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

Non-Newtonian Simulation Issue

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 11, 2017, 15:56
Default Non-Newtonian Simulation Issue
  #1
New Member
 
Max
Join Date: Jun 2017
Posts: 21
Rep Power: 7
max.kozak is on a distinguished road
Hi all,

I am running a Non-Newtonian simulation using a Herschel-Bulkley model. I am wanting to run over a shear range from 0 to 148s^-1. The simulation will only run it from 0.0001 to 148s^-1, but there are a few areas where the shear is actually about 1e^-6 s^-1. However, whenever I lower the range to this value, I do not get the solution to converge. Does anyone have any experience with this, or any ideas regarding how to accurately run the simulation and capture the results where shear rate is this low? Thanks for any advice regarding this.
max.kozak is offline   Reply With Quote

Old   September 11, 2017, 19:56
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,234
Rep Power: 135
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
This is a common problem with non-Newtonian models as they often have divide by zero and similar issues at zero shear strain rate. So I think you will find you cannot run it down to zero SSR as it is not defined. If you look at the CFX implementation it has a maximum and minimum SSR so you can avoid these problems by artificially changing the SSR in these regions to a value where it is defined. Obviously this affects the result, the question is whether it is significant or not.

So I would recommend you do a sensitivity study on the minimum SSR setting and see how low you need to go.
ghorrocks is offline   Reply With Quote

Old   September 12, 2017, 19:33
Default
  #3
New Member
 
Max
Join Date: Jun 2017
Posts: 21
Rep Power: 7
max.kozak is on a distinguished road
Thanks ghorrocks. I'm reading about using a CEL, and I'm wondering if you think this would be a good option? Perhaps it would be reasonable to say if the shear strain rate is less than the minimum CFX is defined for, then the dynamic viscosity value is some constant value? In reality, I'm not sure if this would affect the results too significantly, but I was thinking it could work around the obviously very flawed results I currently have.

Thanks
max.kozak is offline   Reply With Quote

Old   September 12, 2017, 19:54
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,234
Rep Power: 135
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
The Herschel-Bulkley model in CFX has max and min thresholds built in. There is no need to use CEL to do this. Just use the built in functions. But you will need to do a sensitivity analysis to check that the values you are using stop the excessive values which lead to numerical failure, but are wide enough to allow accurate modelling of your case.
ghorrocks is offline   Reply With Quote

Old   September 14, 2017, 19:36
Default
  #5
New Member
 
Max
Join Date: Jun 2017
Posts: 21
Rep Power: 7
max.kozak is on a distinguished road
Hi Glenn,

It seems the values I am using with the built-in range are not stopping the excessive error. I'm wondering, if instead of directly applying the Herschel-Bulkley model I create my own expression, that I could work around this issue?

Is there any way to create an expression where I say,
Dynamic Viscosity=some function at a certain range, but when shear strain rate falls out of the range, say below 0.001s^-1 (where I was having problems), then the Dynamic Viscosity=some value? I was thinking an if or step function could work around this??
max.kozak is offline   Reply With Quote

Old   September 17, 2017, 22:00
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,234
Rep Power: 135
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
If the built in limits are not stopping the error then tighten them further. It is far preferable to use the built in stuff rather than generate your own function, as using discontinuous functions for any material property almost always causes grief.
ghorrocks is offline   Reply With Quote

Reply

Tags
cfx 12

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
Issue with time in transient simulation Oula ANSYS 0 September 5, 2016 10:38
Vof sloshing simulation issue navi CFX 4 August 18, 2016 01:51
Wind Tunnel Flow Simulation Mass Flow BC Issue ledelman SU2 4 August 3, 2014 19:38
mass flow rate issue in supersonic nozzle simulation xkang FLUENT 0 July 31, 2014 17:06
Multiphase and Non Newtonian fluid wanna88 FLUENT 0 May 27, 2013 01:09


All times are GMT -4. The time now is 09:03.