ABSTRACT
Drag coefficients are used to relate flow rate to momentum transfer for both external (over the outside surfaces of a solid such as over a flat plate, a cylinder, a sphere, etc.) and internal flows (inside a conduit such as a pipe). The basic concept is the same in both cases, but with external flows the velocity used is the velocity "far" from the surface, i.e., the velocity of approach or the freestream velocity, and for internal flows the velocity used is usually the areaaverage velocity.
