Well, I have to take issue with Port Pie?s explanation.
Cd = drag coefficient - and is a dimensionless number that is used in calculations to relate a measured drag to the various fluid dynamic conditions existing while the drag was measured. It is really just a fudge factor that allows the drag to be calculated for other fluid dynamic conditions. There is no ?minimum? but smaller is better and generally ranges from zero to small single whole digits.
CdA = is the product of the drag coefficient, Cd, times the (generally frontal) area of the object of interest. It has the dimensions of ?area?, or length squared. It is indicative of the overall drag since it takes into account the frontal area of the object. So, a barn door would have a greater CdA than a clipboard due to its larger area, even if the two rectangles might have similar Cd?s.
CFD refers to computer based fluid dynamic analysis or analysis software in which an object is modeled in the computer and then subjected to simulated flow conditions with the ability to produce useful output such as the Cd, forces, moments, pressure distributions etc. A wind tunnel on the desktop.
