In fluid mechanics, the drag force is determined by the product between the drag coefficient, the dynamic pressure, and the reference area.
My question is how can I know what geometric area to use as the reference area?
(I had read the post regarding the reference area of a parachute but it doesn't give me a clear answer, should I look up the drag coefficient, then calculate back to find the reference area?)
The reference area is arbitrary in principle, but it is a good idea to choose an area that is characteristic for the object you are considering. As an example, in aircraft aerodynamics we typically use the planform area of the wing as the reference area for both lift and drag coefficients of the wing. In automobile aerodynamics, by convention people use the frontal area of the vehicle as the reference area for the drag coefficient.
Of course, different choices of the reference area give different values of the various non-dimensional coefficients, which means that the definition of the reference area needs to be provided along with these values. The same is true, by the way, for the reference velocity used to fix the dynamic pressure.
The drag coefficient cannot be defined unless the reference area is also defined. If you know the drag coefficient (or look it up in a table) you will know how to measure the reference area.
You cannot calculate reference area from drag coefficient unless you also know drag force, speed and velocity. If you know all these quantities, there is little point in knowing the reference area, because it will not tell you anything you do not already know.
