Chapter 11, Problem 1CP

What is drag? What causes it? Why do we usually try to minimize it?

To determine

The definition for the drag.

The causes of the drag.

The reason for the minimization of the drag.

Explanation of Solution

Drag:

The drag or drag force is defined as the force which is exerted by the flowing body on the surface of the body in the flow direction. It is also the component of the resultant force in the direction of the flow.

Write the expression for the drag force.

  $D=C_D\frac{1}{2}\rho U^{2}A$

Here, the coefficient of the drag is $C_D$, the density is $\rho$, the flight speed is $U$ and the frontal area is $A$.

When the fluid is moving over the surface of the body the drag will cause two types of forces.

1. The pressure force which is normal to surface of the body.
2. The shear force which is parallel to the surface of the body.

The drag force is responsible for the production of the frictional effect on the surface of the body which will create undesirable effects. It is also responsible for the high consumption of fuel by the vehicle. Hence, to reduce the fuel consumption the drag must be minimizing. The reduction in the drag force will also increase the safety and durability of the structure subjected to high wind and will also reduce noise and vibration.