Recirculation bubble and wall shear stress

Daniele111 · July 9, 2010, 10:23

I try to use with my case the mesh of pitzDaily tutorials, I only change mesh, and in this case a I haven't overshooting on step. Why on my mesh is there this overshooting?

fgal · July 12, 2010, 07:18

I can see two jumps in the size of the cells in the x directions, of a factor 2 or 3, which is not a good meshing practice. You should use very smooth variations in the cell sizes, in all directions. Variations of 10 % between two consecutive cells, especially in critical areas such as the boundary layer, is sometimes cited, which is not stupid in the experience I had.

Cheers

Francois

Daniele111 · July 12, 2010, 07:24

I eliminate the overshooting I change mesh add a cell where the shape change slope

fgal · July 12, 2010, 07:28

That seems to be better, did you keep the same cell height at the wall because of the y+ criteria ? Any improvement in the results ?

Daniele111 · July 12, 2010, 07:36

No because the three cell near the slope has different areas the solution is bad

fgal · July 12, 2010, 07:41

I am sorry but I don't understand.

Daniele111 · July 12, 2010, 07:46

My English is very bad!

In my previous mesh near the slope i have only two cell. In my new cell three. Three cell with at least a vertex shared with the vertex where th obstacle changes its slope.

fgal · July 12, 2010, 08:11

I don't understand why the mesh configuration could change the geometry. This is more a problem of how you do the mesh.

Daniele111 · July 12, 2010, 08:15

Uhmm I don't understand your question. Sorry

fgal · July 12, 2010, 08:19

I think I did not understand what you said before.

-In my previous mesh near the slope i have only two cell. In my new cell three.
Here you say that you didn't use the same number of cells in the slope, is it that ?

- Three cell with at least a vertex shared with the vertex where th obstacle changes its slope.

And here that this change in the mesh configuration changes the slope of the obstacle.

This is not normal. The geometry should be fixed and then you do the mesh on it. If the way you do the mesh changes the geometry it is because you don't do the mesh in an appropriate manner.

Daniele111 · July 12, 2010, 08:27

No. I didn't change the shape of th obstacle, I divide my domain i four hex in my blockMeshDict, so near the vertex where start the step I have more cell. The cell number, two or three, aren't the number of cell on slope, but the cell number that surrounding the vertex where start the step

Daniele111 · September 20, 2010, 12:54

hi
I return on this problem. I must try to solve it with a monoblock grid instead of a multiblock grid. The problem isn't the separation a use obstacle with high aspect ratio, and I haven't overshooting. I think that the problem is the point with differet value of derivate (left and right). Is possible resolve it?

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Simulation of a single bubble with a VOF-method	Suzzn	CFX	21	January 29, 2018 00:58
Two-phase air water flow problems by activating Wall Lubrication Force	challenger85	CFX	5	November 5, 2009 05:44
air bubble is disappear increasing time using vof	xujjun	CFX	9	June 9, 2009 07:59
gas hold up in bubble	Shelly	Main CFD Forum	0	November 13, 2006 00:28
hwo to model rising bubble interact with wall	Dusky.He	Main CFD Forum	2	December 19, 2005 08:53

July 9, 2010, 10:23		#21
Daniele111 Senior Member Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 16	I try to use with my case the mesh of pitzDaily tutorials, I only change mesh, and in this case a I haven't overshooting on step. Why on my mesh is there this overshooting?

July 12, 2010, 07:18		#22
fgal Member Francois Gallard Join Date: Mar 2010 Location: Toulouse, France Posts: 43 Rep Power: 16	I can see two jumps in the size of the cells in the x directions, of a factor 2 or 3, which is not a good meshing practice. You should use very smooth variations in the cell sizes, in all directions. Variations of 10 % between two consecutive cells, especially in critical areas such as the boundary layer, is sometimes cited, which is not stupid in the experience I had. Cheers Francois

July 12, 2010, 07:28		#24
fgal Member Francois Gallard Join Date: Mar 2010 Location: Toulouse, France Posts: 43 Rep Power: 16	That seems to be better, did you keep the same cell height at the wall because of the y+ criteria ? Any improvement in the results ?

July 12, 2010, 07:36		#25
Daniele111 Senior Member Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 16	No because the three cell near the slope has different areas the solution is bad

July 12, 2010, 07:41		#26
fgal Member Francois Gallard Join Date: Mar 2010 Location: Toulouse, France Posts: 43 Rep Power: 16	I am sorry but I don't understand.

July 12, 2010, 07:46		#27
Daniele111 Senior Member Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 16	My English is very bad! In my previous mesh near the slope i have only two cell. In my new cell three. Three cell with at least a vertex shared with the vertex where th obstacle changes its slope.

July 12, 2010, 08:11		#28
fgal Member Francois Gallard Join Date: Mar 2010 Location: Toulouse, France Posts: 43 Rep Power: 16	I don't understand why the mesh configuration could change the geometry. This is more a problem of how you do the mesh.

July 12, 2010, 08:15		#29
Daniele111 Senior Member Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 16	Uhmm I don't understand your question. Sorry

July 12, 2010, 08:19		#30
fgal Member Francois Gallard Join Date: Mar 2010 Location: Toulouse, France Posts: 43 Rep Power: 16	I think I did not understand what you said before. -In my previous mesh near the slope i have only two cell. In my new cell three. Here you say that you didn't use the same number of cells in the slope, is it that ? - Three cell with at least a vertex shared with the vertex where th obstacle changes its slope. And here that this change in the mesh configuration changes the slope of the obstacle. This is not normal. The geometry should be fixed and then you do the mesh on it. If the way you do the mesh changes the geometry it is because you don't do the mesh in an appropriate manner.

July 12, 2010, 08:27		#31
Daniele111 Senior Member Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 16	No. I didn't change the shape of th obstacle, I divide my domain i four hex in my blockMeshDict, so near the vertex where start the step I have more cell. The cell number, two or three, aren't the number of cell on slope, but the cell number that surrounding the vertex where start the step

September 20, 2010, 12:54		#32
Daniele111 Senior Member Daniele Join Date: Feb 2010 Posts: 134 Rep Power: 16	hi I return on this problem. I must try to solve it with a monoblock grid instead of a multiblock grid. The problem isn't the separation a use obstacle with high aspect ratio, and I haven't overshooting. I think that the problem is the point with differet value of derivate (left and right). Is possible resolve it?