an introduction to mechanical engineering part 2 pdf
Fluid dynamics is the study of the dynamics of fluid flow. Here we learn how flows behave
under different external forces and conditions. In a sense this is similar to rigid body dynamics
in physics, where Newton’s second law is used to describe the motion of rigid bodies. Here,
we must apply Newton’s second law to fluid flows in a different way since fluids do not
behave exactly like rigid bodies. This will be discussed in Section 1.2, where basic equations to
describe fluid motion are derived and explained. Some discussions on laminar and turbulent
flows are also given there, paving the way for what will follow.
The fluid that we deal with in this unit is a viscous fluid, so the velocity of fluid flow becomes
zero at the solid surface. The consequence of this no-slip condition is that flow velocity
changes from zero at the wall to the free-stream value sufficiently far away from the wall
surface. This thin layer is called the boundary layer, an important concept in fluid dynamics,
which explains how the fluid forces are generated. So, in Section 1.3, we learn the basic
behaviour of boundary layers to be able to estimate the viscous drag acting on the solid surface.
The boundary layers over solid bodies behave differently depending on their shape.
For example, the drag force acting on sports cars is much less than that on pickup trucks, where the boundary layer is separated from the body surface of vehicle creating a strong flow disturbance.
In Section 1.4 we study the streamlining strategy to reduce the drag force of immersed bodies.
We also discuss how the drag of immersed bodies is affected by the Reynolds number as well as
the wall roughness.
Pipes and ducts are important engineering components used in many fluid systems. It is
important, therefore, that the flow resistance can be correctly estimated for different type of
ducts and pipes. In general there are two types of flow resistance. One is due to the friction drag,
while the other relates to the loss of energy due to boundary layer separation. In Section 1.5, we study a method of minimizing the flow resistance similar to the streamlining strategy for
immersed bodies. The discussion of pipes and ducts is extended to non-circular shapes by
introducing the concept of hydraulic diameter.
The final section of this unit deals with the non-dimensional numbers of fluid dynamics. We
are already familiar with the Reynolds number, but there are many other non-dimensional
numbers in fluid dynamics. In Section 1.6, we learn how to identify relevant non-dimensional
numbers for different types of fluid flow. We also study how these non-dimensional numbers
are used to carry out model tests. Applications of the similarity principle to fluid machinery are
given, emphasizing the importance of non-dimensional numbers in fluid dynamics.