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

beginner: how to change flow behaviour?

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 26, 2011, 10:24
Default beginner: how to change flow behaviour?
  #1
New Member
 
Join Date: Jul 2011
Posts: 4
Rep Power: 6
yvar is on a distinguished road
Hi!

I am beginner with ansys cfx (v12) and have the next question

My challenge is a big water tank with some thin internal walls (it can be any configuration). Inlet is big hole from one side of the tank. Outlet is a small hole from the other side.
BCs: inlet - opening with some relative pressure; outlet - mass flow rate. Also rotating periodicity is used.
Flow velocity in main volume is slow, but high in the outlet

My problem is:
Solution gives flow trajectories with a lot of local swirlings. Flow moves not in straight direction (as seems to be logical in my model), but turns and flows in any other directions.
BUT I saw CFD analysis of such unit where flow trajectories were straight, uniform and logical...
I tried different turbulence models, remeshed model with smaller size but the pattern did not change significantly. Also tried change boundary conditions - no success..

So the questions are: 1. Can type of elements give so significant difference in flow behaviour? i mean if to remesh model with hexa, not tethra
2. I tried all given in CFX turbulence models, but without any additional controls.. all by default. Could you advice which turbulence model is more suitable for such task?

hope here is enough info.. thanks in advance for any advice!

Regards
yvar is offline   Reply With Quote

Old   July 26, 2011, 15:56
Default
  #2
Member
 
Join Date: Mar 2010
Posts: 42
Rep Power: 7
lindner is on a distinguished road
hi yvar,
Have you tried running with upwind scheme instead of high resolution? The upwind scheme will simplify your flow (it should not be used for final results) and maybe you will see something different.

regards
lindner is offline   Reply With Quote

Old   July 26, 2011, 16:12
Default
  #3
New Member
 
Join Date: Jul 2011
Posts: 4
Rep Power: 6
yvar is on a distinguished road
hi Lindner,
No, I haven't... Thanks, I'll try this scheme!
yvar is offline   Reply With Quote

Old   July 26, 2011, 18:54
Default
  #4
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,826
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
Upwinding will dampen the wobbly bits but also put so much dissipation in the model that your results are inaccurate. It might give you a good initial condition for a simulation with high order discretisation to get the accurate answer.

Can you post some images of what you are seeing?
ghorrocks is offline   Reply With Quote

Old   July 27, 2011, 05:29
Default
  #5
New Member
 
Join Date: Jul 2011
Posts: 4
Rep Power: 6
yvar is on a distinguished road
please find img of one of the results.. It is simple design where inlet (opening) is distributed evenly along outer cylindrical surface, outlet (mass flow) is a hole in center.

I saw cfd of the similar design with the same flow rate. Unfortunately I dont have that picture, but unlike my results, there was flow straight and uniform from outside to center. And i only know that overall dimensions and mass flow in that analysis was the same, I dont have more info...

I am not sure that model was correct, but my results also look strange as to me..

Upwind scheme did not change the pattern significantly
Scheme "specific blend factor 1" and high resolution for turbulence numerics is still running

some more info about attached results (high resolution, required RMS 1e-4)
+--------------------------------------------------------------------+
| Average Scale Information |
+--------------------------------------------------------------------+
Global Length = 1.5834E+00
Minimum Extent = 9.0300E-01
Maximum Extent = 3.3790E+00
Velocity = 2.3506E-03
Advection Time = 6.7361E+02
Reynolds Number = 2.3844E+03
+--------------------------------------------------------------------+
| Equation | Loop # | Peak Residual | Final Residual |
+--------------------------------------------------------------------+
| U-Mom-bottom | 3 | 4.27507E-02 | 5.78801E-03 |
| V-Mom-bottom | 3 | 2.45484E-01 | 9.89888E-03 |
| W-Mom-bottom | 3 | 1.25698E-02 | 4.74994E-03 |
| P-Mass-bottom | 1 | 6.72428E-03 | 2.95278E-08 |
| K-TurbKE-bottom | 1 | 5.22813E-01 | 2.01961E-03 |
| E-Diss.K-bottom | 1 | 9.28826E-01 | 9.74163E-04 |


Now I have no idea except remeshing the model with hexa... Automatic tethra gives too heavy model..
Attached Images
File Type: jpg q1.jpg (46.7 KB, 20 views)
yvar is offline   Reply With Quote

Old   July 27, 2011, 06:07
Default
  #6
Super Moderator
 
Glenn Horrocks
Join Date: Mar 2009
Location: Sydney, Australia
Posts: 10,826
Rep Power: 85
ghorrocks has a spectacular aura aboutghorrocks has a spectacular aura aboutghorrocks has a spectacular aura about
The SST turbulence model is to one to use unless you have a good reason not to.

It looks like you are far from converged. Here is some tips: http://www.cfd-online.com/Wiki/Ansys...gence_criteria
ghorrocks is offline   Reply With Quote

Old   July 28, 2011, 14:54
Default
  #7
New Member
 
Join Date: Jul 2011
Posts: 4
Rep Power: 6
yvar is on a distinguished road
Thanks for help!
seems I ve got needed results..
yvar is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
how nozzle's inlet flow speed change in an ocean situation sogolf FLOW-3D 1 August 3, 2009 15:41
total pressure change in a pipe flow Im FLUENT 4 February 6, 2002 20:09
mass flow inlet Denis Tschumperle FLUENT 7 August 9, 2000 02:19
Question on 3D potential flow Adrin Gharakhani Main CFD Forum 13 June 21, 1999 05:18
computation about flow around a yawed cone Tylor Xie Main CFD Forum 0 June 9, 1999 07:33


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