Chemical Engineering Fluid Mechanics, 3rd Edition

12	External Flows

“You have to learn the rules of the game. And then you have to play better than anyone else.”

—Albert Einstein, 1879–1955, Physicist

I.    THE DRAG COEFFICIENT

When a fluid flows past a solid body, or the body moves through the fluid (e.g., Figure 12.1), the force (F_D) exerted on the body by the fluid is proportional to the relative rate of momentum (mass flow rate × velocity = ρV²A) transported by the fluid. This can be expressed in terms of a dimensionless drag coefficient (C_D), which is defined by the following equation:

$\frac{F_D}{A}=\frac{C_D}{2}\rho V^2$

(12.1)

Here

ρ is the density of the fluid

V is the relative velocity between the fluid and the solid body

A is the cross-sectional area of the body normal to the velocity vector V. For ...