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-   -   Force Function Calculator Ansys CFD Post (http://www.cfd-online.com/Forums/cfx/127104-force-function-calculator-ansys-cfd-post.html)

sharadram1989 December 3, 2013 16:15

Force Function Calculator Ansys CFD Post
 
Hi guys,

My question is very basic.

I read through the Ansys 14.5 manuals but it doesn't seem to answer my question so posting on here!

I have a lorry model sat in a wind tunnel setup (Drawn on XY plane and extruded through Z axis).

Now when I need to find the Drag Force on this lorry, I click on Function Calculator in CFD post and select the Z component for direction and it displays a force.

My question is How does Ansys calculate this force? Does it use the generic Drag Equation = 0.5*rho*Usquared*Area?

And if so what Area is it using? Is it all of the surface area of the lorry or just the cross sectional frontal area?

Thanks in Advance!

ghorrocks December 3, 2013 18:03

No, it is not using drag=0.5*rho*A*U^2 :)

It is integrating the pressure and wall shear over the surface of the body.

sharadram1989 December 3, 2013 18:21

Ah I'm guessing that equation originally comes from Navier stokes equations then. Will need to read into that stuff.

Forgot to mention that I have applied an inflation layer on the lorry. I calculated the boundary layer thickness and applied it as the inflation layer.

But I'm guessing that with or without these layers that is how Ansys will always solve it?

Thanks for the help!! :D

ghorrocks December 3, 2013 18:31

Regardless of how you have meshed it it integrates over the surface. The meshing will "just" determine the accuracy of the simulation, not the approach.

sharadram1989 December 3, 2013 18:35

Thanks ever so much. Has made it clearer now!

kiyoshi9 January 30, 2014 04:10

I want find drag force and force of water axial turbine
 
1 Attachment(s)
I want know value drag force and force at water turbine blade
The water axial turbine have eigth blade.
I use Calculate Function in CFD Post (Ansys 13)
result force in Program is much greater than the experimental result.

Thank a lot,
Natthawut

ghorrocks January 30, 2014 04:51

FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

Spinassien August 14, 2014 03:44

Hello World,
if i calculate the force in any direction, is the answer the sum of pressure force and wall shear or only one of the components?

Spinassien August 14, 2014 03:50

i thin k i found the Answer by myself:
The force on a boundary is calculated using momentum flow data from the results file, if it is available. The result can be positive or negative, indicating the direction of the force. For non-boundary locators, an approximate force is always calculated.
CFD-Post calculates the approximate force as follows:
  • If the locator is a wall boundary, the force is equal to the pressure force.
  • For all other locators, the force is equal to the pressure force plus the mass flow force (due to the advection of momentum).
  • In all cases, if wall shear data exists in the results file, the viscous force is added to the calculated force.


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