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Old   February 18, 2015, 05:43
Default Boundary condition slip
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Hello,

i've got a question about the boundary condition "slip". In the UserGuide OpenFOAM defines slip boundary condition: "zeroGradient if φ is a scalar; if φ is a vector, normal component is fixedValue zero, tangential components are zeroGradient"
But I don't understand the definition "tangential components are zeroGradient" for velocity.
My problem is a test rig to visualize water flow around a zylinder. I've uploaded a picture.
I need the boundary condition for the free surface. I tested "zeroGradient" and "slip", because I won't simulate a multi-phase flow.
What's the better choice? I don't know the exact definition for the slip boundary condition.
Can somebody help me?
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Old   February 18, 2015, 13:26
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Hi,

Quote:
But I don't understand the definition "tangential components are zeroGradient" for velocity.
A zero gradient for the tangencial velocity means a symmetry plane (roughly, it means that you have another identical experiment above yours). Notice that it is different from the no slip condition in which the tangencial velocity itself is zero.

In your experiment, apparently, only the flow around the cylinder is of your interest. Therefore, the experiment should be designed in a way that the upper and bottom boudaries do not affect the flow around the cylinder. If it is true, a symmetry plane in your simmulation would be a good choice.

But I have a question, this is a test rig for a Hele-Shaw flow visualization?

Last edited by thiagopl; February 18, 2015 at 13:34. Reason: Question added in the end
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Old   February 18, 2015, 14:09
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Quote:
Originally Posted by thiagopl View Post

But I have a question, this is a test rig for a Hele-Shaw flow visualization?
Yes, I want to approximate a Hele-Shaw flow. But the distance between the front and the back wall is to large for a Hele-Shaw-Flow (10mm). And the boundary conditions are different. Therefore I will get significant deviations.
I have aalready been thinking to use a symmetry plane for the top of the test rig. But I'm also interested in the effect of the boundary conditions - in especially the boundary condition at the top of the test rig.
So which boundary condition should I use, to show the effect of the boundary conditions to the flow around the cylinder?
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Old   February 18, 2015, 14:21
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PLanger90,

I suggest you to read the Breaking of a dam tutorial at the User Guide. The tutorial shows the treatment for the same boundary condition as yours, a surface free to the atmosphere.
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Old   February 18, 2015, 15:00
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Quote:
Originally Posted by thiagopl View Post
PLanger90,

I suggest you to read the Breaking of a dam tutorial at the User Guide. The tutorial shows the treatment for the same boundary condition as yours, a surface free to the atmosphere.
Hi thiagopl,

i've already read the "Breaking of a dam" tutorial. In my simulation I used the solver simpleFoam.
For the pressure I used this boundary condition:
{
type totalPressure;
p0 uniform 0;
U U;
gamma 1;
value uniform 0;
}
And for the velocity:
{
type pressureInletOutletVelocity;
value uniform (0 0 0);
}
I uploaded a picture with the stream lines.
Obviously, this is wrong. But what's the mistake?

The second picture shows the solution with the "slip" boundary condition at the top of the test rig. When I use symmetryPlane, the solution is equal.

Thank's a lot for your help.
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Old   February 18, 2015, 15:10
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Quote:
Originally Posted by PLanger90 View Post
Hi thiagopl,

i've already read the "Breaking of a dam" tutorial. In my simulation I used the solver simpleFoam.
For the pressure I used this boundary condition:
{
type totalPressure;
p0 uniform 0;
U U;
gamma 1;
value uniform 0;
}
And for the velocity:
{
type pressureInletOutletVelocity;
value uniform (0 0 0);
}
I uploaded a picture with the stream lines.
Obviously, this is wrong. But what's the mistake?

The second picture shows the solution with the "slip" boundary condition at the top of the test rig. When I use symmetryPlane, the solution is equal.

Thank's a lot for your help.
I forgot the second picture.
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Old   February 19, 2015, 07:15
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You could write all your boundary conditions here (using [ CODE] [ / CODE] ), so it would be easier to find a problem. Before, check your fvSolutions or fvSchemes (sometimes we copy/paste some tutorials and forget to check them).
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