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-   -   Pressure drag, friction drag and total drag? (http://www.cfd-online.com/Forums/cfx/23361-pressure-drag-friction-drag-total-drag.html)

Cheng December 1, 2006 03:59

Pressure drag, friction drag and total drag?
 
Am I right about these three definitions?

1. Pressure drag can be obtained from the Function Calculator by integrating the total pressure with area at the flow direction, i.e. using Function: areaInt and Variable: Total Pressure.

2. Friction drag is obtained by integrating the wall shear with area at the flow direction, i.e. using Function: areaInt and Variable: Wall Shear.

3. Drag (pressure plus friction) is obtained from the force function specified in the flow direction.

I've tried to obtain these values and they didn't match, i.e. Pressure drag (def. 1) + Friction drag (def. 2) not equaled to Drag (def. 3).

Thanks in advance for your kind response. Cheng

Joe December 1, 2006 10:20

Re: Pressure drag, friction drag and total drag?
 
1. Pressure drag can be obtained from the Function Calculator by integrating the total pressure with area at the flow direction, i.e. using Function: areaInt and Variable: Total Pressure.

I think think this has to be done as 3 - 2 = 1

2. Friction drag is obtained by integrating the wall shear with area at the flow direction, i.e. using Function: areaInt and Variable: Wall Shear.

areaInt(component of wall shear in direction of interest)

Cheng December 2, 2006 02:24

Re: Pressure drag, friction drag and total drag?
 
Thanks Joe for your response. How about drag? Is it using the Function: Force for the flow direction?


Joe December 2, 2006 09:08

Re: Pressure drag, friction drag and total drag?
 
Yup.

Cheng December 2, 2006 23:19

Re: Pressure drag, friction drag and total drag?
 
By these definitions, and in my case of air flow around a cube, the calculated Pressure Drag is 0.119644 N, Friction Drag is 0.000167 N. Summing these value give 0.119811 N. But the calculated Drag is actually 0.104207 N. Any idea why the value different?

Confused, Cheng


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