
[Sponsors] 
Time dependent solver of unsteady Navier Stokes 

LinkBack  Thread Tools  Search this Thread  Display Modes 
May 17, 2014, 11:45 
Time dependent solver of unsteady Navier Stokes

#1 
Member
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 12 
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 

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 PetrovGalerkin (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  
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,246
Rep Power: 67 
Quote:
"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 timespace discretization 

May 18, 2014, 13:52 

#7  
Member
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 12 
Quote:
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

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  
Member
Sami
Join Date: Nov 2012
Location: Cap Town, South Africa
Posts: 87
Rep Power: 12 
Quote:
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

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.


Tags 
comsol, navier stoke solver, transient flow, unsteady flow 
Thread Tools  Search this Thread 
Display Modes  


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 