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dae4god July 10, 2011 08:07

Spatial Derivative Using CEL Functions

I am trying to find the normal component of the gradient of the vorticity, dw/dn, around the entire surface of an airfoil. Here, w is the vorticity and n is the direction normal to the surface, both at a particular point on the airfoil surface. Is there a way to find this normal vorticity distribution and export the results during a transient analysis? Perhaps using a CEL function?

The vorticity is found from the curl of the velocity, but is there a way to define a CEL function that describes the gradient of the curl of the velocity?

stumpy July 11, 2011 09:13

There already a Vorticity variable available, so no need to calculate this. CEL has a .Gradient operator, so in theory "Vorticity.Gradient X" would work.

dae4god July 12, 2011 19:08

Thanks stumpy. I'll give it a try and let you know how it goes! :)

dae4god July 26, 2011 03:09

Hi Stumpy,

Thanks for your suggestion but I couldn't get it to work. When I entered an expression, like "Velocity.Curl.Gradient", or similar into CFX-Pre, it couldn't evaluate the expression in CFX-Post.

Perhaps I could try to clarify my problem. I'm trying to create an expression/variable/function in CFX-Pre which evaluates the normal component of the vorticity gradient on a curved surface (Wall Boundary Condition) and outputs the results to a file (.csv or similar) while it is solving. Do you know if this is possible?

I was trying to find a way to find the normal unit vector to the surface, and then inner dot product it with the vorticity gradient to get the result, but it wasn't as easy as I thought. Any ideas or thoughts or criticisms would be greatly appreciated!

stumpy July 26, 2011 08:49

The double operator ".Curl.Gradient" probably won't work, which is why I suggested Vorticity.Gradient X. Also ".Gradient" won't work since you haven't specified in which direction you want the gradient, X, Y or Z. CFD-Post cannot evaluate gradients as far as I know, so you should write whatever quantity you want to an Additional Variable. Vorticity is also not written to the res file by default, you'll need to ask for that in the Results sections in CFX-Pre.

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