CONTINUITY EQUATION
Fluid Flow
Figure 2 Pascal’s Law
Control Volume
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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