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

Stabilizing turbulence equation in channel flow

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

Reply
 
LinkBack Thread Tools Display Modes
Old   March 20, 2005, 08:28
Default Stabilizing turbulence equation in channel flow
  #1
Biga
Guest
 
Posts: n/a
Hy,

I'm currently trying to solve a developed turbulent channel flow problem with a FV code. I have several turbulence models implemented into it, such as SA, keps, kom, EASM and Reyolds-stress ones.

I solve the turbulence convection by a 1st-order upwind scheme. Turbulence diffusion is solved by a 2nd-order centred scheme.

In the developed channel flow case, however, convection is definitely zero. Therefore, I won't get any type of artificial dissipation coming from it.

What I'm finding with all turbulence models in this channel flow case is an awkward oscillation of turbulence profiles, very much similar to an odd-even decoupling behaviour.

I can somehow reduce this behaviour by introducing an additional artificial dissipation term to the turbult transport. This additional AD terms are based on a 4th-difference-like operator. For the k equation and eps equation, this approach succeeded with very low amounts of artificial dissipation. For the omega equation, as well as the modified eddy viscosity of Spalart-Allmaras, oscillations can only be removed away from the wall, with higher values of the AD constant, but very large oscillation are still found near the wall.

I would like to know if anyone had this kind of problem. How could I successfully avoid oscillations in the turbulent profiles?

Thanx a lot in advance for your help.
  Reply With Quote

Old   March 20, 2005, 09:29
Default Re: Stabilizing turbulence equation in channel flo
  #2
Bak_Flow
Guest
 
Posts: n/a
Hi,

just one point which you may already have considered. Depending upon how accuratly you solve the linearized equation set (and since very few of us can afford a direct solve) there is the possiblity that we get intermediate solutions during iteration which have negative values of k and/or epsilon. This can happen even if we use a bounded advection discretization just because we don't solve it exactly. This can happen locally with a multigrid method as we send-up coarse grid corrections....and sometimes looks like a odd-even coupling depending on the corrections that are injected! If things work against us then the negative values can lead to negative diffusion. So adding a bit of artificial diffusion keeps the solution on the "right side of the tracks".

What are you using for your linear equation solver? Maybe try a sensativity to grinding in a tighter solution to the linear equations. Also some intermediate checks on negative k, eps, etc.

Just an idea............Bak_Flow
  Reply With Quote

Old   March 21, 2005, 08:37
Default Re: Stabilizing turbulence equation in channel flo
  #3
Biga
Guest
 
Posts: n/a
I'm already considering the clipping of k, eps and so on. That's REALLY necessary! =)

The oscillations I'm observing nevertheless are far from the lower bounds of such variables. In other words, it's not the clipping that's causing the problem. I've also turned multigrid off and the same behaviour is observed. =\

I'm solving the turbulence equations by a first-order implicit Euler scheme.

Thanx!
  Reply With Quote

Old   March 21, 2005, 15:25
Default Re: Stabilizing turbulence equation in channel flo
  #4
Daniel
Guest
 
Posts: n/a
Well, it may be useless what I am saying, but the details you give me makes me think your grid is poor. I wouldn't use any artificial dissipation with turbulent models, and I'd make a try increasing a lot ( but really a lot! ) the number of points.

  Reply With Quote

Old   March 22, 2005, 07:29
Default Re: Stabilizing turbulence equation in channel flo
  #5
Biga
Guest
 
Posts: n/a
I got 60 points inside the boundary layer, and y+ is about 0.3. I don't think that's so poor a grid... is it? =\
  Reply With Quote

Old   March 22, 2005, 21:06
Default Re: Stabilizing turbulence equation in channel flo
  #6
Bak_Flow
Guest
 
Posts: n/a
Hi,

well it sounds like you are have already done a lot of trouble-shooting!

Is there some reason for a "shock" in the turbulence quantities? That is what it would appear similar to..and obviously you have a good grid on this....as you say in your last response!

One useful thing is to take away the fact that these are tubulence scalars, fix a uniform velocity field and solve it as a linear scalar-diffusion equation with sources. You can come up with some appropriate problems by passing a known solution through the differential equations to get the source (RHS) and boundary conditins that are consistient. Then put in this source in your code and see what the solution is, if it reduces regularly (1st order) with grid refinement, etc. You can pick out bugs and something inconsistient.

The other thing I might suspect is boundary condition implementation. How did you do this for the turbulence equtions? Is everything consistient?

Just some ideas.........Bak
  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
DNS of turbulence in a channel flow YANGLIANG OpenFOAM 0 March 4, 2010 09:40
mass flow in is not equal to mass flow out saii CFX 2 September 18, 2009 08:07
Code release: Flow Transition and Turbulence Chaoqun Liu Main CFD Forum 0 September 26, 2008 17:15
turbulent channel flow profile otto mierka Main CFD Forum 3 August 23, 2005 11:56
compressible channel flow.. R.D.Prabhu Main CFD Forum 0 July 17, 1998 17:23


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