CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Post-Processing

wallShearStress, wallGradU are inconsistent

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

Like Tree10Likes
  • 1 Post By dancfd
  • 8 Post By dancfd
  • 1 Post By wyldckat

Reply
 
LinkBack Thread Tools Display Modes
Old   June 18, 2013, 23:57
Default wallShearStress, wallGradU are inconsistent
  #1
Senior Member
 
Daniel
Join Date: Jul 2009
Location: Montreal, Canada
Posts: 151
Rep Power: 8
dancfd is on a distinguished road
Hello all,

I just tried to calculate the wallShearStress entries of my case from wallGradU (dU/dy), rho (all found using OF postprocessors and sample utility), and nu (from transportProperties). Since tau = mu * (dU/dy) = rho * nu * (dU/dy), I figure that tau / (rho * dU/dy) should be constant, however this is not the case.

Can anyone explain why there is a discrepancy? I feel like I should be able to calculate wallShearStress somehow using its constituent parameters. This is important to me since the wallShearStress utility does not work for laminar flow and I want to compare the forces on an airfoil in laminar and turbulent flow for the same case.

Thanks in advance,
Dan
immortality likes this.
dancfd is offline   Reply With Quote

Old   July 14, 2013, 15:07
Default
  #2
Senior Member
 
Daniel
Join Date: Jul 2009
Location: Montreal, Canada
Posts: 151
Rep Power: 8
dancfd is on a distinguished road
Issue resolved - rather, issues:

1) for compressible cases, you must run "wallShearSress -compressible" with that flag in place. The only way to know to do this is to read the code for the wallShearStress utility. Suggestion: the utility should be able to distinguish between compressible and incompressible cases by itself, or force the user to use a flag for compressible and incompressible, to make this utility more user friendly.

2) by looking at the turbulence model (kwSST) code and the wallShearStress code, I found that OF calculates wallShearStress as follows:
tau_w = -(mu + mut) * (wallGradU)

When I made that change, the data matched.

Therefore, to calculate laminar, tau_w = -mu * (wallGradU) since mut = 0.

May others learn from my mistakes!
nimasam, kiddmax, xuan8908 and 5 others like this.
dancfd is offline   Reply With Quote

Old   May 25, 2014, 10:13
Default
  #3
Senior Member
 
Daniel
Join Date: Jul 2009
Location: Montreal, Canada
Posts: 151
Rep Power: 8
dancfd is on a distinguished road
To continue this monologue, I noticed that when running wallShearStress with and without the compressible flag on the same case, the results differed by a factor of rho yet the dimensions in both wallShearStress files were the same. This suggests that the equation for tau_w (wallShearStress) changes as follows for compressible/incompressible cases:

wallShearStress:
\tau_w = \nu_{eff} \frac{\partial U}{\partial y}

wallShearStress -compressible:
\tau_w = \mu_{eff} \frac{\partial U}{\partial y}= \rho \nu_{eff} \frac{\partial U}{\partial y}

Therefore, if I understand correctly:
\tau_{wCompressible} = \rho\tau_{wIncompressible}

Which would not be a problem if the dimensions in both cases did not read the same (m2/s2):
Code:
dimensions      [0 2 -2 0 0 0 0];
This looks to me like a bug. Thoughts?

Regards,
Daniel
dancfd is offline   Reply With Quote

Old   December 6, 2014, 06:30
Default
  #4
Super Moderator
 
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 8,251
Blog Entries: 34
Rep Power: 84
wyldckat is just really nicewyldckat is just really nicewyldckat is just really nicewyldckat is just really nice
Greetings Daniel,

Sorry for the (very) late reply, but only now did I trip over your thread, while looking for the exact answer you've given in post #2

The explanation to your latest question is simple: OpenFOAM uses a simplification of omitting rho to save up some CPU cycles in pointless multiplications by rho, when handling incompressible simulations... at least in most cases. This should also increase accuracy, since with each multiplication, there is always an error involved, no matter how small.

This to say that wallShearStress in incompressible mode does what it can with the data we provide it, namely since rho was not provided, it cannot multiply by an unknown rho
I also explained this with other details a few months ago (which was a few months after your last post), on this post: wall shear stress post #6

Best regards,
Bruno
gebab likes this.
wyldckat 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
getting (0 0 0) wallGradU value when i m probing wallGradU at some points suryawanshi_nitin OpenFOAM 1 December 9, 2013 12:36
wallShearStress Monitoring Mojtaba.a OpenFOAM Running, Solving & CFD 4 November 27, 2013 06:08
Problems with YPlusRAS and wallShearStress Peter85 OpenFOAM Post-Processing 20 May 5, 2013 08:42
wallGradU components amin66 OpenFOAM 8 March 3, 2013 11:12
WallShearStress for settlingFoam Elise OpenFOAM Post-Processing 1 August 28, 2012 05:50


All times are GMT -4. The time now is 12:34.