How the force derived on a 2D model?
 

As we all know, force is the integral on a wall surface, that means, surface area must not be zero. But my question is, how the force derived on a 2D model which has no area.
For example, the problem of a cylinder in a fluid field. The 2D model in FLUENT is only a circle, the force could be reported that to tell you how much Newton of drag force on the circle is. How this be derived?
Is there any clue in the HELP document?

Thanks in advance.

I used to think that the force is based on the existing 2 dimensions of the 2D model and one unit in the other dimension. If the model is built in mm, the third dimension is 1mm. And if in m, it is 1m.

But a model of a 1000mm x 50mm in a 20m/s wind field gets a relatively big drag force, that makes me doubting the derivation.

Problem solved.