FloEFD Adaptive meshing

vaneedenp · July 27, 2011, 05:58

Hi all,

I am learning FloEFD and have started playing around with adaptive meshing. When I run the simulation and I look at the graph of one of my goals, it seems to converge to a certain value. But when I hit the refine option, the value of the goal drops on the graph and after a few iterations it picks up again to the previous value it was converging to. Everytime I do that the dip is there and it seems to be going to the same values. Shouldn't this dip disappear as the simulation converges? How often does one refine? I tried on every 10 iterations, then 20 and then 40. Same pattern.

Any advice?
Pieter

vaneedenp · July 27, 2011, 06:55

I found that increasing the total number of cells when the mesh refines each time makes this drop in value less. I will continue increasing number of cells until I do not get a drop anymore when I do a refine. This should tell me if the mesh is refined enough. Does this make sense? Am I correct in my assumption?

vaneedenp · July 27, 2011, 10:37

Got to a maximum number of cells, and the dip is still there. I am missing something here. Anybody help?

Boris_M · August 1, 2011, 09:17

Hi vaneedenp,
no, please don't refine too often. Usually I refine maximum 3 times, usually I use 2 times. This drop depends on if you look on absolute or normalized values. It is completely normal to have a jump like that. The strength of the jump can be less for other parameters.
You should also always work from a reasonable good mesh as the adaptive refinement is based on the actual results. So if you have a bad mesh you would refine on bad results which doesn't give a good refinement. These refinements are used to refine areas of stronger gradients as for example on a nozzles, flow separations like in a 90° bent or behind a blunt body or in a shok front to better resolve the physics and parameters over such gradients.

If you want to see if you have a mesh convergence (values don't change anymore with finer mesh) you can do a refinement and if the value converges again towards the same as before you won't get any better results after 10 further refinements.

I hope this helps,
Boris

vaneedenp · August 1, 2011, 09:44

Hallo Boris,

Thank you for your reply. I try to set up a proper mesh before I start. For the critical areas I do a local initialised mesh and try to get the spacing between cells as good as possible to have enough cells in small openings. When I run the simulation and refine it looks as if it converges to the same value. Which tells me that the mesh is good if I use your criteria.

Thanx for the info!

July 27, 2011, 05:58	FloEFD Adaptive meshing	#1
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 15	Hi all, I am learning FloEFD and have started playing around with adaptive meshing. When I run the simulation and I look at the graph of one of my goals, it seems to converge to a certain value. But when I hit the refine option, the value of the goal drops on the graph and after a few iterations it picks up again to the previous value it was converging to. Everytime I do that the dip is there and it seems to be going to the same values. Shouldn't this dip disappear as the simulation converges? How often does one refine? I tried on every 10 iterations, then 20 and then 40. Same pattern. Any advice? Pieter

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
adaptive meshing	clifford bradford	Main CFD Forum	14	September 3, 2022 20:13
Adaptive meshing technique	kpsudhakar	CFX	2	January 18, 2018 17:25
Adaptive meshing in CFX-5.5	zahid	CFX	1	June 11, 2003 19:37
Adaptive meshing	Michelle	Main CFD Forum	0	January 26, 2003 20:04
Adaptive Meshing	Chan K I	Main CFD Forum	0	October 27, 2000 03:40

July 27, 2011, 06:55		#2
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 15	I found that increasing the total number of cells when the mesh refines each time makes this drop in value less. I will continue increasing number of cells until I do not get a drop anymore when I do a refine. This should tell me if the mesh is refined enough. Does this make sense? Am I correct in my assumption?

July 27, 2011, 10:37		#3
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 15	Got to a maximum number of cells, and the dip is still there. I am missing something here. Anybody help?

August 1, 2011, 09:17		#4
Boris_M Disabled Join Date: Jul 2009 Posts: 616 Rep Power: 24	Hi vaneedenp, no, please don't refine too often. Usually I refine maximum 3 times, usually I use 2 times. This drop depends on if you look on absolute or normalized values. It is completely normal to have a jump like that. The strength of the jump can be less for other parameters. You should also always work from a reasonable good mesh as the adaptive refinement is based on the actual results. So if you have a bad mesh you would refine on bad results which doesn't give a good refinement. These refinements are used to refine areas of stronger gradients as for example on a nozzles, flow separations like in a 90° bent or behind a blunt body or in a shok front to better resolve the physics and parameters over such gradients. If you want to see if you have a mesh convergence (values don't change anymore with finer mesh) you can do a refinement and if the value converges again towards the same as before you won't get any better results after 10 further refinements. I hope this helps, Boris

August 1, 2011, 09:44		#5
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 15	Hallo Boris, Thank you for your reply. I try to set up a proper mesh before I start. For the critical areas I do a local initialised mesh and try to get the spacing between cells as good as possible to have enough cells in small openings. When I run the simulation and refine it looks as if it converges to the same value. Which tells me that the mesh is good if I use your criteria. Thanx for the info!