OneStopGate.Com
OnestopGate   OnestopGate
   Friday, December 6, 2024 Login  
OnestopGate
Home | Overview | Syllabus | Tutorials | FAQs | Downloads | Recommended Websites | Advertise | Payments | Contact Us | Forum
OneStopGate

GATE Resources
Gate Articles
Gate Books
Gate Colleges 
Gate Downloads 
Gate Faqs
Gate Jobs
Gate News 
Gate Sample Papers
Training Institutes

GATE Overview
Overview
GATE Eligibility
Structure Of GATE
GATE Coaching Centers
Colleges Providing M.Tech/M.E.
GATE Score
GATE Results
PG with Scholarships
Article On GATE
Admission Process For M.Tech/ MCP-PhD
GATE Topper 2012-13
GATE Forum




GATE 2025 Exclusive
Organizing Institute
Important Dates
How to Apply
Discipline Codes
GATE 2025 Exam Structure

GATE 2025 Syllabus
Aerospace Engg..
Agricultural Engg..
Architecture and Planning
Chemical Engg..
Chemistry
Civil Engg..
Computer Science / IT
Electronics & Communication Engg..
Electrical Engg..
Engineering Sciences
Geology and Geophysics
Instrumentation Engineering
Life Sciences
Mathematics
Mechanical Engg..
Metallurgical Engg..
Mining Engg..
Physics
Production & Industrial Engg..
Pharmaceutical Sciences
Textile Engineering and Fibre Science

GATE Study Material
Aerospace Engg..
Agricultural Engg..
Chemical Engg..
Chemistry
Civil Engg..
Computer Science / IT
Electronics & Communication Engg..
Electrical Engg..
Engineering Sciences
Instrumentation Engg..
Life Sciences
Mathematics
Mechanical Engg..
Physics
Pharmaceutical Sciences
Textile Engineering  and Fibre Science

GATE Preparation
GATE Pattern
GATE Tips N Tricks
Compare Evaluation
Sample Papers 
Gate Downloads 
Experts View

CEED 2013
CEED Exams
Eligibility
Application Forms
Important Dates
Contact Address
Examination Centres
CEED Sample Papers

Discuss GATE
GATE Forum
Exam Cities
Contact Details
Bank Details

Miscellaneous
Advertisment
Contact Us


Home » GATE Study Material » Civil Engineering » Laminor and Turbulant Flow

Laminor and Turbulant Flow

Looking for GATE Preparation Material? Join & Get here now!

** Gate 2013 Question Papers.. ** CEED 2013 Results.. ** Gate 2013 Question Papers With Solutions.. ** GATE 2013 CUT-OFFs.. ** GATE 2013 Results.. **

Laminor and Turbulant Flow

1. Real fluids

The flow of real fluids exhibits viscous effect, that is they tend to "stick" to solid surfaces and have stresses within their body.
 

You might remember from earlier in the course Newtons law of viscosity:

 

This tells us that the shear stress, t, in a fluid is proportional to the velocity gradient - the rate of change of velocity across the fluid path. For a "Newtonian" fluid we can write:

 

where the constant of proportionality, m, is known as the coefficient of viscosity (or simply viscosity). We saw that for some fluids - sometimes known as exotic fluids - the value of m changes with stress or velocity gradient. We shall only deal with Newtonian fluids.
 

In his lecture we shall look at how the forces due to momentum changes on the fluid and viscous forces compare and what changes take place.
 

2. Laminar and turbulent flow

If we were to take a pipe of free flowing water and inject a dye into the middle of the stream, what would we expect to happen?
 

This
 

this
 

or this
 

Actually both would happen - but for different flow rates. The top occurs when the fluid is flowing fast and the lower when it is flowing slowly.
 

The top situation is known as turbulent flow and the lower as laminar flow.

In laminar flow the motion of the particles of fluid is very orderly with all particles moving in straight lines parallel to the pipe walls.
 

But what is fast or slow? And at what speed does the flow pattern change? And why might we want to know this?
 

The phenomenon was first investigated in the 1880s by Osbourne Reynolds in an experiment which has become a classic in fluid mechanics.


 








 

He used a tank arranged as above with a pipe taking water from the centre into which he injected a dye through a needle. After many experiments he saw that this expression

 

where r = density, u = mean velocity, d = diameter and m = viscosity

would help predict the change in flow type. If the value is less than about 2000 then flow is laminar, if greater than 4000 then turbulent and in between these then in the transition zone.
 

This value is known as the Reynolds number, Re:

 


 

Laminar flow: Re < 2000

Transitional flow: 2000 < Re < 4000

Turbulent flow: Re > 4000
 

What are the units of this Reynolds number? We can fill in the equation with SI units:
 

 

i.e. it has no units. A quantity that has no units is known as a non-dimensional (or dimensionless) quantity. Thus the Reynolds number, Re, is a non-dimensional number.
 

We can go through an example to discover at what velocity the flow in a pipe stops being laminar.

If the pipe and the fluid have the following properties:
 

water density r = 1000 kg/m3

pipe diameter d = 0.5m

(dynamic) viscosity, m = 0.55x103 Ns/m2
 

We want to know the maximum velocity when the Re is 2000.

 

If this were a pipe in a house central heating system, where the pipe diameter is typically 0.015m, the limiting velocity for laminar flow would be, 0.0733 m/s.
 

Both of these are very slow. In practice it very rarely occurs in a piped water system - the velocities of flow are much greater. Laminar flow does occur in situations with fluids of greater viscosity - e.g. in bearing with oil as the lubricant.
 

At small values of Re above 2000 the flow exhibits small instabilities. At values of about 4000 we can say that the flow is truly turbulent. Over the past 100 years since this experiment, numerous more experiments have shown this phenomenon of limits of Re for many different Newtonian fluids - including gasses.
 

What does this abstract number mean?

We can say that the number has a physical meaning, by doing so it helps to understand some of the reasons for the changes from laminar to turbulent flow.

 


 

It can be interpreted that when the inertial forces dominate over the viscous forces (when the fluid is flowing faster and Re is larger) then the flow is turbulent. When the viscous forces are dominant (slow flow, low Re) they are sufficient enough to keep all the fluid particles in line, then the flow is laminar.
 

In summary:

Laminar flow

  • Re < 2000
  • 'low' velocity
  • Dye does not mix with water
  • Fluid particles move in straight lines
  • Simple mathematical analysis possible
  • Rare in practice in water systems.

 

Transitional flow

  • 2000 > Re < 4000
  • 'medium' velocity
  • Dye stream wavers in water - mixes slightly.

 

Turbulent flow

  • Re > 4000
  • 'high' velocity
  • Dye mixes rapidly and completely
  • Particle paths completely irregular
  • Average motion is in the direction of the flow
  • Cannot be seen by the naked eye
  • Changes/fluctuations are very difficult to detect. Must use laser.
  • Mathematical analysis very difficult - so experimental measures are used
  • Most common type of flow.

3. Pressure loss due to friction in a pipeline.

Up to this point on the course we have considered ideal fluids where there have been no losses due to friction or any other factors. In reality, because fluids are viscous, energy is lost by flowing fluids due to friction which must be taken into account. The effect of the friction shows itself as a pressure (or head) loss.

In a pipe with a real fluid flowing, at the wall there is a shearing stress retarding the flow, as shown below.

 

If a manometer is attached as the pressure (head) difference due to the energy lost by the fluid overcoming the shear stress can be easily seen.

The pressure at 1 (upstream) is higher than the pressure at 2.

 

We can do some analysis to express this loss in pressure in terms of the forces acting on the fluid.

Consider a cylindrical element of incompressible fluid flowing in the pipe, as shown

 


 

The pressure at the upstream end is p, and at the downstream end the pressure has fallen by Dp to (p-Dp).

The driving force due to pressure (F = Pressure x Area) can then be written

 

driving force = Pressure force at 1 - pressure force at 2

 

The retarding force is that due to the shear stress by the walls

 

As the flow is in equilibrium,

 

driving force = retarding force

 

Giving an expression for pressure loss in a pipe in terms of the pipe diameter and the shear stress at the wall on the pipe.

 

The shear stress will vary with velocity of flow and hence with Re. Many experiments have been done with various fluids measuring the pressure loss at various Reynolds numbers. These results plotted to show a graph of the relationship between pressure loss and Re look similar to the figure below:

 

This graph shows that the relationship between pressure loss and Re can be expressed as


 

As these are empirical relationships, they help in determining the pressure loss but not in finding the magnitude of the shear stress at the wall tw on a particular fluid. If we knew tw we could then use it to give a general equation to predict the pressure loss.

4. Pressure loss during laminar flow in a pipe

In general the shear stress tw. is almost impossible to measure. But for laminar flow it is possible to calculate a theoretical value for a given velocity, fluid and pipe dimension.

In laminar flow the paths of individual particles of fluid do not cross, so the flow may be considered as a series of concentric cylinders sliding over each other - rather like the cylinders of a collapsible pocket telescope.

As before, consider a cylinder of fluid, length L, radius r, flowing steadily in the centre of a pipe.

 

We are in equilibrium, so the shearing forces on the cylinder equal the pressure forces.

 

By Newtons law of viscosity we have , where y is the distance from the wall. As we are measuring from the pipe centre then we change the sign and replace y with r distance from the centre, giving

 

Which can be combined with the equation above to give

 

In an integral form this gives an expression for velocity,

 

Integrating gives the value of velocity at a point distance r from the centre

 

At r = 0, (the centre of the pipe), u = umax, at r = R (the pipe wall) u = 0, giving

 

so, an expression for velocity at a point r from the pipe centre when the flow is laminar is

 

Note how this is a parabolic profile (of the form y = ax2 + b ) so the velocity profile in the pipe looks similar to the figure below

 

What is the discharge in the pipe?

 

So the discharge can be written

 

This is the Hagen-Poiseuille equation for laminar flow in a pipe. It expresses the discharge Q in terms of the pressure gradient (), diameter of the pipe and the viscosity of the fluid.

We are interested in the pressure loss (head loss) and want to relate this to the velocity of the flow. Writing pressure loss in terms of head loss hf, i.e. p = rghf
 

 

This shows that pressure loss is directly proportional to the velocity when flow is laminar.

It has been validated many time by experiment.

It justifies two assumptions:

  1. fluid does not slip past a solid boundary
  2. Newtons hypothesis.

 

 



Discussion Center

Discuss/
Query

Papers/
Syllabus

Feedback/
Suggestion

Yahoo
Groups

Sirfdosti
Groups

Contact
Us

MEMBERS LOGIN
  
Email ID:
Password:

  Forgot Password?
 New User? Register!

INTERVIEW EBOOK
Get 9,000+ Interview Questions & Answers in an eBook. Interview Question & Answer Guide
  • 9,000+ Interview Questions
  • All Questions Answered
  • 5 FREE Bonuses
  • Free Upgrades
GATE RESOURCES
 
  • Gate Books
  • Training Institutes
  • Gate FAQs
  • GATE BOOKS
     
  • Mechanical Engineeering Books
  • Robotics Automations Engineering Books
  • Civil Engineering Books
  • Chemical Engineering Books
  • Environmental Engineering Books
  • Electrical Engineering Books
  • Electronics Engineering Books
  • Information Technology Books
  • Software Engineering Books
  • GATE Preparation Books
  • Exciting Offers



    GATE Exam, Gate 2009, Gate Papers, Gate Preparation & Related Pages


    GATE Overview | GATE Eligibility | Structure Of GATE | GATE Training Institutes | Colleges Providing M.Tech/M.E. | GATE Score | GATE Results | PG with Scholarships | Article On GATE | GATE Forum | GATE 2009 Exclusive | GATE 2009 Syllabus | GATE Organizing Institute | Important Dates for GATE Exam | How to Apply for GATE | Discipline / Branch Codes | GATE Syllabus for Aerospace Engineering | GATE Syllabus for Agricultural Engineering | GATE Syllabus for Architecture and Planning | GATE Syllabus for Chemical Engineering | GATE Syllabus for Chemistry | GATE Syllabus for Civil Engineering | GATE Syllabus for Computer Science / IT | GATE Syllabus for Electronics and Communication Engineering | GATE Syllabus for Engineering Sciences | GATE Syllabus for Geology and Geophysics | GATE Syllabus for Instrumentation Engineering | GATE Syllabus for Life Sciences | GATE Syllabus for Mathematics | GATE Syllabus for Mechanical Engineering | GATE Syllabus for Metallurgical Engineering | GATE Syllabus for Mining Engineering | GATE Syllabus for Physics | GATE Syllabus for Production and Industrial Engineering | GATE Syllabus for Pharmaceutical Sciences | GATE Syllabus for Textile Engineering and Fibre Science | GATE Preparation | GATE Pattern | GATE Tips & Tricks | GATE Compare Evaluation | GATE Sample Papers | GATE Downloads | Experts View on GATE | CEED 2009 | CEED 2009 Exam | Eligibility for CEED Exam | Application forms of CEED Exam | Important Dates of CEED Exam | Contact Address for CEED Exam | CEED Examination Centres | CEED Sample Papers | Discuss GATE | GATE Forum of OneStopGATE.com | GATE Exam Cities | Contact Details for GATE | Bank Details for GATE | GATE Miscellaneous Info | GATE FAQs | Advertisement on GATE | Contact Us on OneStopGATE |
    Copyright © 2024. One Stop Gate.com. All rights reserved Testimonials |Link To Us |Sitemap |Privacy Policy | Terms and Conditions|About Us
    Our Portals : Academic Tutorials | Best eBooksworld | Beyond Stats | City Details | Interview Questions | India Job Forum | Excellent Mobiles | Free Bangalore | Give Me The Code | Gog Logo | Free Classifieds | Jobs Assist | Interview Questions | One Stop FAQs | One Stop GATE | One Stop GRE | One Stop IAS | One Stop MBA | One Stop SAP | One Stop Testing | Web Hosting | Quick Site Kit | Sirf Dosti | Source Codes World | Tasty Food | Tech Archive | Software Testing Interview Questions | Free Online Exams | The Galz | Top Masala | Vyom | Vyom eBooks | Vyom International | Vyom Links | Vyoms | Vyom World
    C Interview Questions | C++ Interview Questions | Send Free SMS | Placement Papers | SMS Jokes | Cool Forwards | Romantic Shayari