Convergence problem

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 September 3, 2013, 10:32 Convergence problem #1 Member   Ali Mohamadi Join Date: Aug 2012 Posts: 32 Rep Power: 6 I have worked on a geometry (3D) that has one inlet (velocity Inlet) and three out let (pressure outlet) which the shape is similar to (+), inlet channel (upside) is converging and three others are diverging. The Reynolds number in critical point is about 700 and flow is incompressible. I have solved it with coarse mesh by using first and second order of discretization. By making the mesh finer, I only could solve it in first order with very good convergence criteria (density is below 10^-12); however, I could not solve it in second order. Residual jumped and after descending in some iteration it goes straight forward. I must mention that I made the mesh fine with constant coefficient (1.7) in all directions to preserve aspect ratio and skewness constant (I think my problem is sensitive to aspect ratio). I used the solution of first order as initial condition of second order. I solved it with segregated and coupled algorithm, steady and unsteady with decreasing time step up to (10^-7), also I decreased under relation factors or currant number to lowest possible number. In all these cases residual fall and then go again straight forward with a little fluctuation. There is only one point that the height of my channel is uniform in all parts and it equals to 0.2 mm, when I wanted to solve it in 2D form the flow became completely unstable and sometimes it diverged; however, after changing to 3D I could converged it in some cases but when I increase the mesh from 3 cells to higher number (5,6, …) I have convergence problem. I use fluent 14; do you have any suggestion for me? Is it possible that changing fluent to CFX could be useful for me?

 September 4, 2013, 05:28 #2 Member     Join Date: Jul 2011 Posts: 74 Blog Entries: 1 Rep Power: 11 chek this first __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 September 4, 2013, 05:35 #3 Member   Ali Mohamadi Join Date: Aug 2012 Posts: 32 Rep Power: 6 Thank you very much, I had read your post before, It was good review for me. I think my problem is about mesh; however, its difficult to find proper mesh for this problem.

 September 4, 2013, 05:37 #4 Member     Join Date: Jul 2011 Posts: 74 Blog Entries: 1 Rep Power: 11 What is your geometry? I could not understand it from your first post. Can you give us a screenshot (upload it first to some free server for pictures)? What mesh did you use? __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

September 4, 2013, 05:47
#5
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I use triangular mesh in all domain.
Attached Images
 1.jpg (71.8 KB, 70 views)

 September 4, 2013, 05:50 #6 Member     Join Date: Jul 2011 Posts: 74 Blog Entries: 1 Rep Power: 11 perhaps solution of first order is good enough. My problems also sometimes (with Mach number above 1) have difficulties converging with second order methods. Maybe your mesh is too fine. Try coarsening it a bit __________________ I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.

 September 4, 2013, 06:21 #8 Senior Member   duri Join Date: May 2010 Posts: 160 Rep Power: 8 It seems flow encounters very high adverse pressure gradient. It could be possible that separation in second order creates unsteadiness and possibly damped in first order solution. (picture of flow field would be helpful). Another possible reason could be incompatible boundary condition. Please post more details for better understanding of your issue.

 September 4, 2013, 09:46 #9 Member   Ali Mohamadi Join Date: Aug 2012 Posts: 32 Rep Power: 6 Thank you very much dear "Centurion2011" your information is very useful, in my case I study a lot about mesh. except skewness, aspect ratio is also very important. In the nozzle area, I used aspect ratio between 1 and 1.5 to solve it in first order discretization.

September 4, 2013, 10:18
#10
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Dear Duri

You are completely true, there is separation in existence in the right side and maybe in down part. I have attached the contours of pressure and velocity for both first and second order. I think I should make mesh more finer specially in right and down side. I used only rectangular cell, do you think combination with triangular is better or not?
do you have any suggestion for me?
Attached Images
 pressure-first order.jpg (85.0 KB, 49 views) velocity-first order.jpg (89.3 KB, 38 views) pressure-second order.jpg (81.2 KB, 39 views) velocity-second order.jpg (85.3 KB, 38 views)

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