# Time dependent solver of unsteady Navier Stokes

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 May 17, 2014, 12:20 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,246 Rep Power: 67 what about your flow problem and Re number? Are you solving in DNS? have you numerical viscosity due to upwind?

 May 17, 2014, 12:47 #3 Member   Sami Join Date: Nov 2012 Location: Cap Town, South Africa Posts: 87 Rep Power: 12 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

 May 17, 2014, 18:37 #4 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,246 Rep Power: 67 Are you using low order upwind schemes?

 May 17, 2014, 21:29 #5 Member   Sami Join Date: Nov 2012 Location: Cap Town, South Africa Posts: 87 Rep Power: 12 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.

May 18, 2014, 05:04
#6
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Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,246
Rep Power: 67
Quote:
 Originally Posted by Mehrez 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

May 18, 2014, 13:52
#7
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Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 12
Quote:
 Originally Posted by FMDenaro "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

 May 18, 2014, 14:49 #8 Super Moderator   Praveen. C Join Date: Mar 2009 Location: Bangalore Posts: 342 Blog Entries: 6 Rep Power: 16 You may have to add some disturbance to induce unsteadiness. E.g., perturb the initial condition by a small amount.

May 18, 2014, 15:08
#9
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Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 12
Quote:
 Originally Posted by praveen 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

 May 19, 2014, 06:01 #10 Super Moderator   Praveen. C Join Date: Mar 2009 Location: Bangalore Posts: 342 Blog Entries: 6 Rep Power: 16 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.

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

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