CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > CFX

Problems moving a ball - mesh crashes

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 9, 2016, 09:11
Question Problems moving a ball - mesh crashes
  #1
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
Hi,

I am for the first time venturing in to the unknown land of moving meshes. I have followed the tutorial 22 with the ball check valve. My first problem is a very simple one:

I have a pipe with a ball in it. The ball doesn't fill out the pipe and water (or whatever other fluid I choose to use) can flow past it. I want to move the ball.

First I created a steady state simulation with no movement to use as input for the transient. The transient model I set up more or less exactly like the ball check valve tutorial.

The mesh is done by cutting as close as possible to the ball and doing multizone on the endpieces (theese should be able to deform quite a lot). The domain right around the ball is a tet-mesh. I specified that domain as a subdomain with moving mesh and the rigid body solution as the

I run with quite small time steps (1E-5) and it crashes before I can see any movement of the ball, ie second time step. The negative elements occur in domain on the inlet side, not where they should be compressed.

I am in doubt weather I should create stationary sub domains for the multizone regions.
lbergmann is offline   Reply With Quote

Old   February 9, 2016, 16:47
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

Is this a pre-defined motion or rigid body?
ghorrocks is offline   Reply With Quote

Old   February 10, 2016, 02:58
Default Rigid body
  #3
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
This is a rigid body movement.
lbergmann is offline   Reply With Quote

Old   February 10, 2016, 05:55
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Try running an even smaller time step. Debugging is much easier if you can get a single time step done so you can see the motion.

Also - is the immersed solid model suitable for this application? It is much simpler and does not require remeshing.
ghorrocks is offline   Reply With Quote

Old   February 10, 2016, 08:05
Default four timesteps
  #5
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
By lovering the velocity and the time step I managed to get four timesteps done (I get negativ elements in the fifth) done. I can now see the ball moving in the results file. Also now the negative element occurs on the side where the mesh is being compressed.

I assume I need to optimize the mesh I have an orthogonal angle of 33.9, expansion factor of 142 and an aspect ratio of 160.
The majority of the "bad" elements are due to inflation layers.
lbergmann is offline   Reply With Quote

Old   February 10, 2016, 17:19
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Don't forget about the mesh motion parameters like increase stiffness near walls. This can help keep inflation layers together.
ghorrocks is offline   Reply With Quote

Old   February 12, 2016, 03:52
Default mesh stiffness
  #7
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
Thanks a lot. I managed to get an orthogan angle of 38.1, an expansion factor of 49 and an aspect ratio of 99.

I have tried increasing the stiffness with a model exponent of 2 both near boundaries and at small volumes. The result is the same. What would you suggest having the model exponent at?
lbergmann is offline   Reply With Quote

Old   February 12, 2016, 04:45
Default
  #8
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
It is problem dependant. You have to try some values until you find one which works. Also look at the mesh motion diffusion variable as that can help see what is going on.

But in my experience problems like this are a sign that the meshing for mesh motion is not defined well. Can you show an image of how you have meshed this and what motion you expect to see?
ghorrocks is offline   Reply With Quote

Old   February 12, 2016, 05:00
Default picture of mesh
  #9
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
I have created an Imgur album http://imgur.com/a/1hcit

Also; where do I find the mesh motion diffusion variable. I haven't heard about that one before?
lbergmann is offline   Reply With Quote

Old   February 13, 2016, 05:06
Default
  #10
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
You will have more success with this I suspect if you make the inflation mesh/tet region region move with the sphere rigidly, and the extruded mesh on the left and right stretch to do your motion. It is much easier to keep extruded meshes working in mesh motion than inflation or tet meshes.
ghorrocks is offline   Reply With Quote

Old   February 15, 2016, 02:12
Default
  #11
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
The tet mesh moves with the rigid body. The multizone mesh (is that what you call extruded mesh?) how do I get that to stretch if not just by "letting it be". Do I need to have no inflation on the multizone?
lbergmann is offline   Reply With Quote

Old   February 15, 2016, 03:14
Default
  #12
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
I just tried taking the inflation layers off on the whole geometry, this is not perfect obviously, but it is now running on the 11'th iteration, so it did fix something.
lbergmann is offline   Reply With Quote

Old   February 15, 2016, 04:32
Default
  #13
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Have a close look at the mesh as it is running. You will probably see large mesh deformations in the same region the other mesh was failing at.
ghorrocks is offline   Reply With Quote

Old   February 15, 2016, 04:34
Default
  #14
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
How do I look at the mesh deformation while it is running?

I crashed after 13 iterations
lbergmann is offline   Reply With Quote

Old   February 15, 2016, 04:44
Default
  #15
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
Another problem:

When I want to visualize the movement of the ball I don't see any movement until the last timestep (the one that crashes) This is true for all of the runs.
lbergmann is offline   Reply With Quote

Old   February 15, 2016, 06:06
Default
  #16
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
Quoting from the FAQ (http://www.cfd-online.com/Wiki/Ansys...ent_wrong.3F):
Quote:
On the Output tab, go to "Transient Results Files" and set it to save a results file every time step. To keep the file size manageable, select "Include mesh", but make it "Selected Variables" and select no variables.
If nothing happens then it suddenly starts moving and crashes this could also be a rigid body motion problem. Are you sure you have reasonable values for the mass of the body and all the moments of inertia? Are you sure your fluid model is stable or does it go bezerk? It would be good to put some monitor points looking at things like the rigid body motion and flow conditions so you can see if something goes crazy near the crash.
ghorrocks is offline   Reply With Quote

Old   February 15, 2016, 06:18
Default
  #17
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
I have the same velocity and mass for the ball as in the tutorial I was following. I was monitoring the mass flow and that looks fine. Will try to monitor the velocities as well.
lbergmann is offline   Reply With Quote

Old   February 15, 2016, 08:04
Default
  #18
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
I can't seem to get it to plot the mesh while solving? It just shows the monitors as normal.
lbergmann is offline   Reply With Quote

Old   February 15, 2016, 23:36
Default
  #19
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 16,346
Rep Power: 126
ghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the roughghorrocks is a jewel in the rough
You have to load the results files in the post processor. They are not shown as the simulation progresses
ghorrocks is offline   Reply With Quote

Old   February 16, 2016, 02:20
Default
  #20
New Member
 
Line Bergmann
Join Date: Sep 2013
Posts: 20
Rep Power: 9
lbergmann is on a distinguished road
I am not sure if I managed to plot the mesh or what I did here. If I did it does not compress the mesh in the mapped regions.

I showed the regions and picked "Draw as lines" under Render.

I have created another album with the snaps http://imgur.com/a/TdM8V

Thank you so much for all your help. I feel completely hopeless at this.
lbergmann is offline   Reply With Quote

Reply

Tags
moving mesh.

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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 Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[snappyHexMesh] Add Mesh Layers doesnt work on the whole surface Kryo OpenFOAM Meshing & Mesh Conversion 12 March 11, 2020 16:16
Gambit problems Althea FLUENT 22 January 4, 2017 03:19
Moving mesh still crashes in parallel in 1.6-ext paul b OpenFOAM Running, Solving & CFD 36 February 17, 2014 08:35
Question on InterFoam moving mesh capabilities ziv OpenFOAM Running, Solving & CFD 0 April 23, 2008 09:11
Convergence moving mesh lr103476 OpenFOAM Running, Solving & CFD 30 November 19, 2007 14:09


All times are GMT -4. The time now is 10:46.