Systems & Control Volumes Video Lecture | Fluid Mechanics for Mechanical Engineering

In this lesson, we will discuss the difference
between systems and control volumes.
In all areas of engineering, including fluid
mechanics, the conservation laws are vital
for analyzing the behavior and performance
of devices.
For example, these laws help us determine
the lift produced by an airfoil, the increase
in pressure created by a pump, or the amount
of power we can extract from a turbine.
There are two approaches to analyzing engineering
devices.
The first approach is called the control mass
or “system”, approach, which will be denoted
by the subscript “sys”.
In the system approach, we focus our attention
on a set of fluid particles as they travel
through the device.
The second approach is the control volume
approach, which will be denoted by the subscript
“CV”.
In the control volume approach, we focus on
the region of space occupied by the device.
The control volume is surrounded by a control
surface, which is denoted by the subscript
“CS”.
To help illustrate the difference between
the system and control volume approaches,
let’s take a look at fluid flowing through
a pipe.
The section of pipe we wish to analyze is
outlined by red dashed lines.
At time t1, a specific amount of mass, highlighted
in pink, occupies the section of pipe.
This mass is called a system.
The control surface is marked by the red dashed
lines and the control volume is inside the
control surface.
So initially, both the system and control
volume occupy the same region of space.
At a future time t2, the control volume and
control surface stay in place but some of
the system has flowed out of the control volume.
When analyzing a device, it is usually much
easier to observe the region of space occupied
by the device rather than track the individual
particles as they travel through the device.
Although many devices are fixed in place and
rigid, such as the pipe we just discussed,
that is not the case for all devices.
Sometimes it is necessary to have a control
volume that moves or deforms.
For example, a jet engine is filled with air,
fuel, and exhaust gases at time t1.
The control volume is the region of space
occupied by the jet engine and outlined by
red dashed lines.
The system is the pink shaded area.
At time t1, the control volume and system
occupy the same region of space.
At a later time t2, the control volume has
travelled to the left while the system has
been expelled to the right.
In this case, the control volume has moved,
but not deformed.
On the bottom of the screen we have an example
where the control volume moves and deforms.
A balloon is initially filled with air at
time t1.
The control volume is outlined by the surface
of the balloon and the system is shaded in
pink.
At this time, both the control volume and
system occupy the same region of space.
Air is allowed to escape and at a future time
t2, the control volume has moved and become
smaller, while some of the system has left
the balloon.