Thermodynamics is a branch of physics
which deals with the energy and work of a system.
Thermodynamics deals only with the
large scale response
of a system which we can observe
and measure in experiments.
In aerodynamics, the thermodynamics
of a gas obviously plays an important role in the analysis of
but also in the understanding of
high speed flows. The
of thermodynamics defines the relationship between the various forms of
energy present in a system (kinetic and potential), the
which the system
performs and the
transfer of heat.
The first law states that energy is conserved in all thermodynamic processes.
We can imagine thermodynamic processes which conserve energy
but which never occur in nature. For example, if we bring a hot object into
contact with a cold object, we observe that the hot object
cools down and the cold object
heats up until an equilibrium is reached. The transfer of heat goes from the
hot object to the cold object. We can imagine a system, however, in which the
heat is instead transferred from the cold object to the hot object, and
such a system does not violate the first law of thermodynamics. The cold
object gets colder and the hot object gets hotter, but energy
is conserved. Obviously we don't encounter such a system in nature and to
explain this and similar observations, thermodynamicists proposed a second
law of thermodynamics. Clasius, Kelvin, and Carnot proposed various forms
of the second law to describe the particular physics problem that each was
studying. The description of the second law stated on this slide was taken
from Halliday and Resnick's textbook, "Physics". It begins with the definition
of a new state variable called
Entropy has a variety of
physical interpretations, including the statistical disorder of the system,
but for our purposes, let us consider entropy to be just another property
of the system, like
The second law states that there exists a useful state
variable called entropy S.
The change in entropy delta S is equal to the
heat transfer delta Q divided
by the temperature T.
delta S = delta Q / T
For a given physical process, the combined entropy of
the system and the environment remains a constant if the process can be
If we denote the initial and final states of the system by "i" and "f":
Sf = Si (reversible process)
An example of a reversible process is ideally forcing a
flow through a constricted pipe. Ideal means no boundary layer losses.
As the flow moves through the constriction, the pressure, temperature and
velocity change, but these variables return to their original
values downstream of the constriction. The
of the gas returns to its original conditions and the change of entropy
of the system is zero.
Engineers call such a process an
Isentropic means constant entropy.
The second law states that
if the physical process is irreversible, the combined
entropy of the system
and the environment must increase. The final entropy must be greater than
the initial entropy for an irreversible process:
Sf > Si (irreversible process)
An example of an irreversible process is the problem
discussed in the second paragraph.
A hot object is put in contact with a cold object.
Eventually, they both achieve the same equilibrium temperature. If we then
separate the objects they remain at the equilibrium temperature and
do not naturally return to their original temperatures. The
process of bringing them to the same temperature is irreversible.
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