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August 2, 2006, 06:15 
Pressure Drag not calculated correctly in CFXPost

#1 
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Hello,
I have a problem with my pressure drag. I calculate the total drag just by "force_x()@WALL" and the friction drag on my body by "areaInt(Wall Shear)@WALL". That both seems to work really well, at least the numbers make sense. However when calulating the pressure drag by "areaInt(Absolute Pressure)@WALL" or "areaInt(Pressure)@WALL" the numbers do not make sense at all. I know that actually I can get the pressure drag just by subtracting but I have intended to make sure that all three calculations fit together. Only in order to doublecheck. Has anyone ever had s similar problem or just knows what I have done wrong? Regards Andy 

August 2, 2006, 06:54 
Re: Pressure Drag not calculated correctly in CFX

#2 
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Here's how i've done it in the past:
XViscous force = areaInt(Wall Shear X)@body YViscous force = areaInt(Wall Shear Y)@body ZViscous force = areaInt(Wall Shear Z)@body XPressure force = areaInt(Pressure*Normal X)@body YPressure force = areaInt(Pressure*Normal Y)@body ZPressure force = areaInt(Pressure*Normal Z)@body Out of interest i have checked the CEL computed values with those given in the .out file and guess what, there is a maximum of 10% difference in the values espically in the X component of pressure force (with the CEL giving the smallest value)! Hope this helps. James 

August 2, 2006, 06:59 
Re: Pressure Drag not calculated correctly in CFX

#3 
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Dear Andy If you intend to find pressure drop between two locations in the flow path then you can simply calculate the average pressure (use total pressure if two crosssections are not same) at desired locations and substract as you know. But if you want to find the force on the wall by using areaInt(Absolute Pressure)@WALL" or "areaInt(Pressure)@WALL you will only get the normal force (pressure) acting on the wall not the drag force,as the pressure always acts normal to wall whereas drag is shear force. Bye Ram Dayal


August 2, 2006, 07:28 
Re: Pressure Drag not calculated correctly in CFX

#4 
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Thanks a lot, you guys have helped me a lot!
@James: I have compared the CELvalue with the given value from the outfile. And I got the similar problem. My CELValue for the xpressure component (calculated with your expression) is about 6% too high. However when adding the outfilevalue for the xpressure to the CELvalue for the xviscous force the sum is pretty exactly the same as for the CELvalue of the total xdrag [force_x()@body]. That is really strange. And it is even more weird that they show you the values in the outfile but do not provide them automatically in the POSTfile. I think there is definitely some space for improvement in user friendliness... ;) Anyway, thanks a lot and have a nice day! 

August 2, 2006, 11:06 
Re: Pressure Drag not calculated correctly in CFX

#5 
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Yeah, Andy I found exactly the same thing. The viscous drag is pretty much identical in the .out and the CEL, but the pressure drag is quite a bit different! Perhaps there is some rounding error occurring in the CEL or .out routine.


August 2, 2006, 12:06 
Re: Pressure Drag not calculated correctly in CFX

#6 
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Dear Andy,
Are your boundaries flat? areaInt()@Locator does not do area projection. Pressure is a scalar. However, the force_x/y/z calls now exactly what is being integrated.. Perhaps that is the difference? Opaque 

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