# Boundary condition slip

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February 18, 2015, 04:43
Boundary condition slip
#1
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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?
Attached Images
 20150205_145039.jpg (42.6 KB, 228 views)

February 18, 2015, 12:26
#2
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Thiago Parente Lima
Join Date: Sep 2011
Location: Diamantina, Brazil.
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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 12:34. Reason: Question added in the end

February 18, 2015, 13:09
#3
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Quote:
 Originally Posted by thiagopl 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?

 February 18, 2015, 13:21 #4 Member   Thiago Parente Lima Join Date: Sep 2011 Location: Diamantina, Brazil. Posts: 57 Rep Power: 13 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. __________________ Fields of interest: Heat transfer; Convection heat transfer; Conjugate heat transfer.

February 18, 2015, 14:00
#5
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Quote:
 Originally Posted by thiagopl 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.
Attached Images
 stream lines.jpg (16.2 KB, 121 views)

February 18, 2015, 14:10
#6
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Quote:
 Originally Posted by PLanger90 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.
Attached Images
 stream lines.jpg (16.6 KB, 82 views)

 February 19, 2015, 06:15 #7 Member   Thiago Parente Lima Join Date: Sep 2011 Location: Diamantina, Brazil. Posts: 57 Rep Power: 13 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). __________________ Fields of interest: Heat transfer; Convection heat transfer; Conjugate heat transfer.