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

Boundary conditions for streamfunction

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

Like Tree1Likes

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 12, 2019, 11:40
Default Boundary conditions for streamfunction
  #1
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Hello to all,

I have written SIMPLE algorithm for 2D backwards facing step.

My problem is to calculate streamfunction from Poisson equation. In the attached figure are the boundary conditions used correct?

Thank you.
Attached Images
File Type: jpg 2D Backwards Facing Step.jpg (42.7 KB, 15 views)

Last edited by IbizaFR; July 12, 2019 at 11:42. Reason: corrections
IbizaFR is offline   Reply With Quote

Old   July 12, 2019, 13:21
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by IbizaFR View Post
Hello to all,

I have written SIMPLE algorithm for 2D backwards facing step.

My problem is to calculate streamfunction from Poisson equation. In the attached figure are the boundary conditions used correct?

Thank you.



Yes, they are mathematically correct. You can use a more physically free outflow condition by setting d2 psi /dy2 =0
FMDenaro is offline   Reply With Quote

Old   July 12, 2019, 15:30
Default
  #3
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Yes, they are mathematically correct. You can use a more physically free outflow condition by setting d2 psi /dy2 =0
Thank you.
With these boundary conditions i am solving a poisson equation but the resulting contours of calculated psi do not present the vortex near the bottom of step.
IbizaFR is offline   Reply With Quote

Old   July 12, 2019, 15:31
Default
  #4
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by IbizaFR View Post
Thank you.
With these boundary conditions i am solving a poisson equation but the resulting contours of calculated psi do not present the vortex near the bottom of step.



Maybe you need to check the vorticity field. What is the Re number?
FMDenaro is offline   Reply With Quote

Old   July 12, 2019, 15:37
Default
  #5
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Maybe you need to check the vorticity field. What is the Re number?
The velocity field is correct compared to other works.
The code for vorticity field is the same for the lid driven cavity in which the streamlines are compared good enough with other works. The same code for cavity I copied to facing step but it does not give correct results.
Any help?
IbizaFR is offline   Reply With Quote

Old   July 12, 2019, 15:38
Default
  #6
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by IbizaFR View Post
The velocity field is correct compared to other works.
The code for vorticity field is the same for the lid driven cavity in which the streamlines are compared good enough with other works. The same code for cavity I copied to facing step but it does not give correct results.
Any help?
Re=83-167-333.
IbizaFR is offline   Reply With Quote

Old   July 12, 2019, 16:22
Default
  #7
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by IbizaFR View Post
Re=83-167-333.



Post the three solutions at a convergence state. Have you considered that to see the recirculating region you should plot the streamlines with negative values?
FMDenaro is offline   Reply With Quote

Old   July 12, 2019, 16:56
Default
  #8
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Post the three solutions at a convergence state. Have you considered that to see the recirculating region you should plot the streamlines with negative values?
Yes when I have prepared the diagrams I will post them.

Thank you.
IbizaFR is offline   Reply With Quote

Old   July 13, 2019, 14:10
Default
  #9
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Post the three solutions at a convergence state. Have you considered that to see the recirculating region you should plot the streamlines with negative values?
I post the dimensionless velocity and the dimensionless streamfunction field for Re=83-167-333. The resulting streamfunction results obtained from the poisson equation for the calcalation of Psi does not have any negative values at the recirculating region near the bottom of the step. The poisson equation is solved with either ADI or Jacobi technique.

Thank you.
Attached Images
File Type: jpg Streamlines_Re_83.jpg (166.0 KB, 8 views)
File Type: jpg Streamlines_Re_167.jpg (129.2 KB, 6 views)
File Type: jpg Streamlines_Re_333.jpg (161.2 KB, 6 views)
File Type: jpg Velocities_Re_83.jpg (194.1 KB, 5 views)
File Type: jpg Velocities_Re_167.jpg (197.0 KB, 5 views)
IbizaFR is offline   Reply With Quote

Old   July 13, 2019, 14:11
Default
  #10
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
Post the three solutions at a convergence state. Have you considered that to see the recirculating region you should plot the streamlines with negative values?
Also the velocities for Re=333.
Attached Images
File Type: jpg Velocities_Re_333.jpg (177.3 KB, 5 views)
IbizaFR is offline   Reply With Quote

Old   July 13, 2019, 15:09
Default
  #11
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
From the figures the solutions seem obtained from a irrotational field. Check vorticity and convergence criteria
FMDenaro is offline   Reply With Quote

Old   July 15, 2019, 13:41
Default
  #12
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
From the figures the solutions seem obtained from a irrotational field. Check vorticity and convergence criteria
I corrected the poisson equation for the calculation and I attach the streamlines for Re=8.3-10-50-83-100-167-200-250-300.
Are they in a way correct?

Thank you.
Attached Images
File Type: jpg Streamlines_Re_8_3.jpg (150.7 KB, 4 views)
File Type: jpg Streamlines_Re_10.jpg (150.7 KB, 4 views)
File Type: jpg Streamlines_Re_50.jpg (155.0 KB, 2 views)
File Type: jpg Streamlines_Re_83.jpg (162.0 KB, 3 views)
File Type: jpg Streamlines_Re_100.jpg (157.2 KB, 3 views)
IbizaFR is offline   Reply With Quote

Old   July 15, 2019, 13:43
Default
  #13
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
I also post the results for Re=167-200-250-300.
Attached Images
File Type: jpg Streamlines_Re_167.jpg (158.7 KB, 2 views)
File Type: jpg Streamlines_Re_200.jpg (159.7 KB, 5 views)
File Type: jpg Streamlines_Re_250.jpg (160.1 KB, 3 views)
File Type: jpg Streamlines_Re_300.jpg (183.5 KB, 3 views)
IbizaFR is offline   Reply With Quote

Old   July 15, 2019, 15:58
Default
  #14
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
The results show a better solution but something is still wrong. The recirculation region extends through the outflow. I suggest to extend the domain lenght and verify your reattachment lenght with the results in literature cited here
https://www.researchgate.net/publica...ical_procedure
FMDenaro is offline   Reply With Quote

Old   July 15, 2019, 16:42
Default
  #15
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
The results show a better solution but something is still wrong. The recirculation region extends through the outflow. I suggest to extend the domain lenght and verify your reattachment lenght with the results in literature cited here
https://www.researchgate.net/publica...ical_procedure
I will check it. The reattachment length is the line where the vorticity is zero?
IbizaFR is offline   Reply With Quote

Old   July 15, 2019, 16:59
Default
  #16
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by IbizaFR View Post
I will check it. The reattachment length is the line where the vorticity is zero?



The first point on the bottom wall where the vorticity is zero
FMDenaro is offline   Reply With Quote

Old   July 15, 2019, 17:09
Default
  #17
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by FMDenaro View Post
The first point on the bottom wall where the vorticity is zero
Thank you.
IbizaFR is offline   Reply With Quote

Old   July 15, 2019, 17:12
Default
  #18
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,756
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by IbizaFR View Post
Thank you.



However, check for bugs in the code, as you can see from the figures the streamlines from the inflow are quite strange
FMDenaro is offline   Reply With Quote

Old   July 15, 2019, 17:20
Default
  #19
Senior Member
 
adrin
Join Date: Mar 2009
Posts: 115
Rep Power: 17
adrin is on a distinguished road
Curious, what was wrong with your poisson solver? If the code worked for the cavity flow it would mean your poisson solver is correct - except, of course, for the boundary condition.

Also, would you be able to share the algorithm you use for solving the vorticity transport equation?

adrin
adrin is offline   Reply With Quote

Old   July 15, 2019, 17:30
Default
  #20
Member
 
E
Join Date: Mar 2018
Posts: 77
Rep Power: 8
IbizaFR is on a distinguished road
Quote:
Originally Posted by adrin View Post
Curious, what was wrong with your poisson solver? If the code worked for the cavity flow it would mean your poisson solver is correct - except, of course, for the boundary condition.

Also, would you be able to share the algorithm you use for solving the vorticity transport equation?

adrin
It was an error in boundary conditions for streamfunction for the step. Iterative solvers for poisson equations can be applied.
IbizaFR 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
sliding mesh problem in CFX Saima CFX 46 September 11, 2021 08:38
Radiation in semi-transparent media with surface-to-surface model? mpeppels CFX 11 August 22, 2019 08:30
Basic Nozzle-Expander Design karmavatar CFX 20 March 20, 2016 09:44
GETVAR Error in Multiband Monte Carlo Radiation Simulation with Directional Source silvan CFX 3 June 16, 2014 10:49
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 07:28


All times are GMT -4. The time now is 06:41.