Fluid Mechanics in Civil/Structural Engineering

 1. Introduction

1.1 Course Outline

Goals

The goal is that you will:

1. Have fundamental knowledge of fluids:

a. compressible and incompressible;

b. their properties, basic dimensions and units;

2. Know the fundamental laws of mechanics as applied to fluids.

3. Understand the limitations of theoretical analysis and the determination of correction factors, friction factors, etc from experiments.

4. Be capable of applying the relevant theory to solve problems.

Syllabus

Basics:

• Definition of a fluid: concept of ideal and real fluids, both compressible and incompressible.

• Properties of fluids and their variation with temperature and pressure and the dimensions of these properties.

Hydrostatics:

• The variation of pressure with depth of liquid.

• The measurement of pressure and forces on immersed surfaces.

Hydrodynamics:

• Description of various types of fluid flow; laminar and turbulent flow; Reynolds’s number, critical Reynolds’s number for pipe flow.

• Conservation of energy and Bernoulli’s theorem. Simple applications of the continuity and momentum equations.

• Flow measurement e.g. Venturi meter, orifice plate, Pitot tube, notches and weirs.

• Hagen-Poiseuille equation: its use and application.

• Concept of major and minor losses in pipe flow, shear stress, friction factor, and friction head loss in pipe flow.

• Darcy-Weisbach equation, hydraulic gradient and total energy lines. Series and parallel pipe flow.

• Flow under varying head.

• Chezy equation (theoretical and empirical) for flow in an open channel.

• Practical application of fluid mechanics in civil engineering.

1.2 Programme

Lectures

There are 4 hours of lectures per week. One of these will be considered as a tutorial class – to be confirmed.

The lectures are:

• Monday, 11:00-12:00, Rm. 209 and 17:00-18:00, Rm 134;

• Wednesday, to be confirmed.

Assessment

The marks awarded for this subject are assigned as follows:

• 80% for end-of-semester examination;

• 20% for laboratory work and reports.

1.3 Reading Material

Lecture Notes

The notes that you will take in class will cover the basic outline of the necessary

ideas. It will be essential to do some extra reading for this subject.

Obviously, only topics covered in the notes will be examined. However, it often aids

understanding to hear/read different ways of explaining the same topic.

Books

Books on Fluid Mechanics are kept in Section 532 of the library. However, any of these books should help you understand fluid mechanics:

• Douglas, J.F., Swaffield, J.A., Gasiorek, J.M. and Jack, L.B. (2005), Fluid Mechanics, 5th Edn., Prentice-Hall.

• Massey, B. and Ward-Smith, J. (2005), Mechanics of Fluids, 8th Edn., Routledge.

• Chadwick, A., Morfett, J. and Borthwick, M. (2004), Hydraulics in Civil and Environmental Engineering, 4th Edn., E & FN Spon.

• Douglas, J.F. and Mathews, R.D. (1996), Solving Problems in Fluid Mechanics, Vols. I and II, 3rd Edn., Longman.

The Web

There are many sites that can help you with this subject. In particular, there are

pictures and movies that will aid your understanding of the physical processes behind

the theories.

If you find a good site, please let me know and we will develop a list for the class.

1.4 Fluid Mechanics in Civil/Structural Engineering

Every civil/structural engineering graduate needs to have a thorough understanding of fluids. This is more obvious for civil engineers but is equally valid for structural

engineers:

• Drainage for developments;

• Attenuation of surface water for city-centre sites;

• Sea and river (flood) defences;

• Water distribution/sewerage (sanitation) networks;

• Hydraulic design of water/sewage treatment works;

• Dams;

• Irrigation;

• Pumps and Turbines;

• Water retaining structures.

• Flow of air in/around buildings;

• Bridge piers in rivers;

• Ground-water flow.

As these mostly involve water, we will mostly examine fluid mechanics with this in mind.

Remember: it is estimated that drainage and sewage systems – as designed by civil

engineers – have saved more lives than all of medical science. Fluid mechanics is

integral to our work.

2. Introduction to Fluids

2.1 Background and Definition

Background

• There are three states of matter: solids, liquids, and gases.

• Both liquids and gases are classified as fluids.

• Fluids do not resist a change in shape. Therefore fluids assume the shape of the container they occupy.

• Liquids may be considered to have a fixed volume and therefore can have a free surface. Liquids are almost incompressible.

• Conversely, gases are easily compressed and will expand to fill a container they occupy.

• We will usually be interested in liquids, either at rest or in motion.

The behaviour of fluids in containers

Definition

The strict definition of a fluid is:

A fluid is a substance which conforms continuously under the action of

shearing forces.

To understand this, remind ourselves of what a shear force is:

Application and effect of shear force on a book

Definition Applied to Static Fluids

According to this definition, if we apply a shear force to a fluid it will deform and

take up a state in which no shear force exists. Therefore, we can say:

If a fluid is at rest there can be no shearing forces acting and therefore all

forces in the fluid must be perpendicular to the planes in which they act.

Note here that we specify that the fluid must be at rest. This is because it is found

experimentally that fluids in motion can have a slight resistance to shear force. This is

the source of viscosity.

Definition Applied to Fluids in Motion

For example, consider the fluid shown flowing along a fixed surface. At the surface

there will be little movement of the fluid (it will ‘stick’ to the surface), whilst further

away from the surface, the fluid flows faster (has greater velocity):

Definition Applied to Fluids in Motion

If one layer of is moving faster than another layer of fluid, there must be shear forces

acting between them. For example, if we have fluid in contact with a conveyor belt

that is moving we will get the behaviour shown:

Ideal fluid

When fluid is in motion, any difference in velocity between adjacent layers has the

the same effect as the conveyor belt does.

Therefore, to represent real fluids in motion we must consider the action of shear

forces.

Fluid Mechanics

Consider the small element of fluid shown, which is subject to shear force and has a

dimension s into the page. The force F acts over an area A = BC×s. Hence we have a

shear stress applied:

Stress(τ) = Force(F)/Area(A)

Any stress causes a deformation or strain, and a shear stress causes a shear strain.

This shear strain is measured by the angle φ.

Remember that a fluid continuously deforms when under the action of shear. This is

different to a solid: a solid has a single value of φ for each value of τ . So the longer

a shear stress is applied to a fluid, the more shear strain occurs. However, what is

known from experiments is that the rate of shear strain (shear strain per unit time) is

related to the shear stress:

Shear stress ∝ Rate of shear strain

Shear stress = Constant X Rate of shear strain

We need to know the rate of shear strain. From the diagram, the shear strain is:

φ = X/Y

If we suppose that the particle of fluid at E moves a distance x in time t, then, using

S = Rθ for small angles, the rate of shear strain is:

Where u is the velocity of the fluid. This term is also the change in velocity with

height. When we consider infinitesimally small changes in height we can write this in

differential form, du/dy. Therefore we have:

This constant is a property of the fluid called its dynamic viscosity (dynamic because

the fluid is in motion, and viscosity because it is resisting shear stress). It is denoted

μ which then gives us:

Newton’s Law of Viscosity:

Generalized Laws of Viscosity

We have derived a law for the behaviour of fluids – that of Newtonian fluids.

However, experiments show that there are non-Newtonian fluids that follow a generalized law of viscosity:

Where A, B and n are constants found experimentally. When plotted these fluids show much different behaviour to a Newtonian fluid:

The behaviour of Fluids and Solids

In this graph, the Newtonian fluid is represented by a straight line, the slope of which is

μ. Some of the other fluids are:

• Plastic: Shear stress must reach a certain minimum before flow commences.

• Pseudo-plastic: No minimum shear stress necessary and the viscosity decreases with the rate of shear, e.g. substances like clay, milk and cement.

• Dilatant substances; Viscosity increases with rate of shear, e.g. quicksand.

• Viscoelastic materials: Similar to Newtonian but if there is a sudden large change in shear they behave like plastic.

• Solids: Real solids do have a slight change of shear strain with time, whereas ideal solids (those we idealise for our theories) do not. Lastly, we also consider the ideal fluid. This is a fluid which is assumed to have no viscosity and is very useful for developing theoretical solutions. It helps achieve some practically useful solutions.

2.2 Units

Fluid mechanics deals with the measurement of many variables of many different

types of units. Hence we need to be very careful to be consistent.

Dimensions and Base Units

The dimension of a measure is independent of any particular system of units. For

For example, velocity may be in metres per second or miles per hour, but dimensionally,

it is always length per time or L T = LT−1. The dimensions of the relevant base units

of the Système International (SI) system are:

Système International

Derived Units

From these, we have some relevant derived units (shown on the next page).

Checking the dimensions or units of an equation is very useful to minimize errors.

For example, if when calculating a force and you find a pressure then you know

you’ve made a mistake.

SI Unit

Note: The acceleration due to gravity will always be taken as 9.81 m/s2.

SI Prefixes

SI units use prefixes to reduce the number of digits required to display a quantity.

The prefixes and multiples are:

SI Prefixes

Be very particular about units and prefixes. For example:

• kN means kilo-Newton, 1000 Newtons;

• Kn is the symbol for knots – an imperial measure of speed;

• KN has no meaning;

• kn means kilo-nano – essentially meaningless.

Further Reading

• Sections 1.6 to 1.10 of Fluid Mechanics by Cengel & Cimbala.

2.3 Properties

Further Reading

Here we consider only the relevant properties of fluids for our purposes. Find out

about surface tension and capillary action elsewhere. Note that capillary action only

features in pipes of ≤ 10 mm diameter.

Mass Density

The mass per unit volume of a substance, usually denoted as ρ . Typical values are:

• Water: 1000 kg/m3;

• Mercury: 13546 kg/m3;

• Air: 1.23 kg/m3;

• Paraffin: 800 kg/m3.

Specific Weight

The weight of a unit volume a substance, usually denoted as γ . Essentially density

times the acceleration due to gravity:    γ = ρ g

Relative Density (Specific Gravity)

A dimensionless measure of the density of a substance with reference to the density

of some standard substance, usually water at 4°C:

relative density = (density of substance/density of water) = (specific weight of substance/specific weight of water)

Bulk Modulus

In analogy with solids, the bulk modulus is the modulus of elasticity for a fluid. It is the ratio of the change in unit pressure to the corresponding volume change per unit volume, expressed as:

(Change in Volume/Original Volume) = (Change in pressure/Bulk Modulus)

In which the negative sign indicates that the volume reduces as the pressure

increases. The bulk modulus changes with the pressure and density of the fluid, but

for liquids can be considered constant for normal usage. Typical values are:

• Water: 2.05 GN/m3;

• Oil: 1.62 GN/m3.

The units are the same as those of stress or pressure.

Viscosity

The viscosity of a fluid determines the amount of resistance to shear force.

Viscosities of liquids decrease as temperature increases and are usually not affected

by pressure changes. From Newton’s Law of Viscosity: