CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > COMSOL

Time dependent solver of unsteady Navier Stokes

Register Blogs Community New Posts Updated Threads Search

Like Tree1Likes
  • 1 Post By praveen

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 17, 2014, 11:45
Exclamation Time dependent solver of unsteady Navier Stokes
  #1
Member
 
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 14
Mehrez is on a distinguished road
Hi,
I'm simulating flow of incompressible fluid in periodic media.
I have implemented dimensionless Navier Stokes equations with Comsol PDE module.

1- I resolve my problem with a steady solver (for high Reynolds number which leads to unsteady flow). I get a good solution.

2- I use this steady solution as initial values for my unsteady solver. The problem is that the unsteady solver is giving me a steady solution (for high Re !). The solution is still the same as the steady solver solution.
The unsteady solver : Direct MUMPS solver, Times: range(0,0.002,1), Time stepping method: BDF, Steps taken by solver: Strict, Error estimation: Exclude Algebraic.

Can you advice me please how can I fix this problem ?
Thanks in advance.

Mehrez
Mehrez is offline   Reply With Quote

Old   May 17, 2014, 12:20
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,897
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
what about your flow problem and Re number? Are you solving in DNS? have you numerical viscosity due to upwind?
FMDenaro is offline   Reply With Quote

Old   May 17, 2014, 12:47
Default
  #3
Member
 
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 14
Mehrez is on a distinguished road
Dear FMDenaro,
Thank you for your help.
My geometry is a 2D periodic square (fluid) with a solid square placed in its center. The fluid represents 75% of my geometry.
The governing equations are the time dependent dimensionless Navier Stokes equations (DNS), so I have just to give the Re number.
Boundary conditions:
- no slip BC at the interface fluid/solid.
- periodic BC, vertically and horizontally (The fluid is driven by a volumetric force F=(1,0) ).
I ran simulations with Re equal to 200 but I get stationary solution ! (comparing to OpenFoam, Re=200 gives unsteady flow).
Even for a very high Re number, the Comsol time dependent solver is still giving a steady solution.
Thanks,

Mehrez
Mehrez is offline   Reply With Quote

Old   May 17, 2014, 18:37
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,897
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Are you using low order upwind schemes?
FMDenaro is offline   Reply With Quote

Old   May 17, 2014, 21:29
Default
  #5
Member
 
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 14
Mehrez is on a distinguished road
If I'm not wrong a such scheme is used for stabilization ?, On Comsol I can use streamline upwind Petrov-Galerkin (SUGP) or GLS but trying to compute with and then without stabilization didn't change the results, I still have a stationary solution.
I suspect a problem with my solver.
Mehrez is offline   Reply With Quote

Old   May 18, 2014, 05:04
Default
  #6
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,897
Rep Power: 73
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by Mehrez View Post
If I'm not wrong a such scheme is used for stabilization ?, On Comsol I can use streamline upwind Petrov-Galerkin (SUGP) or GLS but trying to compute with and then without stabilization didn't change the results, I still have a stationary solution.
I suspect a problem with my solver.

"stabilization" is not necessary for viscous flows provided you solve up to the viscous scales...generally, such terms comes from solution of Euler equation with the presence of shocks.
In your case the scheme can be too dissipative, try using a more refined grid or use second order time-space discretization
FMDenaro is offline   Reply With Quote

Old   May 18, 2014, 13:52
Question
  #7
Member
 
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 14
Mehrez is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
"stabilization" is not necessary for viscous flows provided you solve up to the viscous scales...generally, such terms comes from solution of Euler equation with the presence of shocks.
In your case the scheme can be too dissipative, try using a more refined grid or use second order time-space discretization
Dear FMDenaro,
Thank you for these precisions.
If I have well understood the User's Manual, the upwind scheme is used to stabilize the solution (damp the oscillations), am I right ?
Which scheme do you think is too dissipative ?
I am not using a stabilization in my problem and I am solving with a BDF solver which uses backward differentiation formulas with order of accuracy varying from one (that is, backward Euler) to five.
For the velocity discritization, the order is 2 and 1 for pressure.
I can send you my Comsol model if you can run it.

Thank you for your help.

Best regards,

Mehrez
Mehrez is offline   Reply With Quote

Old   May 18, 2014, 14:49
Default
  #8
Super Moderator
 
Praveen. C
Join Date: Mar 2009
Location: Bangalore
Posts: 343
Blog Entries: 6
Rep Power: 18
praveen is on a distinguished road
You may have to add some disturbance to induce unsteadiness. E.g., perturb the initial condition by a small amount.
praveen is offline   Reply With Quote

Old   May 18, 2014, 15:08
Arrow
  #9
Member
 
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 14
Mehrez is on a distinguished road
Quote:
Originally Posted by praveen View Post
You may have to add some disturbance to induce unsteadiness. E.g., perturb the initial condition by a small amount.
Hello dear Praveen,
I use as initial values in the Comsol time dependent solver the solution of the stationary solver. It means that I don't have any perturbation in the initial values.
On the other side, If I do the same procedure on OpenFoam with the icoFoam solver, the results are much more better and I have unsteady flow for high Reynolds number.

Thank you for your help,

Best regards,

Mehrez
Mehrez is offline   Reply With Quote

Old   May 19, 2014, 06:01
Default
  #10
Super Moderator
 
Praveen. C
Join Date: Mar 2009
Location: Bangalore
Posts: 343
Blog Entries: 6
Rep Power: 18
praveen is on a distinguished road
It may happen that openfoam has some sort of asymmetry in the solution process which induces instabilities to develop. If it does not happen in comsol you have to add some disturbance yourself.
Mehrez likes this.
praveen is offline   Reply With Quote

Old   March 20, 2024, 04:57
Default Hi,I'm learning about it now.Can you share me how you implement dimensionless Navier?
  #11
New Member
 
lintaoli
Join Date: Mar 2024
Posts: 2
Rep Power: 0
luoan is on a distinguished road
Quote:
Originally Posted by Mehrez View Post
Hi,
I'm simulating flow of incompressible fluid in periodic media.
I have implemented dimensionless Navier Stokes equations with Comsol PDE module.

1- I resolve my problem with a steady solver (for high Reynolds number which leads to unsteady flow). I get a good solution.

2- I use this steady solution as initial values for my unsteady solver. The problem is that the unsteady solver is giving me a steady solution (for high Re !). The solution is still the same as the steady solver solution.
The unsteady solver : Direct MUMPS solver, Times: range(0,0.002,1), Time stepping method: BDF, Steps taken by solver: Strict, Error estimation: Exclude Algebraic.

Can you advice me please how can I fix this problem ?
Thanks in advance.

Mehrez
Hi,I'm learning about it now.Can you share me how you implement dimensionless Navier? THX
Does it build with weak form PDE ?
luoan is offline   Reply With Quote

Reply

Tags
comsol, navier stoke solver, transient flow, unsteady flow


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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Suggested unsteady, implicit solver stable with arbitrarily large time steps djbungee OpenFOAM Programming & Development 45 March 23, 2015 05:14
Moving mesh Niklas Wikstrom (Wikstrom) OpenFOAM Running, Solving & CFD 122 June 15, 2014 07:20
Unstabil Simulation with chtMultiRegionFoam mbay101 OpenFOAM Running, Solving & CFD 13 December 28, 2013 14:12
AMI interDyMFoam for mixer nu problem danny123 OpenFOAM Programming & Development 8 September 6, 2013 03:34
Orifice Plate with a fully developed flow - Problems with convergence jonmec OpenFOAM Running, Solving & CFD 3 July 28, 2011 06:24


All times are GMT -4. The time now is 15:30.