which is the more familiar thermodynamic form.
The appearance of the temperature T in the equation of state means that it must vary within the
flowfield. Therefore, T(x, y, z) must be treated as a new field variable in addition to ρ(x, y, z). In
the moving CV scenario above, the change in the CV’s volume is not only accompanied by a
change in density, but by a change in temperature as well.
The appearance of the temperature also means that thermodynamics will need to be addressed.
So in addition to the conservation of mass and momentum which were employed in low speed
flows, we will now also need to consider the conservation of energy. The following table
compares the variables and equations which come into play in the two cases.
V, p, ρ, T
mass, momentum, energy, state
A thermodynamic system is defined as a definite quantity of matter or a region in space upon
which attention is focussed in the analysis of a problem. We may want to study a quantity of
matter contained with in a closed rigid walled chambers, or we may want to consider something
such as gas pipeline through which the matter flows. The composition of the matter inside the
system may be fixed or may change through chemical and nuclear reactions. A system may be
arbitrarily defined. It becomes important when exchange of energy between the system and the
everything else outside the system is considered. The judgement on the energetics of this
exchange is very important.
Everything external to the system is surroundings. The system is distinguished from its
surroundings by a specified boundary which may be at rest or in motion. The interactions
between a system and its surroundings, which take place across the boundary, play an important
role in thermodynamics. A system and its surroundings together comprise a universe.
Types of systems
Two types of systems can be distinguished. These are referred to, respectively, as closed systems
and open systems or control volumes. A closed system or a control mass refers to a fixed
quantity of matter, whereas a control volume is a region in space through which mass may flow.
A special type of closed system that does not interact with its surroundings is called an Isolated
Two types of exchange can occur between the system and its surroundings:
1. energy exchange (heat or work) and