[Taltan]

I am a bit confused as to what the difference between these boundary conditions are. Any help would be appreciated. My take on these BC's:

1) SymmetryPlane

Mirrors along a plane so to reduce the amount of cells needed for a full model. For instance you have a model of a car, then you would only need to compute half of the car as the other half is symmetric.

2) Cyclic

Looping back information from one bc to another. For instance a pipe, where the outlet loops back to the inlet.

3) Wedge

This is the one I am the least confident about. Apparently this is only for 2D simulations for axisymmetric problems, for instance a pipe. But I though (hoped) that it was a way to reduce amount of cells in an axisymmetric 3D problem where you would only need to model a fraction of the pipe.

In the case I am working on I have a pipe which is axisymmetric with about 4 million cells and I wish to reduce this amount. Intuitively the wedge approach seemed good as I would only need the mesh for a small section of the pipe. However if this is only for 2D then my simulation would probably fall apart as I intend to model LES.

So hopefully the symmetryPlane approach would work in my case perhaps? Where I would revolve my pipe only 180 degrees and use symmetryPlane bc on the resulting flat surface.

[Gerhard]

Hi Taltan

Be careful of these boundary conditions. They should be used with great care. The symmetry example you gave will only work if the flow is laminar. If it is not, there would be vortices behind the car, which is not symmetrical any more. But the mirror explanation is correct.

The wedge BC is to reduce a 3D case to a 2D case... If the flow is axisymmetric, thus f(r,z), then you should be able to use this boundary condition (not sure about the LES thing).

The symmetry approach might also work and you only need to revolve the pipe by 90 degrees if you think about it

[gautami]

Dear All,

1. To simulate a floating cylinder (using 6DOF rigid body motion solver) with symmetryPlane option, dividing the computational domain into half, does the mass and moment of inertia of cylinder defined in the dynamicMeshDict be divided to half. Taking full mass sinks the cylinder.
2. To simulate a floating cylinder (using 6DOF rigid body motion solver) with 3 DOF motion in heave, surge and pitch, and be restrained with linear spring, should the spring stiffness be taken half value for half the domain using symmetry plane option.
Kindly please advise. Thank You.

Most&More,
Gautami Erukulla.

[quarkz]

Hi,

I have a similar problem involving flow past a cone. I intent to run it as a LES simulation. I am now using 1/4 (90deg rotation) of the cone with the 2 planes using the symmtryplane BC.

Btw, is it possible to reduce further to something of a wedge shape (30 or 45deg rotation) while using the same BC? This would save the computation even further.

Thanks.

[hasha]

Hi quarkz,
Wedge angle shouldn't exceed 5deg in the axis symmetric problems of OpenFOAM.

[vava10]

Hey,

I am simulating a ship. Due to symmetry I am only working with half of the geometry. for Dynamic mesh I have following doubts

1. centre of mass - the centre of half or full geometry
2. moment of inertia -moment of inertia of half geometry or (moment of inertia of full geometry)/2


Thanks in advance
vava10

[SHANRU]

Originally Posted by Gerhard

Hi Taltan

Be careful of these boundary conditions. They should be used with great care. The symmetry example you gave will only work if the flow is laminar. If it is not, there would be vortices behind the car, which is not symmetrical any more. But the mirror explanation is correct.

The wedge BC is to reduce a 3D case to a 2D case... If the flow is axisymmetric, thus f(r,z), then you should be able to use this boundary condition (not sure about the LES thing).

The symmetry approach might also work and you only need to revolve the pipe by 90 degrees if you think about it

There is a good summary from the following post, it explains well the difference between symmetry and cyclic BC.

"Periodic boundary conditions are used when the physical geometry of interest and the expected flow pattern have a periodically repeating nature. This means that the flows across two opposite planes in your computational model are identical. Periodic boundary conditions can be applied to a pair of boundary sections, which are referred to as the inlet and outlet of the periodic condition. The velocity profile for the inlet and outlet will be the same, but a difference of normal force or pressure will be allowed between them.
The symmetry condition is equivalent to imposing zero values for the normal velocity and tangential force."



https://www.researchgate.net/post/Do...haves_the_same

[Rahmat1]

How would you handle a blade passage of turbine?