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

Boundary conditions/solver

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 9, 2012, 23:45
Default Boundary conditions/solver
  #1
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
Hi!

I have to simulate a screw extruder.

I have set up the geometry - I have an inlet and an outlet needle.
I have the inlet as one part, the screw fluid as one part and the needle part.

I meshed within ANSYS workbench. Named the inlet and outlet as well as the screw wall.
I set up: Using laminar flow, defining a new fluid (PCL) with values I found in the literature.
For the cell zone condition I put fluid everywhere. For the screw fluid body I also tick frame rotation with rotation around - y axis with a certain speed.
Boundary conditions: Inlet-pressure 7e5 Pa, outlet-pressure 0 Pa gauge pressure, screw_wall is rotating around - y axis with the same speed as the frame rotation.

Solver is Presto and second order upwind momentum.

Monitors is the convergence turned off.

Then I initialize.
And then run until stable around 4000 iteration steps.

PROBLEMS that I have:
1) it is stable, but in the 10^3 range rather than 10^-4 range.
2) I tried standard/ second as well as standard/first, but the problems remains.
3) I didn't rotate the frame. And the same thing happens (stabilized at around 10^3)
4) If I make the whole extruder out of one part, then I get stabilizing in the 10^-4 to 10^-16 range for standard/second. If I use presto - I get still 10^3 and oscillating like crazy.

That brought me to the thought, that something must be wrong with my parts and their connection maybe.

Please give me some suggestions what I can try out next.

Christiane

P.S.: I just realized that by setting the extruder up out of three parts, the coordinate system is not in the same position, so that the rotation is not around the rotation axis of the screw. Ho do I solve that?

Last edited by chrisausAUS; May 10, 2012 at 00:17.
chrisausAUS is offline   Reply With Quote

Old   May 10, 2012, 01:01
Default
  #2
Member
 
Narendra Gadwal
Join Date: Jul 2010
Location: India
Posts: 35
Rep Power: 6
narendra.gadwal is on a distinguished road
Send a message via Yahoo to narendra.gadwal Send a message via Skype™ to narendra.gadwal
Check the mesh quality, scale in Fluent.
Try to run in first order for some iterations.
Create a monitor point in the domain and plot the values of velocity at that location wrt no of iterations.
narendra.gadwal is offline   Reply With Quote

Old   May 10, 2012, 01:27
Default Mesh Quality
  #3
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
It tells me that maximum cell skewness is greater than 0.98 - I refined the mesh, but it seems that this mistake does not disappear.
chrisausAUS is offline   Reply With Quote

Old   May 10, 2012, 01:30
Default First order
  #4
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
I tried running it in standard and first order (a friend told me till it stabilizes) - but unfortunately it doesn't stabilze in the minus range.
chrisausAUS is offline   Reply With Quote

Old   May 10, 2012, 01:31
Default Monitor point
  #5
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
Do you mean within FLUENT? Can you please explain a bit more - or send me a link whre this is explained better (I'm new to FLUENT)
chrisausAUS is offline   Reply With Quote

Old   May 10, 2012, 01:34
Default Rotation Origin
  #6
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
Hi!

I created a new coordinate system within ANSYS Mesh and could read out the values to the global coordinate system.

Then I tried to change the coordinates within the rotation origin in:
1) both, the cell zone condition for the screw part AND the screw wall in the rotation in the boundary conditions
2) just in the cell zone
3) just in the boundary screw wall
4) making the frame stationary

All didn't converge.

Christiane
chrisausAUS is offline   Reply With Quote

Old   May 10, 2012, 01:58
Default
  #7
Member
 
Krishna
Join Date: Oct 2009
Posts: 34
Rep Power: 7
t.krishnamohan is on a distinguished road
Complete geometry should be a single part
t.krishnamohan is offline   Reply With Quote

Old   May 10, 2012, 02:38
Default
  #8
Member
 
Narendra Gadwal
Join Date: Jul 2010
Location: India
Posts: 35
Rep Power: 6
narendra.gadwal is on a distinguished road
Send a message via Yahoo to narendra.gadwal Send a message via Skype™ to narendra.gadwal
Skewness is greater than 0.98 means your mesh is not of good quality.
Please look at the mesh you created and see that the quality is good.
narendra.gadwal is offline   Reply With Quote

Old   May 10, 2012, 02:38
Default
  #9
Member
 
Narendra Gadwal
Join Date: Jul 2010
Location: India
Posts: 35
Rep Power: 6
narendra.gadwal is on a distinguished road
Send a message via Yahoo to narendra.gadwal Send a message via Skype™ to narendra.gadwal
if possible send me a schematic diagram
narendra.gadwal is offline   Reply With Quote

Old   May 10, 2012, 02:58
Default Schematic setup
  #10
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
Here is the schematic set up.

I model the fluid, such that the screw part is hollow (from the screw inside)

I hope it is understandable - otherwise please ask again.

Christiane
Attached Files
File Type: pdf easy set up.pdf (39.0 KB, 16 views)
chrisausAUS is offline   Reply With Quote

Old   May 10, 2012, 03:14
Default
  #11
Member
 
Narendra Gadwal
Join Date: Jul 2010
Location: India
Posts: 35
Rep Power: 6
narendra.gadwal is on a distinguished road
Send a message via Yahoo to narendra.gadwal Send a message via Skype™ to narendra.gadwal
The geometry doesn't look that complicated. Do you have enough number of cells in the needle ?
The case also is like a basic laminar flow with known properties of the fluid. I don't think it should not create a convergence issue. Do you have ICEM to generate the mesh ?
narendra.gadwal is offline   Reply With Quote

Old   May 10, 2012, 19:58
Default
  #12
New Member
 
Christiane
Join Date: May 2012
Posts: 8
Rep Power: 4
chrisausAUS is on a distinguished road
Number of cells in the needle: I did the face sizing option to mesh in the first place and then I used the refinemnt number 2 to refine the needle. (I thought that this would be good enough)

But even now - standard/second, presto/second, standard/first all go up after 18 iterations step to around 10^3.

What is ICEM? If you mean the ANSYS ICEM CFD something - I can't see in my list of the workbench.
chrisausAUS is offline   Reply With Quote

Old   May 11, 2012, 00:14
Default
  #13
Member
 
Narendra Gadwal
Join Date: Jul 2010
Location: India
Posts: 35
Rep Power: 6
narendra.gadwal is on a distinguished road
Send a message via Yahoo to narendra.gadwal Send a message via Skype™ to narendra.gadwal
Can you send me the geometry ?
narendra.gadwal 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
Implementation of boundary conditions for FVM Tom Main CFD Forum 7 August 26, 2014 06:58
RPM in Wind Turbine Pankaj CFX 9 November 23, 2009 05:05
CFX doesn't continue calculation... mactech001 CFX 6 November 15, 2009 22:25
Boundary conditions? Tom Main CFD Forum 0 November 5, 2002 02:54
Boundary Conditions Jan Ramboer Main CFD Forum 11 August 16, 1999 09:59


All times are GMT -4. The time now is 14:45.