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

Problem in Turbulent modeling of impinging slot jet

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

Reply
 
LinkBack Thread Tools Display Modes
Old   October 5, 2009, 08:40
Default Problem in Turbulent modeling of impinging slot jet
  #1
New Member
 
Join Date: Oct 2009
Posts: 7
Rep Power: 7
amirbabak1979 is on a distinguished road
Dear Friends,

I wrote a code with Fortran for laminar impinging slot jet and I verify it with experimental and numerical references.

Now when I add the low reynolds models of K-e turbulent model to my code,
unfortunately I couldn't get the converge from code and if I could the results is not the same with available numerical results.

Recently, I found some articles which it says we should relaxed the Eddy viscosity and Generation term as well as other variables.(eg. u,v,p,T,k,e)
but I am not sure this additional relaxation factor help me to get converge from code.

If anybody has a same experience in such a Low Reynolds k-e Modeling could help me and tell me what should I do with this problem, I really appreciate that. Because I am at the last semester in my master program and need to get the result in order to be graduate.

Best Regard,

Amir
amirbabak1979 is offline   Reply With Quote

Old   October 21, 2009, 02:05
Default
  #2
Member
 
MAZI
Join Date: Oct 2009
Posts: 79
Rep Power: 7
mazdak is on a distinguished road
Hi Amir.

it's better that you first write code with Standard k-e model then you can change it to Low-Re model.to know that is your codeis correct or no.
Ret and f_mi_u maybe produce some error in code because of their value.

what's the type of your grid? staggered or collocated?

in blow link two persons discused about Low-Re model.maybe it be usefull.


Writing Turbulence Solver, k-e model
mazdak is offline   Reply With Quote

Old   October 21, 2009, 06:52
Default
  #3
New Member
 
Join Date: Oct 2009
Posts: 7
Rep Power: 7
amirbabak1979 is on a distinguished road
Dear Mazdak,

Thanks for your reply.
As I wrote in the last post, I wrote it for collocated grid and in the laminar I got aggreement result with experimental and available numerical result.

As you suggested to read this link http://www.cfd-online.com/Forums/main/63353-writing-turbulence-solver-k-e-model.html
I wondered about the wall treatment and the first grid point subject which they discusted about that.
Before it I thought when we use the Low-Reynolds number model for K-e we don't need any care about the first grid point distance to the wall.

Anyway I really confused about this subject.
If I need to put any special distance to the wall for the first grid, how I can define it? I can't underestand if I wanna put the Y+ term in my equations,should I generate the grid depends on it? how I can define the Y+?

If you have any experience in this subject, I wondered If you can share with me.

Thanks in advanced.

Amir

Quote:
Originally Posted by mazdak View Post
Hi Amir.

it's better that you first write code with Standard k-e model then you can change it to Low-Re model.to know that is your codeis correct or no.
Ret and f_mi_u maybe produce some error in code because of their value.

what's the type of your grid? staggered or collocated?

in blow link two persons discused about Low-Re model.maybe it be usefull.


Writing Turbulence Solver, k-e model
amirbabak1979 is offline   Reply With Quote

Old   October 21, 2009, 07:38
Default
  #4
Member
 
MAZI
Join Date: Oct 2009
Posts: 79
Rep Power: 7
mazdak is on a distinguished road
Good question Amir.

i read one paper, wrote that when you use Low-Re K-E model you must capture y+<5.

Amir Y+ is a solution dependent quantity that it's definition is:

Y+= density*friction velocity*y/viscosity

where y is distance between first grid nodes and the wall.

y: faseleye avalin grid ta divare.yani faseleye nazdiktarin noghte be divare

For example, when you double the mesh (thereby halving the wall distance), the new y+
does not necessarily become half of the y+ for the original mesh.

pleas send me your email address.

Mazdak_parsi2000@yahoo.com
mazdak is offline   Reply With Quote

Old   October 27, 2009, 17:11
Default Please helpppppppp
  #5
New Member
 
Join Date: Oct 2009
Posts: 7
Rep Power: 7
amirbabak1979 is on a distinguished road
Dear Mazdak, Thanks for your comments.
Dear all,
Please Please help me ;(

As I describe my problem before , I am doing 2D numerical of turbulent impinging slot jet.
Actually it doesn't have any different with fully-developed channel. there are jus some difference in boundary conditions.

Anyway let me give u more about my numerical method,
Finite volume based SIMPLE, SIMPLER algorithm are followed and also considered herein are some numerical spatial discretization schemes, e.g., hybrid, power law, second-order upwind, and QUICK schemes, to test their performance in predicting this complex flow. Collocated grid is used and nonuniform grids are used along the walls and at the entrance of jet.

I used different node number in Y direction, from 100 to 240.

But u know I have no idea, If I need to put any special distance to the wall for the first grid, how I can define it? I can't understand if I wanna put the Y+ term in my equations,should I generate the grid depends on it? how I can define the Y+?

If Y+ is a solution dependent quantity that it's definition is:

Y+= density*friction velocity*y/viscosity

where y is distance between first grid nodes and the wall.
so How I can put my first grid near the wall in Y+<5 for low reynolds number models when i don't have the value for friction velocity?!!
before generation the grid base on Y+ how i can run the code and got the value of y+?!!!

I am really confused.
Waiting for your suggestions and comments.

Thanks in advanced.

Amir.
amirbabak1979 is offline   Reply With Quote

Old   October 29, 2009, 22:00
Default
  #6
Senior Member
 
Julien de Charentenay
Join Date: Jun 2009
Location: Australia
Posts: 229
Rep Power: 9
julien.decharentenay is on a distinguished road
Send a message via Skype™ to julien.decharentenay
Hi Amir,

You may want to distinguish the convergence problem from the physical model problem (although both might be linked).

Re the convergence, are you experiencing divergence or erratic convergence? Normally, I would expect that adding turbulence would help convergence due to turbulent viscosity. I would suggest using a simplified boundary condition (maybe high-Reynolds one) to check this aspect.

Re the wall treatment: y is given by your mesh. Y+ is obtained after (during) the calculation: the friction velocity is derived from the velocity field. The normal process is: (a) fix a grid, (b) run a simulation to convergence, (c) extract y+ from the results, (d) modify the grid to have y+ < 5 and redo step b and c. Note that the velocity field may differ for the refined grid, thereby having an impact of y+, i.e. you may have to go through the process a few times.

I am not sure what references/sources you are using, but you may want to have a look at the wiki section on turbulence modeling, a fluent user manual (if you can access one). There was a technical paper from CFX on their adaptive wall function for turbulence and heat transfer (used to be accessible online, but I can't get hold of the link now).

Good luck.
Julien
julien.decharentenay 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
impinging jet natesan CD-adapco 1 February 25, 2005 04:16
Impinging Jet Modeling Problem Anindya FLUENT 1 August 11, 2001 03:16
impinging jet data Andreas Abdon Main CFD Forum 4 January 19, 2000 08:40
IMPINGING JET ........... HELP!!!!!!!! Amir Omoumi Main CFD Forum 10 August 30, 1999 22:11
IMPINGING JET PROBLEM Chuck Leakeas Main CFD Forum 4 January 29, 1999 18:43


All times are GMT -4. The time now is 13:25.