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-   -   Restitution coefficients depending on impact parameters (http://www.cfd-online.com/Forums/cfx/81796-restitution-coefficients-depending-impact-parameters.html)

 flo56 November 7, 2010 07:27

Restitution coefficients depending on impact parameters

Hi!
I use CFX to simulate an air-dust flow in a duct.
I got some computations by setting constant restitution coefficients (perpendicular and parallel) whatever the impact velocity of the particles with the wall, their impact angle and diameters.
I would like to improve the results by entering coefficients which depend on those previous parameters.
Do you know any simple method to do it?
I think it is possible to code a FORTRAN routine but I am not at all familiar with this code...

Regards

 ghorrocks November 7, 2010 18:35

Can you describe what you intend? Do you want the restitution coefficients to depend on previous wall impacts?

 flo56 November 8, 2010 00:43

I get polynomials formulations on restitution coefficients in literature, and would like to implement them in CFX.
For example, the normal coefficient depends on impact angle of the particle on the wall, and the velocity of the particle at the impact; whereas the tangential coefficient depends on impact angle and particle diameter.
Then I would like the software to compute those coefficients for each particle which impacts on the walls of my mesh by following these expressions.

 ghorrocks November 8, 2010 17:36

This can all be implemented as CEL functions. Do some tutorials and have a look at how they use CEL expressions.

 Julian K. November 14, 2011 18:48

Hi!

I was hoping, that you can answer my question:

When does CFX realize, that a particle hits a wall? When the particle's center coordinates coincide with the surface coordinates, or when the distance btw. particle center coordinates and surface coordinates is less than 1/2'particle_diameter?

You can find my main post here: http://www.cfd-online.com/Forums/cfx...n-surface.html

 ghorrocks November 14, 2011 23:59

I do not know as I cannot access the source code but I would think it obvious that wall collisions occur when the particle position is beyond the element face defined as a wall. It does not include the particle diameter.

 Julian K. November 15, 2011 10:32

If this is the case, then CFX small particles which perfectly follow the streamlines, but due to a finite radius touch the collector's surface an get captured (Haugen et al. 2011). This occurs for small Stokes numbers. This capture mode is crucial for my simulations. Thus,

1. How can I find out, if the radius is used or not, in order to check for the particle hitting a wall?

2. If the radius is not used, is there a way to make CFX consider the particle's radius (CEL)? How would I do that?

 Julian K. November 15, 2011 18:22

Quote:
 Originally Posted by Julian K. (Post 332212) 1. How can I find out, if the radius is used or not, in order to check for the particle hitting a wall?
I printed the tracking files and look at the final coordinates of captured particles. I found out, that this final coordiante lies on the collecting surface. Thus, CFX does not consider the particle's radius when checking for impaction (wall BC: Stick-to-Wall; Equation dependent with both restitution coeff. = 0 and wall roughness model with all dimensions set to 0 m).

This answers my first question, but my second question still remains, which is:

Who can I make CFX capture a particle, when the distance between particle centre and collecting surface is less than the particle's radius? Would I need to implement my own algorithm using a CEL? How would I do that? How would I start?

 ghorrocks November 15, 2011 20:01

Correct, particles which are small enough to follow streamlines will only rarely get to the wall due to their small slip, or more likely numerical errors.

Quote:
 1. How can I find out, if the radius is used or not, in order to check for the particle hitting a wall?
Either ask CFX support or do an experiment and see what happens - you have already done the experiment and found out.

Quote:
 2. If the radius is not used, is there a way to make CFX consider the particle's radius (CEL)? How would I do that?
I am not familiar with the particle collision modelling in CFX but suspect it does not work for walls. You might be able to write a user fortran function to do it. I doubt you can do this in CEL. Again, a call to CFX support would be useful.

 Julian K. November 18, 2011 13:07

Thanks! I posted my problem desciption here.

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