Reference area for 2.5D circular cylinder flow
 

I rarely run 2D simulations since CFX does not have a 2D solver in it. However, it is possible to run effectively 2D simulations with a 1 element thick fluid domain i.e. 2.5D. Now having just downloaded the ANSYS student version on my laptop for toying about on smaller meshes and now solving the flow around a 2D circular cylinder for the drag coefficient what is the usual convention for the reference area? Is it the: 1) cylinder diameter (since that is the 2D frontal projected area), 2) cylinder diameter x the 1 element thickness for 2.5D frontal area, 3) cylinder circumference (since that is the 2D wetted area) or 4) cylinder circumference x the 1 element thickness for the 2.5D wetted area? In this case I am not 100% sure what the CFD-Post force_x is best used with

Thanks