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.

Drag coefficient cannot be negative in steady flow and a negative value of drag coefficient in steady flow indicates a computational error. It can however be zero in unsteady flow instantaneously. Negative drag is thrust.

When drag is termed “positive”, that means that it is exerting a force against the force of thrust. Negative drag means the opposite, that there is a reduction in the drag force. When drag is positive, we can assume the aircraft is slowing down. When drag is negative, we can assume it is accelerating.

Figure 1 graphs the dependence of drag coefficient for a sphere and a cylinder in crossflow on the Reynolds Number Re = ρuD/η, where D is the sphere (cylinder) diameter, η the viscosity of liquid, and .

The Reynolds number is a dimensionless number. High values of the parameter (on the order of 10 million) indicate that viscous forces are small and the flow is essentially inviscid.

The Reynolds number (Re) of a flowing fluid is calculated by multiplying the fluid velocity by the internal pipe diameter (to obtain the inertia force of the fluid) and then dividing the result by the kinematic viscosity (viscous force per unit length).

Complete answer: Since, the Reynolds number is just a ratio of 2 forces, hence it is a dimensionless quantity. Where: ρ is the density of the fluid. Its dimensional formula is: [ρ]=[M1L−3T0].

In fluid mechanics, the Reynolds number (Re) is a dimensionless number that gives a measure of the ratio of inertial (resistant to change or motion) forces to viscous (heavy and gluey) forces and consequently quantifies the relative importance of these two types of forces for given flow conditions.