# Flows with Moving and Rotating Objects — Lesson 4

All the fluid flows we have analyzed so far have constant velocity, such as an aircraft in cruise mode. To analyze the flow field, we simply change the reference frame to the aircraft pilot and solve the problem as if the air is now flowing around the aircraft at the same constant velocity.

However, constant velocity is not always the case. Most of the time, moving objects are either accelerating or decelerating. Consider a tennis ball moving in the air. Right after it makes contact with the racket, the ball is moving forward and falling (under the influence of gravity).

Moreover, in engineering, certain objects undergo rotational acceleration. A perfect example of this is the blades of a helicopter. These blades, which are fixed on the rotor, are spinning about the rotor axis at different angular velocities to generate the necessary lift. Turbomachinery offers many such examples where the fluid is undergoing angular acceleration.

In both these cases, we cannot use the regular fluid analysis to understand the fluid flow. In this lesson, we introduce different ways of analyzing the flow field to describe the flow over moving and rotating objects. In addition to outlining these methods, we also describe examples of when they are most applicable.