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Gab July 4, 2005 21:17

nodes number within boundary layer
 
Hello,

As discussed in previous posts about SST model, sufficient nodes(5-20) are needed for boundary layer.I was wondering how to DIRECTLY check the nodes number in CFX to make sure that sufficient nodes are provided in boundary layer. In cfx-post? How to do that?

Can someone explain this? Thanks!

regards!

Gab

Paul July 5, 2005 06:51

Re: nodes number within boundary layer
 
Hi Gab,

The easiest way is to create a plane in CFX post which cuts through the boundary layer you are interested in. Under the "render" tab for the plane select "draw lines", this will display the mesh that is coincident with the plane. Colour the plane with the velocity tangential to the surface.

This will visualise the boundary layer and the mesh, you can then simply count the number of points in the boundary layer region.

Cheers

Paul

Gab July 5, 2005 11:03

Re: nodes number within boundary layer
 
Thanks a lot, Paul

Best regards!

Gab

Gab July 5, 2005 11:46

Re: nodes number within boundary layer
 
Hi, Paul

I am still not sure what you mean 'Colour the plane with the velocity tangential to the surface'. Do you mean to show the mesh and the velocity contour in the same plane or in different planes?

I just tried to show the mesh and velocity contour in one plane. In some regions I can see the great color difference between wall areas and main flow(Is this the way to identify the boundary layer?). However in other regions, no obvious color difference. How to understand this?

Thanks again!

Gab

Paul July 5, 2005 12:16

Re: nodes number within boundary layer
 
Hi Gab,

You've done it correctly, show the mesh and velocity in the same plane.

The boundary layer is the region where you see the colour change as you approach the wall. Depending on the turbulence model you are using you should have between 10 and 15 node points to resolve this area.

Where you are seeing no change in contour level near the wall: If the wall boundary condition is "no slip" then the velocity at the wall is zero. I assume you have some free stream velocity so there must be a gradient there somewhere.

What is the situation you are trying to model?

Regards

Paul

Gab July 5, 2005 13:57

Re: nodes number within boundary layer
 
Hi, Paul.

I appreciate your quick reply and helpful comments. Now I have understood how to ckeck the nodes number.

I am working on a multiphase modelling (gas and water) and always not able to reach sufficient converge level. So I am trying to refine my mesh and change turbulence model from k-e to SST.

The locations that have no change in contour level near the wall were set as "free slip" boundary condition. So it should be normal.

Thanks again!

Regards!

Gab



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