Spatial Derivative Using CEL Functions

dae4god · July 10, 2011, 09:07

Hi,

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, 10:13

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, 20:08

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

dae4god · July 26, 2011, 04:09

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, 09:49

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.

July 10, 2011, 09:07	Spatial Derivative Using CEL Functions	#1
dae4god New Member Josh Yen Join Date: Jul 2011 Posts: 14 Rep Power: 15	Hi, 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?

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July 11, 2011, 10:13		#2
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	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.

July 12, 2011, 20:08		#3
dae4god New Member Josh Yen Join Date: Jul 2011 Posts: 14 Rep Power: 15	Thanks stumpy. I'll give it a try and let you know how it goes!

July 26, 2011, 04:09		#4
dae4god New Member Josh Yen Join Date: Jul 2011 Posts: 14 Rep Power: 15	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!

July 26, 2011, 09:49		#5
stumpy Senior Member Join Date: Apr 2009 Posts: 531 Rep Power: 21	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.