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

Multiphase flow through a nozzle: turbulence model

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

Like Tree1Likes
  • 1 Post By ghorrocks

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 30, 2016, 13:02
Default Multiphase flow through a nozzle: turbulence model
  #1
New Member
 
Leonardo Asfora de Oliveira
Join Date: May 2016
Posts: 9
Rep Power: 9
leonardoasfora is on a distinguished road
Hi,

Im modeling a homogeneous two phase flow through a convergent nozzle (liquid water as continuous phase an air as discrete phase). This study is part of my master degree. I just recently started using CFX (two months now) and i didn't have time to learn all its peculiarities and necessary knowledge (to be honest, i just have three more months).

My question is about which turbulence model should i use in order to model this kind of flow. I read that it's only necessary to use a low reynold number models (like SST) when there's, for instance, adverse pressure gradient or flow separation. Is this case i would need inflation layers with y+~1. In a convergent nozzle that's not the case, and i was wondering if k-e model with scalable wall function would be fine, as long as i guarantee 30 < y+ < 200 (is that right)?

Sorry if that's a stupid question, but as I said, i didn't have much time to learn everything. Sorry for my bad english

Thanks in advance.
leonardoasfora is offline   Reply With Quote

Old   May 30, 2016, 21:21
Default
  #2
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 17,695
Rep Power: 143
ghorrocks is just really niceghorrocks is just really niceghorrocks is just really niceghorrocks is just really nice
My advice is always "Use SST unless you have a very good reason not to." You have not stated a clear reason why not to use it - so use SST.

Some other comments:
Quote:
I read that it's only necessary to use a low reynold number models (like SST) when there's, for instance, adverse pressure gradient or flow separation.
Low Re models do not necessarily handle adverse pressure gradients or separations better.

Quote:
Is this case i would need inflation layers with y+~1. In a convergent nozzle that's not the case, and i was wondering if k-e model with scalable wall function would be fine, as long as i guarantee 30 < y+ < 200 (is that right)?
This is a very common misconception with SST - you do not need y+=1 for SST. With automatic wall functions it works fine using wall functions and can handle y+>30 and maybe up to 200 (or further).

So my recommendation is:
* Use SST
* Use wall functions, so y+ in the range of 30-200
* Do a mesh sensitivity study to see if that is OK. If it is then you are fine. Only go to y+=1 if you fail to get mesh insensitivity.
BlnPhoenix likes this.
ghorrocks is online now   Reply With Quote

Old   May 31, 2016, 09:42
Default
  #3
New Member
 
Leonardo Asfora de Oliveira
Join Date: May 2016
Posts: 9
Rep Power: 9
leonardoasfora is on a distinguished road
Thank you very much

I didn't want to use SST because it took more time to converge, but it isn't a big deal, so i'll use it.
leonardoasfora is offline   Reply With Quote

Old   May 31, 2016, 10:43
Default
  #4
Senior Member
 
Join Date: Jun 2009
Posts: 1,800
Rep Power: 32
Opaque will become famous soon enough
I am a bit surprised by your observations.

It is known that k-omega based models converge better than k-epsilon based models. The stiffness of the epsilon equation, and its behavior near the wall makes it nearly impossible to solve with extremely fine meshes. You can try that by solving the k-epsilon for a fully developed velocity profile in a pipe. It is a 1-D equation, and the solution is hard to converge as the Reynolds number is increased.

As Glenn suggested, stick to SST as long as it works for your case.

Just my 2 cents..
Opaque 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
mass flow in is not equal to mass flow out saii CFX 12 March 19, 2018 05:21
Multiphase Flow with NIST Real Gas Model Annka FLUENT 7 May 23, 2017 07:58
Flow over a blunt body - SST turbulence model bharath CFX 5 January 29, 2016 10:16
compressible flow in a counterflow nozzle d.vamsidhar FLUENT 0 November 24, 2005 01:45


All times are GMT -4. The time now is 20:28.