Drag of Bodies

When a body moves in a fluid, it experiences forces and moments due to the relative motion of the flow taking place around it. If the body has an arbitrary shape and orientation, the flow will exert forces and moments about all the three coordinate axes, as shown in Figure 2.21. The force on the body along the flow direction is called drag.

The drag is essentially a force opposing the motion of the body. Viscosity is responsible for a part of the drag force, and the body shape generally determines the overall drag. The drag caused by the viscous effect is termed the frictional drag or skin friction. In the design of transport vehicles, shapes experiencing minimum drag are considered to keep the power consumption at a minimum. Low drag shapes are called streamlined bodies and high drag shapes are termed bluff bodies.

Drag arises due to (a) the difference in pressure between the front and rear regions and (b) the friction between the body surface and the fluid. Drag force caused by the pressure imbalance is known as pressure drag, and (b) the drag due to friction is known as skin friction drag or shear drag. A body for which the skin friction drag is the major portion of the total drag is called streamlined body, and that with the pressure drag as the major portion of the total drag is called a bluff body.