
[Sponsors] 
December 18, 2013, 08:34 
velocity vectors

#1 
Member
Join Date: Dec 2013
Posts: 53
Rep Power: 4 
Hi
I have simulated 2D flow applying velocity inlet and pressure outlet boundary conditions.Later I applied periodic boundary conditions at inlet making the outlet conditions as shadow zone.After convergence, I see the velocity vectors coming out of the outlet zone.Why is this happening? 

December 18, 2013, 13:03 

#2 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,305
Rep Power: 22 
Why should it not happen?


December 20, 2013, 02:21 

#3 
Member
Join Date: Dec 2013
Posts: 53
Rep Power: 4 
Thank you for the question that made me to think.I was initially in an impression that all the vectors will be represented in the closed domain.I came to that they need not be .
I have another problem now. Iam simulating the flow over solids(squares).For this, I created a domain(big rectangle) with solids squares inside.I gave velocity inlet boundary condition at the entrance and pressure outlet at the exit.I have meshed it and transferred to fluent.I have applied periodic boundary conditions at the inlet making the outlet shadow. I have run the case for different grids with convergence criterion of 106.As I refine the grid the the curve goes flatter.Why is this happening? Which value is to be taken for further analysis?please find attached a pic of the grid independent study result. 

December 20, 2013, 05:42 

#4 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,305
Rep Power: 22 
I have no idea which values you put into this figure, so it is impossible to comment on the results.
And please, attach figures in a file format that everybody can read without much effort, something like .png or .jpg would be nice. 

December 20, 2013, 10:31 
Grid independent study

#5 
Member
Join Date: Dec 2013
Posts: 53
Rep Power: 4 
Thanks for the response.
Please find the following files attached 1.Model with boundary conditions with out mesh 2.Model after mesh generation a.After mesh generation ,mesh is transferred to FLUENT. b.Periodic boundary conditions are applied at inlet making the outlet shadow.Convergence criterion is kept at 106. c.After convergence,a vertical rake exactly at the center of the model is created .Velocity magnitude along the rake is exported to excel sheet. Then the mesh is refined and send to FLUENT and steps (b) and (c) are repeated.I have done this with many(6) times.The velocity magnitude curves go flatter with refinement. If I change my convergence criterion to 107,all curves coincide how coarser or how finer I change my grid. What convergence should I take? Which curve value should I take for my study? I am confused.Why is this happening? Sorry,I could'nt send the pics in the format which you mentioned. Thank you 

December 20, 2013, 13:01 

#6 
Senior Member
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,305
Rep Power: 22 
If the curves change as you lower the convergence criteria, the solution can not be considered converged.
Instead of comparing the errors introduced by coarse meshes, you compare the errors from insufficient iterations. My tip for steadystate simulations in an academic context: Run the case until the residuals level out. Then you can compare the errors caused by the discretization or any other influence you want to investigate. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
problem with velocity magnitude and vectors  Hayder Mohammed  FLUENT  0  August 7, 2013 03:08 
I need help about velocity vectors direction in cylinder modeling!!!  aminepoxnet  FLUENT  0  March 17, 2013 18:41 
Inaccurate velocity vectors  Arun Prasad  FLUENT  4  August 2, 2012 00:43 
how to display a clear vectors of velocity  clauss  FLUENT  1  October 24, 2006 07:30 
Velocity vectors clipping  Vidya  FLUENT  0  July 31, 2006 21:53 