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-   -   Pressure drag, friction drag and total drag? (https://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).

 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

 albaslan May 3, 2017 19:27

Drag breakdown?

Hi. I am working to Ahmed Body. I want to see drag breakdown values. Which mean; contributions to pressure drag from front part (Ck), slant rear end (Cs) and vertical rear end base (Cb). How can I see these values?

Thank you.

 ghorrocks May 4, 2017 07:09

1) Work out the formulas which those numbers are defined by
2) Enter formulas as CEL expressions
3) Get values from CEL expressions as monitor points in a simulation or in post processing in CFD-Post

 albaslan May 4, 2017 07:54

Quote:
 Originally Posted by ghorrocks (Post 647625) 1) Work out the formulas which those numbers are defined by 2) Enter formulas as CEL expressions 3) Get values from CEL expressions as monitor points in a simulation or in post processing in CFD-Post
Hi Gleen. Firstly thank you for your reply. I am working on CFD Fluent, Ahmed Body model. I am examining Ahmed's thesis. I am trying to converge to the drag values in this thesis.But in Ahmed's thesis he did not enter the formulas of Ck, Cs and Cb. Ahmed made this thesis in the wind tunnel during his time. Can I get these values from Ansys without entering the formula? I just found the Cd value. I found 0,227 when Ahmed Cd was 0,230. But I can not find other drag forces. Sorry for my English.

Best regards.

 ghorrocks May 4, 2017 17:48

Try the fluent forum

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