# solver problems at extreme low velocities

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

 August 31, 2006, 06:50 solver problems at extreme low velocities #1 matthias Guest   Posts: n/a Dear all, I am working on CFX to model a laminar flow through a micro channel with a diameter of 1 mm. Because of the narrow channel and a high viscosity the velocity is very low (<0.02m/s). I am working with Reynolds numbers about one. For Reynolds numbers higher then 3, I had no problems achieving convergence. By decreasing the velocity, the RMS values aren't running below a value of 1e^-4. This problem seems to be grid independent. I increased the number of elements with factor 80 and still had the same problems. Because of the solution of those not converged runs showed arbitrary behaviour of the flow field I think altering the convergence criteria is not a good idea. Maybe someone has an idea how I can handle this problem with CFX. Or is there any other possibility to solve it with a different method (e.g. DNS)? Thanks Matthias

 August 31, 2006, 08:10 Re: solver problems at extreme low velocities #2 Robin Guest   Posts: n/a Hi Matthias, Assuming your grid is fine enough, you are already doing DNS. Running the solver in double precision may help. Regards, Robin

 August 31, 2006, 17:10 Re: solver problems at extreme low velocities #3 Glenn Horrocks Guest   Posts: n/a Hi, Or to put it another way, your Reynolds number is so low that the flow is laminar with no turbulence. Therefore there are no turbulent structures to resolve so DNS is not applicable. If your simulation is mesh independant and fully converged then a laminar simulation should be very accurate. As Robin says, try double precision numbers. That is certainly the first thing to try. Glenn Horrocks

 September 1, 2006, 06:40 Re: solver problems at extreme low velocities #4 Manu Guest   Posts: n/a Hi Robin and Glenn, In connection with the same problem i would like to ask what should be done if double precision is also not working.Let me make my problem more clear: I am simulating a backward facing step with full geometry(NO SYMMETRY) with inlet ,outlet and wall.The level of grid size is less than what people have used for LES. Now my problem is also after switching on the double precision , the solution is not converging and oscilatting between 10^-3 and 10^-4. Why it is so?Half of the geometry putting symeetry converged well for the same reynolds number. Please suggest. Regards Manu

 September 4, 2006, 05:03 Re: solver problems at extreme low velocities #5 matthias Guest   Posts: n/a Ok, with double precision it works perfekct. Thanks PS.: Manu, I would suggest your grid is not accurate enough and symmetry is easier to solve, because the boundary values are constant (in angle direction) for every iteration step. Without the symmetry they might change from cell to cell for each iteration step to the next step which comes from the less accurate grid.

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post antonio CFX 3 July 14, 2011 19:33 Frank Main CFD Forum 0 July 24, 2006 13:48 JvK CFX 5 August 9, 2002 13:33 Roued CFX 1 October 2, 2001 16:49 eddy Main CFD Forum 3 September 7, 2000 06:15

All times are GMT -4. The time now is 22:55.