FloEFD Adaptive meshing

vaneedenp · July 27, 2011, 04:58

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, 05:55

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, 09:37

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, 08:17

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, 08:44

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, 04:58	FloEFD Adaptive meshing	#1
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 14	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

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July 27, 2011, 05:55		#2
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 14	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, 09:37		#3
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 14	Got to a maximum number of cells, and the dip is still there. I am missing something here. Anybody help?

August 1, 2011, 08:17		#4
Boris_M Disabled Join Date: Jul 2009 Posts: 616 Rep Power: 23	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, 08:44		#5
vaneedenp New Member Pieter van Eeden Join Date: Jul 2011 Posts: 4 Rep Power: 14	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!