[mostanad]

Hi everyone,


I have a wall and I wanna make it stress free. So, as I know slip boundary condition can provide a wall with zero normal velocity and zero tangential gradient. But OF has two options for that:


1- boundary type wall, boundary condition slip;
2- boundary type patch, boundary condition slip;


Although this slip boundary condition acts like a symmetry boundary, but:


"It is very similar to the symmetry. Keep in mind that the symmetry is a boundary type, whereas the slip is a boundary condition which can be applied on boundary patches of type wall and patch respectively."


So which one, patch or wall boundary type, closer to a wall and far from symmetry?

[mostanad]

Any reply to help me in this regard will be appreciated.

[mAlletto]

For a slip condition I usually use a wall patch

[mostanad]

Originally Posted by mAlletto

For a slip condition I usually use a wall patch

Thank you Michael for your reply. Can you explain to me what is the difference?

[mAlletto]

https://cfd.direct/openfoam/user-guide/v6-boundaries/

[Marpole]

Did you say that you were modelling a wall boundary condition? Then it is wall type slip boundary condition.

[mostanad]

Originally Posted by Marpole

Did you say that you were modelling a wall boundary condition? Then it is wall type slip boundary condition.

Yes, I want to consider it a wall without any penetration and stress-free. But the question is what is the difference between patch-slip and wall-slip BCs?

[Marpole]

I understand that "slip" is a treatment to velocity at a boundary, where velocity component normal to the geometric patch is set to zero and velocity component tangential to the geometric patch is maintained.

The term patch in patch-slip has a special meaning. It is the type of boundary. Wall is another type of boundary. Patch and geometric patch are different.

For a wall type of boundary, it is a simple solid wall.

For a patch type boundary, it is more flexible. It can be used for inlet, outlet, though, with slip, this is not possible. But for some special cases such as a membrane where one or more species can pass through it, it needs a patch type slip boundary condition but not wall type. Patch-slip is rarely used.

[mostanad]

Originally Posted by Marpole

I understand that "slip" is a treatment to velocity at a boundary, where velocity component normal to the geometric patch is set to zero and velocity component tangential to the geometric patch is maintained.

The term patch in patch-slip has a special meaning. It is the type of boundary. Wall is another type of boundary. Patch and geometric patch are different.

For a wall type of boundary, it is a simple solid wall.

For a patch type boundary, it is more flexible. It can be used for inlet, outlet, though, with slip, this is not possible. But for some special cases such as a membrane where one or more species can pass through it, it needs a patch type slip boundary condition but not wall type. Patch-slip is rarely used.

Thanks Charles for your nice explanation. So in terms of wall, there is no difference between patch-slip and wall-slip. However it is logical to use wall boundary type for my wall case. Anyway, I found wall-slip type rarely in tutorials.
But can you tell me what pressure condition should be considered for wall-slip? ZeroGradient or slip?

[Marpole]

Great! For pressure boundary condition, zeroGradient and fixedFluxPressure are both popular. You can also use slip. I haven't tested it myself but believe it will give you the same results.

[mostanad]

Originally Posted by Marpole

Great! For pressure boundary condition, zeroGradient and fixedFluxPressure are both popular. You can also use slip. I haven't tested it myself but believe it will give you the same results.

OK. Thanks Charles. Your help is much appreciated.
Cheers,
Mohammad

[mostanad]

Originally Posted by Marpole

Great! For pressure boundary condition, zeroGradient and fixedFluxPressure are both popular. You can also use slip. I haven't tested it myself but believe it will give you the same results.

Sorry Charles for asking question again. In the tutorials,I can see a few cases using wall-slip BC. But there are multiple cases with patch-slip, like motorbike for front and back sides. When I look at these sides, the normal velocity is not completely zero:





while the U magnitude shows:




How this can be justified?


Mohammad

[mAlletto]

Is the wall normal direction y or z?

[mostanad]

Originally Posted by mAlletto

Is the wall normal direction y or z?

Wall normal is actually the y direction. As you see, I have also shown y velocity.

[mAlletto]

Are you showing the boundary patches?

[mostanad]

Originally Posted by mAlletto

Are you showing the boundary patches?

Yes. These are the front and back planes.

[mAlletto]

From the figure it seems that the boundarie are too close to the body. If you move the boundaries away you will get smaller wall normal velocities

[mostanad]

Originally Posted by mAlletto

From the figure it seems that the boundarie are too close to the body. If you move the boundaries away you will get smaller wall normal velocities

Although it is showing a very low value, there should be no normal velocity for these walls. At the same time, this case considers patch-slip instead of wall-slip. Shoukd it be dependent of distance from body? What is your opinion?

[mAlletto]

Usually the disturbance to the flow caused by the body decreases with distance to the body

[mostanad]

Originally Posted by mAlletto

Usually the disturbance to the flow caused by the body decreases with distance to the body

But it is said that slip boundary condition considers zero normal velocity.