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-   -   Defining area value in CFX-post (https://www.cfd-online.com/Forums/cfx/223564-defining-area-value-cfx-post.html)

itb7 January 16, 2020 10:36

Defining area value in CFX-post
 
Hello all,
I am trying to create a chart in CFX-post that will display the drag coefficient. I've created an expression to determine the drag coefficient:
(2*force_x()@airflowdomain )/ (AR*den*(InletVel)^2)
where AR is the surface area of the object in question and den is the fluid density. However, the value being returned is a Cd of 0.0038, which seems very very low.It seems to me there must be a mistake I've made in determining the force being exerted over the front of the object, but I'm not sure where.
Any suggestions/advice would be greatly appreciated!

aero_head January 13, 2021 00:25

Hello,

There seems to be an error in your equation.

If the velocity is aligned with the x-axis, you could obtain the drag coefficient as Cd = force_x()@Surface / (0.5*Rho*U^2*A)

There is also a thread that you could go through to troubleshoot your issue(s); http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

evcelica January 13, 2021 12:54

1.) As aero_head states: location for force_x() should be the surface object in question, not the domain.
2.) "AR" should not be the surface area, but instead the cross sectional area normal to the flow direction.


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