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Drag/Lift coefficient in 3D Fluid Flow

Posted Feb 20, 2012, 12:30 a.m. EST Version 4.1 3 Replies

Mohd Asamudin Rahman
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Hello everyone,

I'm doing a 3D simulation for a circular cylinder in fluid flow. Basically expand from 2D cylinder flow model in the library. How can i find the Drag/Lift coefficient on the cylinder in 3D domain?I tried to use -reacf and keep on getting error.

Thanks.
3 Replies Last Post Feb 20, 2012, 3:58 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Feb 20, 2012, 1:26 a.m. EST
Hi

normally the force is obtained by the integration of the reaction force on structure boundary (there is a variable predefined, but you can also call the reacf() operator.

To debug and define the correct variables it's often easier to start with a 2D example taken from a book exercice to compare the results. Then only to move to 3D (and I would suggest to use latest version as the CFD module has been greatly improved/extended since 4.1)

--
Good luck
Ivar
Hi normally the force is obtained by the integration of the reaction force on structure boundary (there is a variable predefined, but you can also call the reacf() operator. To debug and define the correct variables it's often easier to start with a 2D example taken from a book exercice to compare the results. Then only to move to 3D (and I would suggest to use latest version as the CFD module has been greatly improved/extended since 4.1) -- Good luck Ivar
Mohd Asamudin Rahman
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Posted: 1 decade ago Feb 20, 2012, 1:40 a.m. EST
Hi Dr Ivar,

Thanks for your reply. before this I tried to integrate the boundary itself instead of using -reacf operator. But still I can't plot the value of the forces. I went through the model from the library for 2D cylinder flow and tried to extend it in 3D. Is it possible to integrate the point to get the force instead of boundary? Should it be in x,y-axis or z-axis?

thanks
Hi Dr Ivar, Thanks for your reply. before this I tried to integrate the boundary itself instead of using -reacf operator. But still I can't plot the value of the forces. I went through the model from the library for 2D cylinder flow and tried to extend it in 3D. Is it possible to integrate the point to get the force instead of boundary? Should it be in x,y-axis or z-axis? thanks
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Feb 20, 2012, 3:58 a.m. EST
Hi

I'm not sure I catch you fully.
Forces can be obained primarly on boundaries, as one can define normals and directions w.r.t. a boundary, which is far more difficult for a point as there is no preferred direction one can define from a sinle point.

This is similar to the numerical singularities one get easily at any point being the intersection of two straight lines. A normal can be define on a line, for each line, but at the point of intersection ,which "normal" should one use ? Often its not easy to find out (a sharp edge in a plane perhaps, but in 3D at the intersection of 3 planes ?

--
Good luck
Ivar
Hi I'm not sure I catch you fully. Forces can be obained primarly on boundaries, as one can define normals and directions w.r.t. a boundary, which is far more difficult for a point as there is no preferred direction one can define from a sinle point. This is similar to the numerical singularities one get easily at any point being the intersection of two straight lines. A normal can be define on a line, for each line, but at the point of intersection ,which "normal" should one use ? Often its not easy to find out (a sharp edge in a plane perhaps, but in 3D at the intersection of 3 planes ? -- Good luck Ivar

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