Place a parachute in a wind tunnel and measure the air speed using a suitable instrument such as a pitot tube. Measure the drag force using a special instrument called a “force balance.” Directly calculate the coefficient of drag.

The drag coefficient Cd is equal to the drag D divided by the quantity: density r times half the velocity V squared times the reference area A. The drag coefficient then expresses the ratio of the drag force to the force produced by the dynamic pressure times the area.

So the drag coefficient is a function of the drag force on the car (Fd), the density of the air (ρ), the frontal area of the car (A) and the speed that the car is travelling (V).

Two objects having the same reference area moving at the same speed through a fluid will experience a drag force proportional to their respective drag coefficients. Coefficients for unstreamlined objects can be 1 or more, for streamlined objects much less.

The drag coefficient will change with size because the Reynolds number will change and the ratio of viscous and inertial forces will be different. One can sometimes alter the fluid conditions so that a scale model will have a Reynolds number near that of the actual body and the drag coefficient will be more meaningful.

Mercedes A-Class

Drag increases with speed (v). An object that is stationary with respect to the fluid will certainly not experience any drag force. Start moving and a resistive force will arise. Get moving faster and surely the resistive force will be greater.

Drag force is proportional to the velocity for a laminar flow and the squared velocity for a turbulent flow. Even though the ultimate cause of a drag is viscous friction, the turbulent drag is independent of viscosity. Drag forces always decrease fluid velocity relative to the solid object in the fluid’s path.

Lift decreases and drag increases when Reynolds number is decreased. (2)For the airfoil with GF, Reynolds number has adverse effects on lift coefficient, while drag coefficient of the airfoil with GF has some beneficial effects compared to the airfoil without GF.

Reynolds number (Rn) is defined as Rn=(ρVL)/μ=(VL)/ν, where V is the fluid velocity relative to the body and L is a typical length.