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

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)

Thanks Joe for your response. How about drag? Is it using the Function: Force for the flow direction?

Yup.

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]

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]

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]

Originally Posted by ghorrocks

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]

Try the fluent forum

[nawres]

The total drag on the car can be expressed as the sum of two components: pressure drag and friction drag. Pressure drag is the drag caused by the difference in pressure between the front and rear of the car. It is proportional to the square of the car’s speed and the surface area of the car that is perpendicular to the airflow. Friction drag is the drag caused by the friction between the car’s surface and the air it moves through. It is proportional to the car’s surface area and the square of the car’s speed.
Therefore, the total drag on the car can be expressed as:
D total​=D pressure​+D friction​
where D total​ is the total drag, D pressure​ is the pressure drag, and D friction​ is the friction drag.