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

Maximum length scale for area density

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   January 28, 2014, 11:29
Default Maximum length scale for area density
  #1
New Member
 
Join Date: Oct 2013
Posts: 21
Rep Power: 12
ThomasN is on a distinguished road
Hello everybody,

i'm simulating a multiphase flow with the free surface model. The simulation is very unstable because there is much dynamic in the system. I figured out that a little value of "Maximum length scale for area density" stabilizes my calculation. I also see an effect in the solution. Unfortunately the documentation gives no informations about the meaning of this setting. In the forum I also didn't found anything related to this theme.

Could anybody explain the physical relationship please?

Greetings

Thomas
ThomasN is offline   Reply With Quote

Old   September 1, 2014, 04:41
Default
  #2
New Member
 
Join Date: Oct 2013
Posts: 21
Rep Power: 12
ThomasN is on a distinguished road
Does nobody have an idea??
ThomasN is offline   Reply With Quote

Old   September 1, 2014, 07:44
Default
  #3
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,665
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
I am not familiar with that setting so cannot comment.

But your instability is probably due to more than just that one single parameter. This FAQ lists a number of things to look at when you simulation is unstable: http://www.cfd-online.com/Wiki/Ansys...do_about_it.3F
ghorrocks is offline   Reply With Quote

Old   September 1, 2014, 10:49
Default
  #4
New Member
 
Michael
Join Date: Jan 2012
Location: Erlangen, Germany
Posts: 3
Rep Power: 14
mrueger is on a distinguished road
Hello Thomas,
i hope the citation from ANSYS HELP is allowed in this forum:
The parameter is explained in the CFX Theory Guide (cit. without formula):

>>5.2.1.1.3. The Free Surface Model
>>
>>The free surface model attempts to resolve the interface between the
>>fluids. If there are just two phases in the simulation, the following
>>equation is used for interfacial area density:
>>
>>
>>(5–16)
>>
>>This area density is clipped to be no smaller than L_max, where L_max is
>>controlled by the parameter Maximum Length Scale for Area Density,
>>which takes a default value of 1 m.

The interfacial area density is explained in chapter 5.2.1.1

Greetings

Michael
mrueger is offline   Reply With Quote

Old   September 2, 2014, 03:54
Default
  #5
New Member
 
Join Date: Oct 2013
Posts: 21
Rep Power: 12
ThomasN is on a distinguished road
Hi Michael,

thanks for your advice. In the meantime I found the section too. So I influence the "interficial area density" with this parameter. That means: If the value is very low the interficial area density in my flow field is constant and very high, which is nonsense but it stabilises my calculation. I've no explanation for that behaviour. Maybe the interfaces are resolved too bad and that produces problems. What does the interfacial area density influence? The drag force? The heat transfer?

I also figured out, that a value of the drag coefficient which is much higher than the default value of 0,44 has a similar effect. The convergence is much better and the flow field looks like if I reduce the "max. length scale for area density".

Does anybody have an advice for this value? 0,44 is related to the flow around a spherical.

Thomas
ThomasN is offline   Reply With Quote

Reply

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
Turbulence length scale and integral length scale rizhang CFX 2 April 22, 2016 08:22
Integral Length Scale vs Turbulent Length Scale Scott Nordsen FLUENT 2 January 20, 2014 02:17
how to calculate Turbulent Length Scale nuimlabib Main CFD Forum 2 August 18, 2009 05:22
REAL GAS UDF brian FLUENT 6 September 11, 2006 09:23
Warning 097- AB Siemens 6 November 15, 2004 05:41


All times are GMT -4. The time now is 05:30.