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**FLUID MECHANICS FOR CIVIL ENGINEERING Chapter 3: Basic of Fluid Flow**

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**SEQUENCE OF CHAPTER 3 Introduction Objectives**

3.1 Uniform Flow, Steady Flow 3.1.1 Laminar, Turbulent Flow 3.1.2 Relative Motion 3.1.3 Compressible or Incompressible 3.1.4 One, Two or Three-dimensional Flow 3.1.5 Streamlines 3.1.6 Streamtubes 3.2.1 Mass Flow Rate 3.2.2 Volume Flow Rate 3.3 The Fundamental Equations of Fluid Dynamics 3.3.1 Continuity (Principle of Conservation of Mass) 3.3.2 Work and Energy (Principle of Conservation of Energy) Summary

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Introduction Discusses the analysis of fluid in motion: fluid dynamics. When a fluid flows through pipes and channel or around bodies such as aircraft and ships, the shape of the boundaries, the externally applied forces and the fluid properties cause the velocities of the fluid particles to vary from point to point throughout the flow field. The motion of fluids can be predicted using the fundamental laws of physics together with the physical properties of the fluid. The geometry of the motion of fluid particles in space and time is known as the kinematics of the fluid motion. A fluid motion may be specified by either tracing the motion of a particle through the field of flow or examining the motion of all particles as they pass a fixed point in space. This course will use the second method where the emphasis is on the spatial position rather than on the particle, or known as Eulerian Approach.

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Objectives comprehend the concepts necessary to analyse fluids in motion. identify differences between steady/unsteady, uniform/non-uniform and compressible/incompressible flow. construct streamlines and stream tubes. appreciate the Continuity principle through Conservation of Mass and Control Volumes. derive the Bernoulli (energy) equation. familiarise with the momentum equation for a fluid flow.

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**3.1 Uniform Flow, Steady Flow**

uniform flow: flow velocity is the same magnitude and direction at every point in the fluid. non-uniform: If at a given instant, the velocity is not the same at every point the flow. (In practice, by this definition, every fluid that flows near a solid boundary will be non-uniform - as the fluid at the boundary must take the speed of the boundary, usually zero. However if the size and shape of the of the cross-section of the stream of fluid is constant the flow is considered uniform.) steady: A steady flow is one in which the conditions (velocity, pressure and cross-section) may differ from point to point but DO NOT change with time. unsteady: If at any point in the fluid, the conditions change with time, the flow is described as unsteady. (In practice there is always slight variations in velocity and pressure, but if the average values are constant, the flow is considered steady.)

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**3.1 Uniform Flow, Steady Flow (cont.)**

Steady uniform flow: Conditions: do not change with position in the stream or with time. Example: the flow of water in a pipe of constant diameter at constant velocity. Steady non-uniform flow: Conditions: change from point to point in the stream but do not change with time. Example: flow in a tapering pipe with constant velocity at the inlet-velocity will change as you move along the length of the pipe toward the exit. Unsteady uniform flow: At a given instant in time the conditions at every point are the same, but will change with time. Example: a pipe of constant diameter connected to a pump pumping at a constant rate which is then switched off. Unsteady non-uniform flow: Every condition of the flow may change from point to point and with time at every point. Example: waves in a channel.

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**3.1.1 Laminar and Turbulent Flow**

Laminar flow all the particles proceed along smooth parallel paths and all particles on any path will follow it without deviation. Hence all particles have a velocity only in the direction of flow. Typical particles path Figure 3.1a: Laminar flow

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**Figure 3.1b: Turbulent flow**

the particles move in an irregular manner through the flow field. Each particle has superimposed on its mean velocity fluctuating velocity components both transverse to and in the direction of the net flow. Transition Flow exists between laminar and turbulent flow. In this region, the flow is very unpredictable and often changeable back and forth between laminar and turbulent states. Modern experimentation has demonstrated that this type of flow may comprise short ‘burst’ of turbulence embedded in a laminar flow. Particle paths Figure 3.1b: Turbulent flow

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**Figure 3.2: Relative motion**

Observer Boat moving Flow pattern moves along channel with boat changes with time UNSTEADY Boat stationary Fluid moving past boat pattern stationary relative to boat does not change with time STEADY Figure 3.2: Relative motion

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**3.1.3 Compressible or Incompressible**

All fluids are compressible - even water - their density will change as pressure changes. Under steady conditions, and provided that the changes in pressure are small, it is usually possible to simplify analysis of the flow by assuming it is incompressible and has constant density. As you will appreciate, liquids are quite difficult to compress - so under most steady conditions they are treated as incompressible.

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**3.1.4 One, Two or Three-dimensional Flow**

In general, all fluids flow three-dimensionally, with pressures and velocities and other flow properties varying in all directions. In many cases the greatest changes only occur in two directions or even only in one. In these cases changes in the other direction can be effectively ignored making analysis much more simple.

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Flow is one dimensional if the flow parameters (such as velocity, pressure, depth etc.) at a given instant in time only vary in the direction of flow and not across the cross-section. The flow may be unsteady, in this case the parameter vary in time but still not across the cross-section. An example of one-dimensional flow is the flow in a pipe. Note that since flow must be zero at the pipe wall - yet non-zero in the centre - there is a difference of parameters across the cross-section. Should this be treated as two-dimensional flow? Possibly - but it is only necessary if very high accuracy is required. A correction factor is then usually applied.

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Flow is two-dimensional if it can be assumed that the flow parameters vary in the direction of flow and in one direction at right angles to this direction. Streamlines in two-dimensional flow are curved lines on a plane and are the same on all parallel planes. An example is flow over a weir for which typical streamlines can be seen in the figure below. Over the majority of the length of the weir the flow is the same - only at the two ends does it change slightly. Here correction factors may be applied.

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3.1.5 Streamlines In analysing fluid flow it is useful to visualise the flow pattern. This can be done by drawing lines joining points of equal velocity - velocity contours. These lines are known as streamlines. Figure 3.5: A streamline

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**Close to a solid boundary streamlines are parallel to that boundary**

At all points the direction of the streamline is the direction of the fluid velocity: this is how they are defined. Close to the wall the velocity is parallel to the wall so the streamline is also parallel to the wall. It is also important to recognise that the position of streamlines can change with time - this is the case in unsteady flow. In steady flow, the position of streamlines does not change.

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**Some things to know about streamlines**

Because the fluid is moving in the same direction as the streamlines, fluid can not cross a streamline. Streamlines can not cross each other. If they were to cross this would indicate two different velocities at the same point. This is not physically possible. The above point implies that any particles of fluid starting on one streamline will stay on that same streamline throughout the fluid.

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3.1.6 Streamtubes A useful technique in fluid flow analysis is to consider only a part of the total fluid in isolation from the rest. This can be done by imagining a tubular surface formed by streamlines along which the fluid flows. This tubular surface is known as a streamtube. In a two-dimensional flow, we have a streamtube which is flat (in the plane of the paper).

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**The "walls" of a streamtube are made of streamlines. **

Figure 3.7: (a) A streamtube The "walls" of a streamtube are made of streamlines. As we have seen above, fluid cannot flow across a streamline, so fluid cannot cross a streamtube wall. The streamtube can often be viewed as a solid walled pipe. A streamtube is not a pipe - it differs in unsteady flow as the walls will move with time. And it differs because the "wall" is moving with the fluid.

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**time taken to collect the fluid**

3.2.1 Mass flow rate mass flow rate = m = time = mass of fluid time taken to collect the fluid mass mass flow rate

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**3.2.2 Volume flow rate - Discharge**

volume of fluid time discharge = Q = = mass of fluid density x time mass volume density = ( ) & mass fluid rate density m r =

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**3.2.3 Discharge and mean velocity**

Figure 3.8: Discharge in pipe If the area of cross section of the pipe at point X is A, and the mean velocity here is um, during a time t, a cylinder of fluid will pass point X with a volume A um t. The volume per unit time (the discharge) will thus be : Q = Q= or um= Let um= V um = V =

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**Figure 3.9: A typical velocity profile across a pipe**

Note how carefully we have called this the mean velocity. This is because the velocity in the pipe is not constant across the cross section. Crossing the centre line of the pipe, the velocity is zero at the walls, increasing to a maximum at the centre then decreasing symmetrically to the other wall. This variation across the section is known as the velocity profile or distribution. A typical one is shown in the figure This idea, that mean velocity multiplied by the area gives the discharge, applies to all situations - not just pipe flow.

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**time taken to collect the fluid**

Example 3.1 An empty bucket weighs 2.0 kg. After 7 seconds of collecting water the bucket weighs 8.0 kg, then: mass flow rate = ṁ = = = kg/s (kg s-1) mass of fluid in bucket time taken to collect the fluid 7

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Example 3.2 If we know the mass flow is 1.7 kg/s, how long will it take to fill a container with 8 kg of fluid? time = = mass mass flow rate 8 1.7 = 4.7s

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Example 3.3 If the density of the fluid in the above example is 850 kg/m3 what is the volume per unit time (the discharge)? Q = = = m3/s (m3s-1) = 10-3 m3/s but 1 litre = 1.0 10-3m3, so Q = l/s mass fluid rate density m r = 0.857 850

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Example 3.4 If the cross-section area, A, is 1.2 x 10-3 m2 and the discharge, Q is 24 l/s, what is the mean velocity, of the fluid? Let mean velocity, um = V um = V = = = 2.0 m/s 2.4 x 10-3 m3/s 1.2 x 10-3

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**3.3 The Fundamental Equations of Fluid Dynamics**

1. The law of conservation of matter stipulates that matter can be neither created nor destroyed, though it may be transformed (e.g. by a chemical process). Since this study of the mechanics of fluids excludes chemical activity from consideration, the law reduces to the principle of conservation of mass.

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**2. The law of conservation of energy**

states that energy may be neither created nor destroyed. Energy can be transformed from one guise to another (e.g. potential energy can be transformed into kinetic energy), but none is actually lost. Engineers sometimes loosely refer to ‘energy losses’ due to friction, but in fact the friction transforms some energy into heat, so none is really ‘lost’.

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**3. The law of conservation of momentum**

states that a body in motion cannot gain or lose momentum unless some external force is applied. The classical statement of this law is Newton's Second Law of Motion, i.e. force = rate of change of momentum

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**3.3.1 Continuity (Principle of Conservation of Mass)**

Matter cannot be created nor destroyed - (it is simply changed in to a different form of matter). This principle is known as the conservation of mass and we use it in the analysis of flowing fluids. The principle is applied to fixed volumes, known as control volumes or surfaces CONTROL VOLUME Control surface Outflow Inflow Figure 3.10: A control volume

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**Figure 3.11: A streamtube section**

For any control volume the principle of conservation of mass says Mass entering = Mass leaving + Increase of mass in the control per unit time per unit time volume per unit time For steady flow: (there is no increase in the mass within the control volume) Mass entering per unit time = Mass leaving per unit time Mass entering per unit time at end 1 = Mass leaving per unit time at end 2 Figure 3.11: A streamtube section

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**(density of fluid, ) (volume of fluid entering per second Q) **

flow is incompressible, the density of the fluid is constant throughout the fluid continum. Mass flow, m, entering may be calculated by taking the product (density of fluid, ) (volume of fluid entering per second Q) Mass flow is therefore represented by the product Q, hence Q (entering) = Q (leaving) But since flow is incompressible, the density is constant, so Q (entering) = Q (leaving) (3.5a) This is the ‘continuity equation’ for steady incompressible flow.

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**Therefore, the continuity equation may also be written as**

If the velocity of flow across the entry to the control volume is measured, and that the velocity is constant at V1 m/s. Then, if the cross-sectional area of the streamtube at entry is A1, Q (entering) = V1 A1 Thus, if the velocity of flow leaving the volume is V2 and the area of the streamtube at exit is A2, then Q (leaving) = V2A2 Therefore, the continuity equation may also be written as V1A1 = V2A2 (3.5b)

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**Application of Continuity Equation**

We can apply the principle of continuity to pipes with cross sections which change along their length. A liquid is flowing from left to right and the pipe is narrowing in the same direction. By the continuity principle, the mass flow rate must be the same at each section - the mass going into the pipe is equal to the mass going out of the pipe. So we can write: 1 A1V1= 2 A2V2 As we are considering a liquid, usually water, which is not very compressible, the density changes very little so we can say 1 =2 =. This also says that the volume flow rate is constant or that Discharge at section 1 = Discharge at section 2 Q1 = Q2 A1V1 = A2V2 or V2 = Figure 3.12: Pipe with a contraction A1V1 A2

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As the area of the circular pipe is a function of the diameter we can reduce the calculation further, V2 = V2 = (3.6) A1 A2 V1= pd12/4 pd22/4 d12 d22 V1 d12 d22 V1 ( ) 2

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**Another example is a diffuser, a pipe which expands or diverges as in the figure below**

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The continuity principle can also be used to determine the velocities in pipes coming from a junction. Total mass flow into the junction = Total mass flow out of the junction 1Q1 = 2Q2 + 3Q3 When the flow is incompressible (e.g. water) 1 = 2 = Q1 = Q2 + Q3 A1V1 = A2V2 + A3V3 (3.7)

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Example 3.5 If the area in Figure A1 = 10 10-3 m2 and A1 = 10 10-3 m2 and and the upstream mean velocity, V1 = 2.1 m/s, what is the downstream mean velocity? V2 = = 7.0 m/s A1V1 A2 10 x 10-3 x 2.1 3 x 10-3 =

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Example 3.6 If the diameter of a diffuser (Figure 3.13) at section 1 is d1 = 30 mm and at section 2 d2 = 40 mm and the mean velocity at section 2 is V2 = 3.0 m/s. Calculate the velocity entering the diffuser. V2 = ( ) 40 30 3.0 = 5.3m/s 2

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Example 3.7 For a junction (Figure 3.14), if pipe 1 diameter = 50 mm, mean velocity 2 m/s, pipe 2 diameter 40 mm takes 30% of total discharge and pipe 3 diameter 60 mm. What are the values of discharge and mean velocity in each pipe? Q1 = A1V1 = = m3/s But Q2 = 0.3Q1 = m3/s Also Q1 = Q2 + Q3 Q3 = Q1 – 0.3Q1 = 0.7Q1 = m3/s V2 = Q2 / V2 = m/s V3 = Q3 / V3 = m/s

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**3.3.2 Work and Energy (Principle Of Conservation Of Energy)**

friction: negligible sum of kinetic energy and gravitational potential energy is constant. Recall : Kinetic energy = ½ mV2 Gravitational potential energy = mgh (m: mass, V: velocity, h: height above the datum).

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To apply this to a falling body we have an initial velocity of zero, and it falls through a height of h. Initial kinetic energy = 0 Initial potential energy = mgh Final kinetic energy = ½ mV2 Final potential energy = 0 We know that, kinetic energy + potential energy = constant Initial kinetic Energy }+{ Initial potential Energy }={ Final Kinetic Energy }+{ Final Potential Energy { } mgh = ½ mV2 or

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**continuous jet of liquid**

Figure 3.15 : The trajectory of a jet of water a continuous jet of water coming from a pipe with velocity V1. One particle of the liquid with mass m travels with the jet and falls from height z1 to z2. The velocity also changes from V1 to V2. The jet is traveling in air where the pressure is everywhere atmospheric so there is no force due to pressure acting on the fluid. The only force which is acting is that due to gravity. The sum of the kinetic and potential energies remains constant (as we neglect energy losses due to friction) so : mgz1 + mV12 = mgz2 + mV22 As m is constant this becomes : V12 + gz1 = V22 + gz2

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**Flow from a reservoir Initial kinetic energy = 0**

The level of the water in the reservoir is z1. Considering the energy situation - there is no movement of water so kinetic energy is zero but the gravitational potential energy is mgz1. If a pipe is attached at the bottom water flows along this pipe out of the tank to a level z2. A mass m has flowed from the top of the reservoir to the nozzle and it has gained a velocity V2. The kinetic energy is now ½mV22 and the potential energy mgz2. Summarising : Initial kinetic energy = 0 Initial potential energy = mgz1 Final kinetic energy = ½ mV22 Final potential energy = mgz2 So mgz1 = ½ mV22 + mgz2 mg ( z1 - z2 ) = ½ mV22 V2 = (3.8) Figure 3.16 : Flow from a reservoir

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Example 3.8 A reservoir of water has the surface at 310 m above the outlet nozzle of a pipe with diameter 15mm. What is the velocity; the discharge out of the nozzle; and mass flow rate. (Neglect all friction in the nozzle and the pipe) Solution: a) b) Volume flow rate is equal to the area of the nozzle multiplied by the velocity Q = AV = = m3/s c) The density of water is 1000 kg/m3 so the mass flow rate is ṁ = density volume flow rate = Q = 1000 = kg/s

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Bernoulli's Equation We see that from applying equal pressure or zero velocities we get the two equations from the section above. They are both just special cases of Bernoulli's equation. Bernoulli's equation has some restrictions in its applicability, they are: Flow is steady; Density is constant (which also means the fluid is incompressible); Friction losses are negligible. The equation relates the states at two points along a single streamline, (not conditions on two different streamlines).

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**Figure 3.19 : A contracting expanding pipe**

A fluid of constant density = 960 kg/m3 is flowing steadily through the above tube. The diameters at the sections are d1 = 100mm and d2 = 80mm. The gauge pressure at 1 is P1 = 200 kN/m2 and the velocity here is V1 = 5m/s. What is the gauge pressure at section 2. Bernoulli equation is applied along a streamline joining section 1 with section 2. The tube is horizontal, with z1 = z2 so Bernoulli gives us the following equation for pressure at section 2: P2 = P1 + (V12 – V22)

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**So we can now calculate the pressure at section 2**

But we do not know the value of V2. We can calculate this from the continuity equation: Discharge into the tube is equal to the discharge out i.e. = m/s So we can now calculate the pressure at section 2 p2 = = N/m2 = kN/m2 P2 = P (V12 – V22) = (52 – ) 960 2 r

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**Modifications of Bernoulli Equation**

In practice, the total energy of a streamline does not remain constant. Energy is ‘lost’ through friction, and external energy may be either : added by means of a pump or extracted by a turbine. Consider a streamline between two points 1 and 2. If the energy head lost through friction is denoted by Hf and the external energy head added (say by a pump) is or extracted (by a turbine) HE, then Bernoulli's equation may be rewritten as : H1± HE = H2 + Hf (3.11) or HE = energy head added/loss due to external source such as pump/turbines This equation is really a restatement of the First Law of Thermodynamics for an incompressible fluid. (3.12)

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The Power Equation In the case of work done over a fluid the power input into the flow is : P = gQHE (3.13) where Q = discharge, HE = head added / loss If p = efficiency of the pump, the power input required, Pin = (3.14)

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Summary This chapter has outlined and discussed on the fundamental of fluid in motion. Students are aspect to be able to discuss and visualise on the following aspect: Able to classify FOUR (4) types of flow- Steady uniform flow, Steady non-uniform flow, Unsteady uniform flow and Unsteady non-uniform flow The differences between Laminar Flow, Turbulent Flow and also Transition Flow The idea of using the streamline to visualise the flow pattern The calculation of mass flow rate, volume flow rate and the mean velocity of the flow Able to explain and apply the THREE (3) laws- conservation of matter (conservation of mass); conservation of energy and conservation of momentum The important of Bernoulli Equation and the derivation

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