# SimpleFoam convergence problems

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 June 24, 2005, 05:09 As a newbie to OpenFoam I star #1 New Member   Klaus Schnitzlein Join Date: Mar 2009 Posts: 7 Rep Power: 10 Sponsored Links As a newbie to OpenFoam I started to set up a simple example, i.e. laminar flow (no turbulence model) in a rectangular channel with inlet at one end and outflow at the other. At the wall the respective boundary condition is prescribed. Despite the solvers for U seem to converge quite rapidly (I set the relative errors to 0 for U and p iterations) I encountered an ever increasing value for the continuity error. I tried to modify the grid, the superficial velocity, etc. but I failed to obtain a converged solution. Any help is greatly appreciated.

 June 24, 2005, 06:01 Compare your case to e.g. simp #2 Super Moderator   Mattijs Janssens Join Date: Mar 2009 Posts: 1,419 Rep Power: 18 Compare your case to e.g. simpleFoam/pitzDaily. Run that one without turbulence and see what happens. Do you have empty patches? If so is your mesh only one cell thick and perfectly aligned?

 June 24, 2005, 06:33 I ran pitzDaily without turbul #3 New Member   Klaus Schnitzlein Join Date: Mar 2009 Posts: 7 Rep Power: 10 I ran pitzDaily without turbulence, i.e. turbulenceModel laminar turbulence off and obtained similar results, i.e. ever increasing values for the time continuity errors.

 June 24, 2005, 07:00 So is your case steady 'in rea #4 Super Moderator   Mattijs Janssens Join Date: Mar 2009 Posts: 1,419 Rep Power: 18 So is your case steady 'in real life'? Is it turbulent? Are you trying to simulate a non-physical problem (e.g. simulate a turbulent flow without a model for the turbulence) simpleFoam might converge if your flow is steady. But if it isn't you can run turbFoam and see if that reaches a steady state.

 June 24, 2005, 07:15 Not surpised that pitzDaily bl #5 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 Not surpised that pitzDaily blows up, it is a turbulent flow. If you do not have enough damping in the form of modelling and/or numerics you will get a build up of turbulent energy in your velocity field. Basically, the flow will just become more and more unstable untill it blows up. There are many good reasons why turbulence models have to be used, and this is one of them. Try increasing your viscosity until your duct is in the laminar regime i.e. Re(b) < 1000~3000.

 June 24, 2005, 07:20 And/Or the instabilities are c #6 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 14 And/Or the instabilities are causing inflow through your outlet boundary, which is probably why you are getting the continuity errors. Using a properly specified inletOutlet boundary should "fix" this.

 June 24, 2005, 09:51 Thank you for your comments an #7 New Member   Klaus Schnitzlein Join Date: Mar 2009 Posts: 7 Rep Power: 10 Thank you for your comments and suggestions. According to preliminary test runs the problem may be may be solved at least partially by specifying an inletOutlet boundary.

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