# Convergence problems

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

May 12, 2010, 14:33
Convergence problems
#1
New Member

Join Date: Apr 2010
Posts: 10
Rep Power: 9
Hi, I'm new to CFD and I'm having some convergence problems with my simulation so wonder if someone here can give me some advice.

What I'm trying to do is to simulate the flow in a refiner for processing paper fibers. The refiner consists of two plates facing each other, one with bars and one flat plate. The flat plate rotates at around 750 rpm and the other plate is not moving. The distance between them is around 0.2 mm and the diameter of the plates is 600 mm.

The narrow gap between the plates and the velocity of the flat plate makes it difficult to get a converged solution so I wonder if you have any tips on for instance what visocity model to use or if there are any other things I can do to improve the results?

I've tried using k-epsilon and LES models but only get converged results for velocities around 1 rpm. The mesh I'm using has around 1.2 million elements and I don't think I can add so many more elements or the calculations will take too long.

I'm adding some pictures of the mesh so you can understand the problem better. The first picture shows the whole model (a coarser mesh than used in the simulation) and the second one is an enlargement of the gap-region (actual mesh size used in the simulation).

Thanks!
Attached Images
 Mesh.jpg (88.5 KB, 31 views) Gap.jpg (97.3 KB, 31 views)

 May 14, 2010, 10:43 #2 Senior Member   Chris Join Date: Jul 2009 Location: Ohio, USA Posts: 169 Rep Power: 10 You said that the solution won't converge, but what specifically is it doing? Also, what are your boundary conditions? With respect to your grid, it looks like you used a uniform grid spacing along the flat plate. I would suggest clustering grid near the entrance/exit of the gaps, as there are gradients there that you need to resolve. The sharp corners might be causing you some problems, too.

June 2, 2010, 08:35
#3
New Member

Join Date: Apr 2010
Posts: 10
Rep Power: 9
Hi Chris, thanks for your help!

I've tried your suggestion and modified the mesh (hopefully the way you suggested, not sure if I understood you correctly). I am using pressure boundary conditions on both inlet and outlet.

Because the rotating wall is adjacent to the inlet in the model above I got a reversed flow in that area (because of the centrifugal force on the fluid close to the rotor I guess) so I extended the inlet a bit which solved the problem. However, despite the modification of the mesh and the inlet I still have a problem with the convergence. When I use the first order solver I can get the largest residual (continuity) down to approximately 4e-3 (for 600 rpm, when I decrease the under-relaxation factors of the pressure to 0.1 and the momentum to 0.4) but when I try using the second order solver the residuals jump up again.

Below are some pictures to illustrate how the solution behaves. The first 4000 iterations I have used a first order steady solution and then in the first picture I have utilized that results to initialize a second order transient solution with 1 ms time step. In the second picture I've used the first order steady solution to initialize a second order steady solution. Sorry about the monochrome pictures - the largest residual is the continuity in every simulation. I also attached some pictures of the new mesh.
Attached Images
 stationary+transient.jpg (48.4 KB, 18 views) stationary+stationary.jpg (40.2 KB, 16 views) Mesh 1.jpg (95.9 KB, 12 views) Mesh 2.jpg (49.1 KB, 13 views) Mesh 3.jpg (97.7 KB, 11 views)

June 2, 2010, 08:37
#4
New Member

Join Date: Apr 2010
Posts: 10
Rep Power: 9
Here is a picture of the model aswell
Attached Images
 Model.jpg (60.0 KB, 14 views)

 June 2, 2010, 10:58 #5 Senior Member   Chris Join Date: Jul 2009 Location: Ohio, USA Posts: 169 Rep Power: 10 What are you monitoring besides the residuals? Have you looked at the solution to see what's going on?

 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 franzdrs Main CFD Forum 0 June 15, 2009 18:17 hedge FLUENT 0 November 27, 2007 10:24 Fonzie CFX 1 March 23, 2007 09:46 Chetan FLUENT 3 April 15, 2004 19:13 Emilien FLUENT 3 May 3, 2002 08:43