# how to calculate the drag force on a 3D sphere in OF?

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May 5, 2015, 16:33
how to calculate the drag force on a 3D sphere in OF?
#1
Member

David Long
Join Date: May 2012
Location: Germany
Posts: 55
Rep Power: 5
Hi Foamers,

I want to test the drag coefficient of regular (e.g. sphere) and irregular objects using the CFD method.

The problem is that how to calculate the fluid force acting the Object.

A still sphere is in the fluid and the velocity and pressure field are solved (currently pimpleFoam):

The resulting force would be: (assuming the sphere has N sub-surfaces, treated as wall)
Quote:
 F_fluild = Sum(Pi * Ai* n), i = 1, 2, .... N
where Pi/AI- pressure/area of ith surface of the sphere/obstacle, n the normal of ith surface .

If we subtract buoyancy force Fb from F_fluid,
Quote:
 F_(drag/life) = F_fluid - Fb
The issue is: how to loop on the certain boundary and access the pressure or velocity information on this boundary (object/obstacle).

Any tips or recommendation would be highly appreciated.

Last edited by keepfit; May 5, 2015 at 19:00.

 May 6, 2015, 05:20 #2 Senior Member   Tom Fahner Join Date: Mar 2009 Location: Delft, Netherlands Posts: 226 Rep Power: 10 I would suggest to use the forces and forceCoeffs functionObjects. There are many threads on this forum that discuss it....

May 6, 2015, 12:04
#3
Member

David Long
Join Date: May 2012
Location: Germany
Posts: 55
Rep Power: 5
Quote:
 Originally Posted by tomf I would suggest to use the forces and forceCoeffs functionObjects. There are many threads on this forum that discuss it....
thanks tomf, I will give a try on the library fucntions.

 Tags drag coefficient, sphere falling

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