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James Date May 19, 2005 10:40

Cf and Cvp Contributions to Cd Using CEL

I'm sure this is straight forward and I'm just missing something. I know that when CFX-5.7.1 completes solving the integrated viscous and pressure forces are output in the .out file in the X, Y & Z coordinate directions. The total drag on the body is hence the sum of the viscous and pressure forces acting on the body.

Is there a command in post which will allows the user to output the viscous or the pressure contributions of force, rather than just their sum using "force_x()@body)". Or do I have to write an expression which sums the tangential shear force around the body to give the viscous drag?


Glenn Horrocks May 19, 2005 18:38

Re: Cf and Cvp Contributions to Cd Using CEL
Hi James,

I think you are right. To split the force into pressure and shear components you will have to integrate over the boundary surface.

Of course in CFX this is quite easy, a CEL expression something like:

Pressure Force = areaInt(p)@Surface


Shear Force = areaInt(Wall Shear)@Surface

Should do the trick.

Regards, Glenn Horrocks

James Date May 20, 2005 07:56

Re: Cf and Cvp Contributions to Cd Using CEL
Glenn Cheers for the advice, i figured that was probably the best way to do it. The commands i used in the end were:

X-Viscous force = areaInt(Wall Shear X)@body

Y-Viscous force = areaInt(Wall Shear Y)@body

Z-Viscous force = areaInt(Wall Shear Z)@body

X-Pressure force = areaInt(Pressure*Normal X)@body

Y-Pressure force = areaInt(Pressure*Normal Y)@body

Z-Pressure force = areaInt(Pressure*Normal Z)@body

Out of interest i checked the CEL computed values with those given in the .out file and guess what, there was a maximum of 10% difference in the values espically in the X component of pressure force (with the CEL giving the smallest value)! So which values are correct?

Any ideas?


Nepal June 8, 2005 11:10

Re: Cf and Cvp Contributions to Cd Using CEL
Hi, I think maybe there're something wrong,

<font color="blue">X-Pressure force = areaInt(Pressure*Normal X)@body</font> I think it should be: <font color="blue"> X-Pressure force = areaInt_x(Pressure)@body </font> I think your equation means: integral[(<font color="red">pressure fraction on X direction</font> * Area)] But the force should be integral[(pressure * area)*fraction_x]

Rui June 8, 2005 12:15

Re: Cf and Cvp Contributions to Cd Using CEL

I think that <font color="blue">areaInt(Pressure*Normal X)@body</font> is exactly the same as <font color="blue">areaInt_x(Pressure)@body</font>

From CFX-5.7.1 documentation about <font color="blue">areaInt</font>: "If a direction is selected, the result is an integration over the projected area of each face onto a plane normal to that direction.""Function: areaInt, Location: Plane1, Variable: Pressure, Direction:X. This example integrates Pressure over the projected area of Plane1 onto a plane normal to the X-axis. The result is the Pressure force acting in the X-direction on Plane1."



Nepal June 8, 2005 19:04

Re: Cf and Cvp Contributions to Cd Using CEL
Hi Rui,
I create a sphere inside a cube to test it.

1-areaInt_x(Pressure)@body=<font color="blue">0.00361582 N</font>
2-areaInt(Pressure*Normal X)@body=<font color="blue">0.00362184 N</font>
3-If I use calculater icon in CFX571,
function :areaInt
location :body
variable :Pressure
Direction:GLobal X
Then the result is 0.00361582

So I think for NormalX case, its area is not projected onto the direction X, its value is larger always. I'm not so sure that I'm right. I think it worth to paying more attention to make it clear for correct report.


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