CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

caculation of friction factor

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Display Modes
Old   April 12, 2001, 01:33
Default caculation of friction factor
  #1
Dilip
Guest
 
Posts: n/a
hi everybody,

I know this is pretty simple question to experts here. But i find variable results in calculating average friction factor and nusselt number in turbulent flow through sinusoidal duct. Friction factor in a duct is calculated by

f = -(dp/dz)*hydraulic_diameter/(0.5*density*mean_vel**2).

Now to calculate average friction factor for a length L,

(1) can i approximate dp/dz=(mean_pres_at2-mean_pres_at1)/L?

(2) can i calculate mean pressure at any cross section by, mean_pres_at1=sum(pres.dA)/A1? Would it be same for separated region?

Is there any better way to calculate overall friction factor and nusselt number from pressure, velocity and temperature field??

Thanks in advance for your help.

Dilip
  Reply With Quote

Old   April 13, 2001, 13:11
Default Re: caculation of friction factor
  #2
John C. Chien
Guest
 
Posts: n/a
(1). You need to go back to the momentum equations and look at the convection terms, pressure gradient terms and the viscous diffusion terms. (2). The friction factor is related to the viscous diffusion terms, and is evaluated at the wall. (3). But I guess, if the contribution from the convection terms is zero for fully developed pipe flow, then the viscous loss will be equal to the pressure gradient term, from the momentum equations. In that case, you can evaluate the viscous loss by calculating the pressure gradient term. This is the case in your friction factor formula. (4). In general, the convection terms are not zero, therefore, you will have to evaluate the friction factor directly from the viscous terms, that is from the skin friction at the wall.
  Reply With Quote

Old   April 14, 2001, 09:36
Default Re: caculation of friction factor
  #3
clifford bradford
Guest
 
Posts: n/a
Dilip, It seems to me that you're formula for the average friction factor is correct. Also static pressure should be area averaged as you propose. This procedure should be appropriate even with reversed flow, however, I'd choose my 1 and 2 points so that there is as little as possible.

By definition you should be able to calculate the average friction factors and Nusselt numbers in this way (using a control volume). To check one can of course calculate the local values and integrate over the length on interest. If I recall correctly for the Nusselt number one should calculate the local heat transfer coefficient, find the average value of that and then find the average Nu. Consult an introductory heat transfer book.
  Reply With Quote

Old   April 15, 2001, 15:58
Default Re: caculation of friction factor
  #4
Dilip
Guest
 
Posts: n/a
Thanks john for your response. I always appreciate your nice explanation on this forum. I understand that in case of fully developed flow skin friction coefficient calculated from wall shear stress is same with the friction factor calculated from pressure gradient. But for developing flow where the convection part is not zero, the skin friction coefficient is different from the friction factor. In that case, how the friction factor is to be calculated from skin friction?? I think that is calculated from pressure difference only.

Thanking you,

dipak
  Reply With Quote

Old   April 15, 2001, 16:04
Default Re: calculation of friction factor
  #5
Dilip
Guest
 
Posts: n/a
Thanks for your response. But i couldn't understand, "appropriate even with reversed flow, however, I'd choose my 1 and 2 points so that there is as little as possible."....as little as possible what??

Waiting for your reply.

Dilip

  Reply With Quote

Old   April 16, 2001, 09:02
Default Re: calculation of friction factor
  #6
S.P.ASOK
Guest
 
Posts: n/a
Hello!By the by I would like to get clarified on one the following points: (1)For fully developed flow through pipes the friction coefficient is given by the symbol: f Some authors call it as Darcy's coefficient also.Some others put up a term "Co-efficient of friction" and multiply it by 4 to get the friction factor value.Some autors use both the names at will as if they are the same.This is a problem for both teacher and students.Now the question is whether they represent the same term or different? (2)In the formula for finding the specific speed of cenrifugal pumps the value of discharge is substituted in the unit of litres/sec in some books and in cubic metres/sec in some other books.Is it because of the unit system followed or one of the above should not be used? As a teacher of fluid mechanics I have been facing the problem for some time and authentic replies will be of great help.
  Reply With Quote

Old   April 16, 2001, 13:10
Default Re: calculation of friction factor
  #7
clifford bradford
Guest
 
Posts: n/a
As little reversed flow as possible.

  Reply With Quote

Old   April 19, 2001, 04:41
Default Re: calculation of friction factor
  #8
Esa Utriainen
Guest
 
Posts: n/a
Hi!

The Darcy (D) friction factor is related to the Fanning (f) friction factor as: D=4f /Esa
  Reply With Quote

Old   May 5, 2001, 00:42
Default Re: calculation of friction factor
  #9
clifford bradford
Guest
 
Posts: n/a
Dilip sorry for not getting back to you sooner. i meant as little reversed flow as possible
  Reply With Quote

Old   February 23, 2016, 06:00
Default friction factor in ansys fluent
  #10
New Member
 
ksr
Join Date: Feb 2016
Posts: 1
Rep Power: 0
smithwill7793@gmail.com is on a distinguished road
how to find friction factor in through ansys fluent simulation.. ?? plzzz reply asap...
smithwill7793@gmail.com is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
friction factor in an annulus with an inner rough surface George89 CFX 0 May 29, 2011 13:48
LES channelFoam solver underpredicts friction factor utexasche OpenFOAM Running, Solving & CFD 0 May 16, 2011 20:47
How to incorporate the friction factor.... Dewson FLUENT 0 April 16, 2007 15:03
friction factor calculation in Fluent Sandy FLUENT 0 February 9, 2007 13:17
Skin friction factor Ali FLUENT 2 December 11, 2005 04:25


All times are GMT -4. The time now is 03:10.