# Cf and Cvp Contributions to Cd Using CEL

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 May 19, 2005, 10:40 Cf and Cvp Contributions to Cd Using CEL #1 James Date Guest   Posts: n/a Hi 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? James

 May 19, 2005, 18:38 Re: Cf and Cvp Contributions to Cd Using CEL #2 Glenn Horrocks Guest   Posts: n/a 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 and Shear Force = areaInt(Wall Shear)@Surface Should do the trick. Regards, Glenn Horrocks

 May 20, 2005, 07:56 Re: Cf and Cvp Contributions to Cd Using CEL #3 James Date Guest   Posts: n/a 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? James

 June 8, 2005, 11:10 Re: Cf and Cvp Contributions to Cd Using CEL #4 Nepal Guest   Posts: n/a Hi, I think maybe there're something wrong, X-Pressure force = areaInt(Pressure*Normal X)@body I think it should be: X-Pressure force = areaInt_x(Pressure)@body I think your equation means: integral[(pressure fraction on X direction * Area)] But the force should be integral[(pressure * area)*fraction_x]

 June 8, 2005, 12:15 Re: Cf and Cvp Contributions to Cd Using CEL #5 Rui Guest   Posts: n/a Hi, I think that areaInt(Pressure*Normal X)@body is exactly the same as areaInt_x(Pressure)@body From CFX-5.7.1 documentation about areaInt: "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." Regards, Rui

 June 8, 2005, 19:04 Re: Cf and Cvp Contributions to Cd Using CEL #6 Nepal Guest   Posts: n/a Hi Rui, I create a sphere inside a cube to test it. 1-areaInt_x(Pressure)@body=0.00361582 N 2-areaInt(Pressure*Normal X)@body=0.00362184 N 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. Regards,

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