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

Symmetry boundary considerations

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

Like Tree1Likes
  • 1 Post By agd

Reply
 
LinkBack Thread Tools Display Modes
Old   August 22, 2016, 04:37
Post Symmetry boundary considerations
  #1
New Member
 
Mario Alberto
Join Date: Sep 2015
Location: Russian Federation
Posts: 10
Rep Power: 4
mrwts is on a distinguished road
Hello everybody,
A couple of questions regarding symmetry boundaries and mesh with mirror planes.
I am running multiple simulations to determine the best location of the engine and the wing for an amphibian aircraft (considering Cd and Cl values for many points within a grid)

1) My geometric model is symmetrical relative to the XY plane, would it be physically correct to apply a Symmetry Boundary in that plane and therefore only work with half the geometry/mesh?
As far as I understand, the symmetry boundary does not allow any diffusion flux across a symmetry plane. So, since I am expecting 3D swirling vortices right behind the engine and the pylon, I think I may be limiting the 3D flows of the vortices accros the symmetry plane. Wouldn't I?

2) Is there any possibility to cut a mesh in FLUENT along a symmetry plane?

Thanks in advance for your insights.
mrwts is offline   Reply With Quote

Old   August 22, 2016, 07:40
Default
  #2
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 3,504
Rep Power: 40
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by mrwts View Post
Hello everybody,
A couple of questions regarding symmetry boundaries and mesh with mirror planes.
I am running multiple simulations to determine the best location of the engine and the wing for an amphibian aircraft (considering Cd and Cl values for many points within a grid)

1) My geometric model is symmetrical relative to the XY plane, would it be physically correct to apply a Symmetry Boundary in that plane and therefore only work with half the geometry/mesh?
As far as I understand, the symmetry boundary does not allow any diffusion flux across a symmetry plane. So, since I am expecting 3D swirling vortices right behind the engine and the pylon, I think I may be limiting the 3D flows of the vortices accros the symmetry plane. Wouldn't I?

2) Is there any possibility to cut a mesh in FLUENT along a symmetry plane?

Thanks in advance for your insights.

imposing symmetry is a strong assumption on the physics of the problem that can be acceptable only in particular cases... what kind of formulation are you using? give more details..
FMDenaro is offline   Reply With Quote

Old   August 22, 2016, 08:57
Default
  #3
agd
Senior Member
 
Join Date: Jul 2009
Posts: 247
Rep Power: 12
agd is on a distinguished road
To add to FMDenaro's answer, the use of a symmetry plane implies that you have mirror-image flow across the plane. Thus, any asymmetric 3D behavior will be lost. At low angles of attack, such an assumption is not unreasonable. But at high angles of attack asymmetric shedding becomes very pronounced, even at zero yaw. Any mutual vortex induction between the pylon/engine and its mirror is also affected by the use of a symmetry boundary condition, so if that is a large effect (even at small angles of attack) a symmetry condition may not be a good choice. It's one of those areas that requires some experience and judgment - I would suggest trying both a symmetry plane and full 3D and comparing the results. That's really one of the best ways to answer your question.
FMDenaro likes this.
agd is offline   Reply With Quote

Old   August 23, 2016, 04:45
Default
  #4
Senior Member
 
sbaffini's Avatar
 
Paolo Lampitella
Join Date: Mar 2009
Location: Italy
Posts: 768
Blog Entries: 17
Rep Power: 21
sbaffini will become famous soon enough
I try to add a more practical perspective to the previous correct answers.

The feasibility of a boundary condition is just dependent from the final solution. In the specific case of symmetry, it simply depends from a single question:

for a full simulation (no symmetry), do you expect the final solution to be symmetric across that plane?

If the geometry is symmetric and all the remaining boundary conditions are symmetric with respect to that plane than what really remains as possible source of asymmetry are the equations.

Do you expect the equations to produce asymmetry under symmetric conditions? RANS equations are not expected to do so, so symmetry is ok for them. This is also true for any experimental campaign satisfying such simmetry conditions (even if, in practice, this might be impossible to achieve), the statistics extracted from the experimental data are expected to satisfy the symmetry within the experimental uncertainty.

Any vortex in a RANS solution is a an average feature of the flow and, if everything else is symmetric, an equal counter-rotating vortex is expected from the other side of the plane, so that their net effect is null on the symmetry plane.

The matter is different for unsteady simulations (or even steady laminar ones), where the objections of the previous posters effectively apply.
sbaffini is offline   Reply With Quote

Reply

Tags
aircraft design., fluent, symmetry bc

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
3D Windturbine simulation in SU2 k.vimalakanthan SU2 13 June 20, 2016 05:27
Basic Nozzle-Expander Design karmavatar CFX 20 March 20, 2016 09:44
Problem in setting Boundary Condition Madhatter92 CFX 12 January 12, 2016 05:39
Difficulty In Setting Boundary Conditions Moinul Haque CFX 4 November 25, 2014 18:30
Import problem ARC Open Source Meshers: Gmsh, Netgen, CGNS, ... 0 February 27, 2010 11:56


All times are GMT -4. The time now is 07:45.