CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > Siemens > STAR-CCM+

unusual behavior with DEM-CFD

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 7, 2018, 04:23
Unhappy unusual behavior with DEM-CFD
  #1
New Member
 
george bergantz
Join Date: Mar 2009
Location: Seattle, Washington, USA
Posts: 9
Rep Power: 14
bergantz is on a distinguished road
When we do a simple DEM particle settling run in a closed box with no fluid (granular only) it works just fine. However when we do that same run with a fluid, once the particles have settled they start "hopping" and moving in a non-physical manner even when there is no other fluid motion, the fluid should be static.

Has anyone else encountered this bizarre behavior?

Thanks
bergantz is offline   Reply With Quote

Old   March 8, 2018, 04:17
Question
  #2
Senior Member
 
ashokac7's Avatar
 
Ashok Chaudhari
Join Date: Aug 2016
Location: Pune, India
Posts: 253
Rep Power: 8
ashokac7 is on a distinguished road
Send a message via Skype™ to ashokac7
What do you mean by particles are settled. Does is mean that they are settled on the wall or hanging in domain. And what is desired physical behavior you are looking for ?
ashokac7 is offline   Reply With Quote

Old   March 8, 2018, 11:49
Default
  #3
New Member
 
george bergantz
Join Date: Mar 2009
Location: Seattle, Washington, USA
Posts: 9
Rep Power: 14
bergantz is on a distinguished road
By settled I mean sedimentation to the floor. Think of simple spherical particles settling under gravity. Once they are resting on the floor no more motion should occur. And the fluid velocity should go to zero. However what I find is that once the particles are resting on the floor some randomly start to suddenly rise a short distance, maybe 5 particle diameters, then fall back down. This is obviously non-physical. Sometimes the whole particle bed will start to do this.

Has anyone else encountered this?
bergantz is offline   Reply With Quote

Old   March 9, 2018, 01:57
Smile
  #4
Senior Member
 
ashokac7's Avatar
 
Ashok Chaudhari
Join Date: Aug 2016
Location: Pune, India
Posts: 253
Rep Power: 8
ashokac7 is on a distinguished road
Send a message via Skype™ to ashokac7
Quote:
Originally Posted by bergantz View Post
By settled I mean sedimentation to the floor. Think of simple spherical particles settling under gravity. Once they are resting on the floor no more motion should occur. And the fluid velocity should go to zero. However what I find is that once the particles are resting on the floor some randomly start to suddenly rise a short distance, maybe 5 particle diameters, then fall back down. This is obviously non-physical. Sometimes the whole particle bed will start to do this.

Has anyone else encountered this?
Did you set the wall boundary condition for Lagrangian phase to "Stick"?. If they are stick once they come down to wall then they should not move. If not change this BC in Physics node for wall boundary. Even if they are not sticking to the wall then you can use Escape condition for particles at wall. But it may not be that accurate.
ashokac7 is offline   Reply With Quote

Old   March 9, 2018, 02:14
Default
  #5
New Member
 
george bergantz
Join Date: Mar 2009
Location: Seattle, Washington, USA
Posts: 9
Rep Power: 14
bergantz is on a distinguished road
Thank you, but consider this, if one does the DEM run under granular conditions, heer there is no fluid, the non-physical behavior does not occur. Particles settle and remain as they should without artificially forcing them to stick. So something in the fluid-particle interaction produces a non-physical force imbalance that makes the particles rise up- even those that are not in direct contact with the floor but are resting on other particles.
bergantz is offline   Reply With Quote

Old   March 9, 2018, 03:33
Smile
  #6
Senior Member
 
ashokac7's Avatar
 
Ashok Chaudhari
Join Date: Aug 2016
Location: Pune, India
Posts: 253
Rep Power: 8
ashokac7 is on a distinguished road
Send a message via Skype™ to ashokac7
Quote:
Originally Posted by bergantz View Post
Thank you, but consider this, if one does the DEM run under granular conditions, heer there is no fluid, the non-physical behavior does not occur. Particles settle and remain as they should without artificially forcing them to stick. So something in the fluid-particle interaction produces a non-physical force imbalance that makes the particles rise up- even those that are not in direct contact with the floor but are resting on other particles.
Yes. But if wall BC is Rebound then coefficient of restitution would be 1 and particles will rebound back in domain. Imparting accurate value of restitution would be the key to accurate solution then. That's why we use stick character to force the particle stick to the wall. Which in my thinking more accurate than rebound for the cases like spraying or painting etc. You can relate your case accordingly.
ashokac7 is offline   Reply With Quote

Old   May 15, 2018, 15:01
Default
  #7
Member
 
Casey
Join Date: Jun 2009
Posts: 98
Rep Power: 14
meangreen is on a distinguished road
Quote:
Originally Posted by bergantz View Post
Thank you, but consider this, if one does the DEM run under granular conditions, heer there is no fluid, the non-physical behavior does not occur. Particles settle and remain as they should without artificially forcing them to stick. So something in the fluid-particle interaction produces a non-physical force imbalance that makes the particles rise up- even those that are not in direct contact with the floor but are resting on other particles.
Have you checked your particle overlap? I wonder if when you turn on the fluid something changes in the solver and a large overlap aloud in granular phase turns to a large restoring force when continuum phase is coupled.
meangreen is offline   Reply With Quote

Reply

Tags
dem, dem-cfd, multiphase

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
CFD Online Celebrates 20 Years Online jola Site News & Announcements 22 January 31, 2015 01:30
Where do we go from here? CFD in 2001 John C. Chien Main CFD Forum 36 January 24, 2001 22:10
ASME CFD Symposium, Atlanta, July 2001 Chris R. Kleijn Main CFD Forum 0 August 21, 2000 05:49
Which is better to develop in-house CFD code or to buy a available CFD package. Tareq Al-shaalan Main CFD Forum 10 June 13, 1999 00:27
public CFD Code development Heinz Wilkening Main CFD Forum 38 March 5, 1999 12:44


All times are GMT -4. The time now is 16:17.