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Old   March 16, 2010, 14:01
Default BC for object moving in water
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maply
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Hi,

I am trying to model a needle moving in a cylinder of water.
The water is supposed to be in rest (no flow) and the needle has to move vertically inside the water cylinder.
I am not really familiar with CFD computation, but I was wondering what kind of Boundary conditions should I set at the surface of the needle.
should it be a moving wall (with certain velocity) or there is another way to describe it.
What should be the BC in the top and the lateral surface of the cylinder??

Thanks in advance,
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Old   March 17, 2010, 00:16
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Hamid Zoka
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when CFD modeling, fluid velocity vector on solid surface will be euqal to that of solid body. in other words, if solid is stationary fluid velocity will be zero. but when solid is moving with a constant velocity, fluid velocity will be equal to solid body velocity. in the latter case you should use moving wall boundary conditions for all solid boundaries. is the needle moving with a constant velocity? which software do you use to simulate that?

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Old   March 17, 2010, 07:48
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Thanks for your reply,

I am using Comsol for the simulation.
I have set the BC on the needle to the velocity which is considered as constant.
But the problem for me is the other boundaries:
The top of the cylinder and the lateral one (height of the cylinder).

I have set the bottom of the cylinder to slip wall. knowing that the needle has to approach the bottom of the cylinder.
Any suggestions would be helpful.

Thanks in advance.
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Old   March 18, 2010, 00:27
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hi;
top and lateral surfaces of cylinder are solid walls and since it is stationary, these faces should be set to solid boundary type with zero velocity. what are other boundaries? does the fluid pass through the needle? is it just an internal circulating flow?
if you send me a pic of your geometry i can give better proposals.

regards,
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Old   March 18, 2010, 07:47
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Hi,

I have set an open boundary on the top of the cylinder with stress = pressure ~0
the BC @ lateral boundary is a slip wall
The BC at the needle is moving wall with a certain velocity.

I am afraid that if I set the top and the lateral boundary to no slip (u=0) it could perturb the velocity field. Knowing that the real size of the cylinder should be bigger than that.

Any comments or suggestions are welcome.
Thanks,
The following animation shows the velocity field distribution.

Last edited by maply007; March 18, 2010 at 12:31.
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Old   March 18, 2010, 12:30
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I have also set the top and the lateral boundaries as perforated Wall with Ur and Uz as velocities. (these are the unknown)
I didn't set any point condition for the pressure.
And the result look like this: Note the difference in the velocity distribution near the walls.
Comments, suggestions are welcome!!
Thanks
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Old   March 20, 2010, 05:41
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dear maply;
the relative positions of needle and cyliner is not clear to me according to your sent files. why the flow is to leave the boundaries? you have outflow from boundaries except one. please check it.
your grid size also seems to be coarse.
can you send a picture of your meshing? please specify the type of boundries for each faces and specify the position of needle.
thanks.
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