FLUID MECHANICS

In the built environment pipes are used to convey In the built environment pipes are used to convey gas, water and other fluids. gas, water and other fluids.

In sanitary and drainage works the pressures In sanitary and drainage works the pressures developed by fluids in motion determine the size developed by fluids in motion determine the size and gradients to be employed.and gradients to be employed.

To make appropriate and efficient use of fluids it is crucial that we understand their properties and can describe, at least approximately, their behaviour under varying conditions.

It is also necessary to control the flow of fluids under these varying conditions and so be able to specify equipment capable of that control.

Edexcel Level 5 BTEC HNC in Construction….

Required knowledge

Fluids flowing through enclosed pipes behave in several ways; some Fluids flowing through enclosed pipes behave in several ways; some of these ways are shown here.of these ways are shown here.

Regular laminar flowLaminar flow with friction slowing molecules at the pipe walls

Turbulent flow

Irregular flow across pipe diameter causing increase in velocityvelocity

Quantity of flowQuantity of flow

The quantity of fluid flowing past a fixed point can be expressed in terms of:

  

litres per second or cubic metres per second (cumecs).

 

The quantity of fluid is found by the use of the formula: 

Q = AVWhere:

A is the cross sectional area of the pipe in metres.

Q is the quantity of fluid.

V is the velocity of the flow.

Fluid in pipelines

The quantity and velocity of fluid in a pipeline and its relationship with the cross sectional area allows us to calculate and predict the flow of fluid at any point. Using the formula Q = VA

If certain information is available we can determine the rate of flow (Q, in cumecs) and the velocity (in metres per sec) and the diameter of the pipe calculated in metres or millimetres.

Friction in the pipeline

As a fluid flows through a pipeline there is a slowing of the particles closest to the interface between the internal walls and the fluid due to friction.

A number of formulae have been developed to quantify and predict the amount of head loss resulting from the friction within a pipeline running full bore.

One of the most commonly used formulas is the D'Arcy formula.

 

h = (4 f L V2) (2g D)

D'ARCY formula: (Write it down)

Where: h

is the head loss over a given length f is the coefficient of friction (fluid to internal surface) Unless there is evidence to the contrary assume 0.0084

V is the velocity of flow in m/sec

G is gravitational acceleration assumed to be 9.81 m/sec

D is the internal diameter of the pipe in metres.

Task:- use the formula to calculate the head loss in a 19 mm diameter cold water supply pipe 8 metres long if the flow is known to be 0.75 m/sec.

h = (4(4 x 0.0084 x 8 x 0.75x 0.0084 x 8 x 0.7522) ) (2 x 9.81 x 0.019) (2 x 9.81 x 0.019)   

h = 0.406 metres.h = 0.406 metres.

Tasks :

Calculate the head loss in a 50 mm diameter pipe 25 metres long if the flow is known to be 2.53 m/sec. (5.48 metres)

Find the velocity of flow when the head loss is 1.16 metres over a 10 metre length in a 25 mm diameter pipe. (1.3 m/sec)

Determine the diameter of a pipe where a head loss of 0.19 metres is experienced over a 16 metre length having a velocity of 0.5 m/sec. (36 mm)

http://www.engineersedge.com/fluid_flow/head_loss.htm

Head loss

SoundSoundThe Nature of SoundThe Nature of Sound

RememberRemember Area of hemisphere = 2Area of hemisphere = 2r²r²

II22 = = Sound PowerSound Power Surface area this is Surface area this is

called…called…

This Sound intensity This Sound intensity 14

Assignment task 4 cAssignment task 4 cCalculate the Sound Pressure Level (SPL) in decibels (dB) of a Percussion drill 3 metres away with a sound power output of 50 watts (M)

The weakest sound that can be The weakest sound that can be heard by a human is termed the heard by a human is termed the threshold of sound and although threshold of sound and although this differs for each individual, a this differs for each individual, a base value of 1 x 10base value of 1 x 10-12-12

15

To calculate the level of sound in terms of decibels (dB),you To calculate the level of sound in terms of decibels (dB),you need the formula...need the formula...

dB = dB = 10 Log 10 Log I I22

II11

  Where:Where: dBdB is the Sound Pressure Level in decibelsis the Sound Pressure Level in decibels LogLog is built into the calculator is built into the calculator II22 is the intensity of a given sound.is the intensity of a given sound.

II11 is the base value of the intensity of sound. (typically 10is the base value of the intensity of sound. (typically 10--

1212))

16

Now put your figures into Now put your figures into the formulathe formula

Remember r = 3Remember r = 3

Watts of the drill = 50Watts of the drill = 50 Find IFind I 1 1 and I and I 2 2

II 1 1 = 10 = 10 -12-12

II 2 2 == 50 / 2 x ∏ x 9 50 / 2 x ∏ x 9

dB = dB = 10 Log x10 Log x I I22

II11

Train problem …

Sound consultancy taskSound consultancy task

Typical Noise ConsultantsTypical Noise Consultants. We provide . We provide services to industry and the public sector in a wide in a wide range of aspects of acoustics, noise and vibration. range of aspects of acoustics, noise and vibration.

Our service includes environmental acoustics, Our service includes environmental acoustics, litigation services (particularly for hearing loss litigation services (particularly for hearing loss claims), specialist measurements, noise control claims), specialist measurements, noise control and building design. and building design.

Train noise nuisance Train noise nuisance investigationinvestigation

CalculateCalculate the Sound Pressure Level the Sound Pressure Level (SPL) in decibels (dB) of a train 10 (SPL) in decibels (dB) of a train 10 metres away with a sound power output metres away with a sound power output of 100 watts. (Method: Find intensity of 100 watts. (Method: Find intensity then place values in dB equation.)then place values in dB equation.)

FLUID MECHANICS112

Exercise: Working in groups of two or three use the Internet,

resource materials and any other sources of information available to you undertake the following tasks.

Select appropriate lighting systems for two the following areas;

A lecture theatre to seat 80 students The ground floor of a department store of approximately

100 m² A sports hall consisting of six 5-a-side football pitches A manufacturer’s workshop specialising in fabricating

computer components The car park of a medium sized college campus

Prepare a short presentation (5 – 10 minutes) to explain and elaborate on your selection

Identify merits and demerits of the lamps of your choice

Use print outs, powerpoint, and any other aids to augment your presentation

Present your work