# Boundary conditions/solver

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

 May 9, 2012, 22:45 Boundary conditions/solver #1 New Member   Christiane Join Date: May 2012 Posts: 8 Rep Power: 7 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 9, 2012 at 23:17.

 May 10, 2012, 00:01 #2 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 8 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.

 May 10, 2012, 00:27 Mesh Quality #3 New Member   Christiane Join Date: May 2012 Posts: 8 Rep Power: 7 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.

 May 10, 2012, 00:30 First order #4 New Member   Christiane Join Date: May 2012 Posts: 8 Rep Power: 7 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.

 May 10, 2012, 00:31 Monitor point #5 New Member   Christiane Join Date: May 2012 Posts: 8 Rep Power: 7 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)

 May 10, 2012, 00:34 Rotation Origin #6 New Member   Christiane Join Date: May 2012 Posts: 8 Rep Power: 7 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

 May 10, 2012, 00:58 #7 Member   Krishna Join Date: Oct 2009 Posts: 34 Rep Power: 9 Complete geometry should be a single part

 May 10, 2012, 01:38 #8 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 8 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.

 May 10, 2012, 01:38 #9 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 8 if possible send me a schematic diagram

May 10, 2012, 01:58
Schematic setup
#10
New Member

Christiane
Join Date: May 2012
Posts: 8
Rep Power: 7
Here is the schematic set up.

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

Christiane
Attached Files
 easy set up.pdf (39.0 KB, 17 views)

 May 10, 2012, 02:14 #11 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 8 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 ?

 May 10, 2012, 18:58 #12 New Member   Christiane Join Date: May 2012 Posts: 8 Rep Power: 7 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.

 May 10, 2012, 23:14 #13 Member   Narendra Gadwal Join Date: Jul 2010 Location: India Posts: 35 Rep Power: 8 Can you send me the geometry ?

 Thread Tools Display Modes Linear Mode

 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 OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post Tom Main CFD Forum 7 August 26, 2014 05:58 Pankaj CFX 9 November 23, 2009 05:05 mactech001 CFX 6 November 15, 2009 22:25 Tom Main CFD Forum 0 November 5, 2002 02:54 Jan Ramboer Main CFD Forum 11 August 16, 1999 08:59

All times are GMT -4. The time now is 03:05.