book can give you is a personal integration of the subject which will allow you to identify what analytical or design technique is necessary in a particular part of a hydraulic problem.

If these two common mistakes are avoided, and if it is always realised that hydraulics is an integral part of civil engineering, the subject can usually be studied with, at least, a fair degree of enjoyment and certainly with a sense of personal achievement.

The ability to control water for the advantage of the community has historically been highly prized and even today this is still the case.

The relative imprecision of hydrodynamics

Hydrostatics, is an exact science.

In contrast, hydrodynamic problems can be analysed only to an approximate accuracy, and the examples given to you in your studies will, to some degree, be of situations simplified from reality. This will be most apparent in the treatment of open channel flow where the bias will be to man-made channels of regular geometric cross-sections, such as rectangular or trapezoidal.

Natural river channels, whose cross-sectional areas and bed roughnesses vary significantly from place to place on the river, are simply too complex for the theory that is available.  The reason for this relative imprecision is that additional forces appear in fluid motion and these prevent an engineer having the same detailed knowledge as they have in hydrostatic problems.  As a result, the theory available in hydrodynamics often has to be supplemented by experimental evidence and the introduction of coefficients of discharge and friction factors.  This point will no doubt be stressed by your tutor and other texts which you should read.

Despite this difficulty of analysing detail, it should not be believed that major engineering works cannot be safely and precisely designed.  The available theory applied with sound judgement and commonsense is adequate, and many practising engineers prefer hydrodynamic to hydrostatic design, simply because it does offer the opportunity for personalised judgement.

The reason, above all others, for the difficulty in obtaining precise details of a hydrodynamic problem is the existence of a property called viscosity which is possessed by all fluids to some extent.

Viscosity is the ability of a fluid to stick to solid surfaces (the walls of a pipe, the bed of a channel, the edges of a bridge pier, the outer skin of a car or aircraft etc.) and to exert a drag on them, which in turn has to be overcome by the using up of some of the energy in the fluid or some of the energy in forcing a body through a fluid.

Very viscous liquids, such as tars, oils, glues, paints and treacle, are thick, slow-flowing fluids which cling to any solid surface.  Anyone who has stirred a tin of paint will realise how difficult it is to make the paint run completely off the stirring rod.  As a matter of interest, the most viscous fluid known to man is glass.  In other words, glass is not a solid but a very viscous fluid.

Other fluids, such as air and water, appear not to show this effect, but even these low-viscosity fluids exert a drag on solid boundaries.  If this were not so, then balloons and boats would remain stationary in a wind or in a moving river, without the need for anchors.

The practical effect of viscosity, which of course does not occur in hydrostatic problems where no relative movement between the fluid and its container takes place, is that the elements of the fluid closest to the solid boundaries are slowed down by viscous drag on the boundaries.  This produces the situation where fluid particles at increasing distance away from the boundaries move at greater and greater velocities.  The zone of near stationary fluid against the solid surface is termed the boundary layer and can vary from a few millimetres to several metres in thickness, depending on the fluid's viscosity and the roughness of the solid surface.

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Last Edited :  04 August 2011 13:34:40