CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Passive scalar is diffusive without mesh motion

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 23, 2015, 08:59
Default Passive scalar is diffusive without mesh motion
  #1
Senior Member
 
Tobi's Avatar
 
Tobias Holzmann
Join Date: Oct 2010
Location: Leoben (Austria)
Posts: 1,449
Blog Entries: 5
Rep Power: 25
Tobi will become famous soon enoughTobi will become famous soon enough
Send a message via ICQ to Tobi Send a message via Skype™ to Tobi
Hi all,

I have a question to all of you. Maybe you are more familiar with that problem. The case is very simple. A 2d rectangular with an outlet velocity of 1m/s, p is fixed at the inlet with fixedValue; the walls are zeroGradient for U and p. The passive scalar is set to zeroGradient at each boundary. With setFields I generate an area with values = 1 for the passive scalar (everywhere else its zero).

Case 1 has no mesh motion
Case 2 has mesh motion due to the velocity field at the inlet (flux after the iteration = 0 at the inlet face).

The position of the scalar is the same (as expected) but why is without mesh motion the scalar diffusive? The transport of the scalar is only done by time and convection
(no diffusion term). In the mesh motion I added the mesh flux to the solver, but can this be the reason for less diffusion? But if I check the code:
Code:
fvm::ddt(S)
+fvm::div(phi, S)
There should be no diffusion in the case with no mesh motion. S is only transported by the flux, which is everywhere constant. :/

Here you can check the results: www.holzmann-cfd.de/cfd-online/passiveScalar.avi


Any hint is appreciated.
Thanks in advance,
Tobi
__________________
Best regards,
Tobias Holzmann

Some interesting OpenFOAM tutorials and videos on www.Holzmann-cfd.de
Tobi is online now   Reply With Quote

Old   July 23, 2015, 09:46
Default
  #2
Senior Member
 
Join Date: Oct 2013
Posts: 364
Rep Power: 6
chriss85 is on a distinguished road
Are you sure there should be no diffusion at all? As far as I know, there is always some artificial, numerical diffusion (though I'm not sure if this requires that an actual laplacian is present in the equation).

I have no idea why the diffusion is smaller with mesh motion, but I have no experience with dynamic meshes at all.
chriss85 is offline   Reply With Quote

Old   July 23, 2015, 10:32
Default
  #3
Senior Member
 
Tobi's Avatar
 
Tobias Holzmann
Join Date: Oct 2010
Location: Leoben (Austria)
Posts: 1,449
Blog Entries: 5
Rep Power: 25
Tobi will become famous soon enoughTobi will become famous soon enough
Send a message via ICQ to Tobi Send a message via Skype™ to Tobi
Hi chriss,

you are right. There is always Diffusion, hence there is only convection. But like you mentioned, I also don't know why I get the better result with mesh motion. Maybe it's due to the fact that the mesh flux reduces the convection term itself. (U-Umesh) which is smaller within the mesh motion case.
__________________
Best regards,
Tobias Holzmann

Some interesting OpenFOAM tutorials and videos on www.Holzmann-cfd.de
Tobi is online now   Reply With Quote

Old   July 23, 2015, 12:45
Default
  #4
Senior Member
 
kmooney's Avatar
 
Kyle Mooney
Join Date: Jul 2009
Location: Amherst, MA USA - San Diego, CA USA
Posts: 320
Rep Power: 10
kmooney is on a distinguished road
I would say that the moving mesh case has less diffusion because there is near zero relative advection taking place. Like you said, with (U-Urel)~=0, there really isn't much math going on to even allow for numerical diffusion.
kmooney is offline   Reply With Quote

Old   July 31, 2015, 04:18
Default less diffusive scheme
  #5
Senior Member
 
Fabian Roesler
Join Date: Mar 2009
Location: Germany
Posts: 209
Rep Power: 10
fabian_roesler is on a distinguished road
Right, I agree with kmooney. The relative advection in the mesh motion case is smaller and so the artificial diffusion is smaller as well.
What schemes did you apply and have you tried with a less diffusive scheme?

Cheers

Fabian
fabian_roesler is offline   Reply With Quote

Old   July 31, 2015, 05:18
Default
  #6
Senior Member
 
Tobi's Avatar
 
Tobias Holzmann
Join Date: Oct 2010
Location: Leoben (Austria)
Posts: 1,449
Blog Entries: 5
Rep Power: 25
Tobi will become famous soon enoughTobi will become famous soon enough
Send a message via ICQ to Tobi Send a message via Skype™ to Tobi
Hi Fabian,

at the beginning I was wondering why its less diffusive but the reason for that is exactly what we are talking about. (U - Urel) ~ 0. I also tried with different schemes, second order, 1st order, not much difference.

Thanks for all feedbacks,
__________________
Best regards,
Tobias Holzmann

Some interesting OpenFOAM tutorials and videos on www.Holzmann-cfd.de
Tobi is online now   Reply With Quote

Old   August 5, 2015, 09:51
Default
  #7
Member
 
Hannes
Join Date: Apr 2009
Location: Schleswig, Germany
Posts: 38
Rep Power: 9
Hannes_Kiel is on a distinguished road
See here for an explanation
__________________
FluidEngineeringSolutions
Hannes_Kiel 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
how to set periodic boundary conditions Ganesh FLUENT 13 January 22, 2014 05:11
Mesh motion with Translation & Rotation Doginal CFX 2 January 12, 2014 07:21
Difficulty in calculating angular velocity of Savonius turbine simulation alfaruk CFX 8 December 3, 2013 17:51
3D Hybrid Mesh Errors DarrenC ANSYS Meshing & Geometry 11 August 5, 2013 06:42
Prescribed mesh motion does not correspond to actual motion rbarrett CFX 8 June 30, 2011 13:22


All times are GMT -4. The time now is 08:23.