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

Diffusor testcase

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 30, 2001, 04:09
Default Diffusor testcase
  #1
Marat Hoshim
Guest
 
Posts: n/a
Hello,

I have a question concerning the testcase 8.2 of the 8th ERCOFTAC workshop in Helsinki, 1999. It's a 2D asymmetric diffusor flow.

I'm not quite sure about the inlet condition. There's said the inlet condition is a fully developed channel flow with a Reynolds-Number of 20000 based on the centreline velocity and the channel height.

Is the centreline velocity the max. velocity of the channel velocity profile ?

If my channel has a height of 1m and the fluid is air am I right if the the centreline velocity is:

v=Re*vis/rho/height=20000*1.8e-5/1.16/1=0.3134 m/s.

In the testcase there was separation but in my calcs there's attached flow. That why I'm so doubtful towards my boundary conditions.

Thanks for your help, Marat

  Reply With Quote

Old   January 30, 2001, 04:56
Default Re: Diffusor testcase
  #2
John C. Chien
Guest
 
Posts: n/a
(1). I don't know what you do with your calculation, the formulation?, the turbulence model?, etc. (2). The accurate prediction of separated diffuser flow is very difficult, because the existing two-equation k-epsilon turbulence model performs poorly in adverse-pressure gradient conditions, which exists in diffuser flows.
  Reply With Quote

Old   January 30, 2001, 05:06
Default Re: Diffusor testcase
  #3
Marat Hoshim
Guest
 
Posts: n/a
Hi John,

thanks for your advise. But what's your opinion towards my two questions ?

Regards, Marat
  Reply With Quote

Old   January 30, 2001, 10:32
Default Re: Diffusor testcase
  #4
sylvain
Guest
 
Posts: n/a
> Is the centreline velocity the max. velocity of the channel velocity profile ?

The answer is yes.

>If my channel has a height of 1m and the fluid is air am I right if the the centreline velocity is:

> v=Re*vis/rho/height=20000*1.8e-5/1.16/1=0.3134 m/s.

The answer is yes again.

> In the testcase there was separation but in my calcs there's attached flow.

As John said, this is a very difficult test case for turbulence models (so it's a good one in fact). I'm not so surprise that the numerical result doesn't show the correct behavior.

The proceedings of this workshop show how it was difficult to correctly predict the recircultaion zone.
  Reply With Quote

Old   January 30, 2001, 13:42
Default Re: Diffusor testcase
  #5
frederic felten
Guest
 
Posts: n/a
hi there,

i don't exactly know all the details of your calculation, but you should be able to find some very usefull informations in the following publication:

H.-J. Kaltenbach, M. Fatica, R. Mittal , T. S. Lund, and P. Moin. "Study of flow in a planar asymmetric diffuser using Large Eddy Simulation" J. Fluid Mech. 390, 151-185, 1999.

I hope this helps. Sincerely,

Frederic Felten CFD Lab, UT Arlington. http://utacfdb.uta.edu/
  Reply With Quote

Old   January 30, 2001, 14:11
Default Re: Diffusor testcase
  #6
John C. Chien
Guest
 
Posts: n/a
(1). I don't see anything wrong with the maximum centerline velocity, if it is fully-developend. Sometimes, people use the averaged inlet velocity. (2). My estimate of the velocity is around 1m/sec, which is close to your number.
  Reply With Quote

Old   January 31, 2001, 03:18
Default Re: Diffusor testcase
  #7
Marat Hoshim
Guest
 
Posts: n/a
John,

your estimate is around 1m/sec. But that 3 times higher than my estimate ! Where does that difference come from ?

Regards, Marat
  Reply With Quote

Old   January 31, 2001, 05:38
Default Re: Diffusor testcase
  #8
John C. Chien
Guest
 
Posts: n/a
(1). It comes from my estimate of the kinematic viscosity of the air.
  Reply With Quote

Old   January 31, 2001, 12:51
Default Re: Diffusor testcase
  #9
Marat Hoshim
Guest
 
Posts: n/a
So, what is your estimate for the kinematic viscosity of air ?

Regards, Marat
  Reply With Quote

Old   January 31, 2001, 16:15
Default Re: Diffusor testcase
  #10
John C. Chien
Guest
 
Posts: n/a
(1).In the book of Boundary Layer Theory, by Schlichting, the kinematic viscosity of air at 68degreeF, 14.7psi, is 160x10E-06 FT*FT/SEC. (2). You get 3.2 FT/SEC when U is calculated from U=Re*nu/L, with RE=20,000. So, U is around 1m/sec.
  Reply With Quote

Old   February 1, 2001, 11:48
Default Re: Diffusor testcase
  #11
sylvain
Guest
 
Posts: n/a
I'm sorry, but you get U=3.2 FT/SEC if L=1FT but if L=1m=3.2FT, it comes U=0.097FT/SEC, which leads to U=0.31m/s.

Best regards,

Sylvain
  Reply With Quote

Old   February 1, 2001, 13:04
Default Eureka!
  #12
John C. Chien
Guest
 
Posts: n/a
(1). I think, you are right. (2). That's why I said, it was an estimate. It's important to double check the result. Thank you. By now everybody knows how to compute the Reynolds number.
  Reply With Quote

Old   February 12, 2001, 13:27
Default Re: Diffusor testcase
  #13
clifford bradford
Guest
 
Posts: n/a
Marat, as John pointed out your issue is probably with your turbulence model. I guess you're using k-e (because it's so popular) unfortunately k-e is known to be poor for adverse pressure gradients. you may want to use the k-omega. See David Wilcox's "Turbulence Modelling for CFD"
  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



All times are GMT -4. The time now is 06:26.