In thermodynamics, a control volume was
defined as a fixed region in space where one studies the
masses and energies crossing the boundaries of the region.
This concept of a control volume is
also very useful in analyzing fluid flow problems. The
boundary of a control volume for fluid flow
is usually taken as the physical boundary of the part through
which the flow is occurring. The
control volume concept is used in fluid dynamics
applications, utilizing the continuity, momentum,
and energy principles mentioned at the beginning of this
chapter. Once the control volume
and its boundary are established, the various forms of energy
crossing the boundary with the
fluid can be dealt with in equation form to solve the fluid
problem. Since fluid flow problems
usually treat a fluid crossing the boundaries of a control
volume, the control volume approach
is referred to as an "open" system analysis, which
is similar to the concepts studied in thermodynamics.
There are special cases in the nuclear field where fluid does
not cross the control
boundary. Such cases are studied utilizing the
"closed" system approach.
Regardless of the nature of the flow, all
flow situations are found to be subject to the established
basic laws of nature that
engineers have expressed in equation form. Conservation of
mass and conservation of
energy are always satisfied in fluid problems, along with
Newton’s laws of motion.
In addition, each problem will have physical constraints,
referred to mathematically as boundary
conditions, that must be satisfied before a solution to the
problem will be consistent with
the physical results
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