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Fig.A-7
Schematic representation of an element
of pipeline system (Fig. A-6), treated as a "black box"
for which the transfer characteristics are of interest:
these are the connexions between the states at input
and states at its output. This approach is typical for
modern mechatronics and it is of particular value if the
element is acting or is acted upon by elements of a dif-
ferent character (such as mechanical or electrical). Study
of these relations may be described as 0-dimensional approach
because spatial relations are not studied (they are limited
to topological relations between individual elements).
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- the capacitance,
dissipance, inertance.
On the other hand, the present textbook abandons
antiquarian items - such as all those definitions and units of viscosity
usually mentioned in standard textbooks, and also
the concepts such as the velocity or pressure heads,
surviving from the times of Bernoullis, founders of hydrodynamics,
who - at the beginning of 18th century - could not, of course, work
with much later idea of energy.
Here we shall work directly with energy of fluid
and its components (kinetic component, pressure component)
which have the advantage of being directly and always governed by concervation law.
In particular, this advantage will be paramount in cases where the
investigated fluid flow is to be calculated in connection with e.g. mechanical action.

Name "fluid" is a generalised description
meant to include both liquids and gases. It also includes their
multiphase mixtures, not treated in the present text (because of
its introductory character).
Fluids are characterised - in contrast to solids - by the fact that its particles (molecules)
may easily move, change its position relative to each other: fluids can flow.
Fluid mechanics has practical importance in almost all human activities,
from meteorology (flow in atmosphere) and, in fact, even astronomy (motion
of interstellar gas) up to medicine (flow of fluids inside human body).
Even if we limit ourselves to technology, there are many fields in which
fluids and laws of their motion have central importance - let us say
in aeronautics or shipbuilding. It would be really difficult to name
a single technological field in which problems of this sort are irrelevant:
even e.g. in civil engineering we cannot avoid study of wind effects upon buildings.
Let us just mention the fundamental importance fluids have in energy conversions:
the flow problems are present if we study fuel combustion on one side and water
evaporation on the other side in a boiler, flow of heat-transporting medium
in a nuclear reactor, in the blade system of a turbine, in recirculation
and coolant pumps - and also in the processes taking place in the cylinder
of an internal combustion engine. Note that the reason for transporting
fluid need not be an intention to transport power: in hydraulic an pneumatic
control system fluid motion serves to transport and even process information.
According to the methods it applies, fluid mechanics (together e.g. with the theory
of stress and strain of deformable bodies) belongs into that part of mechanics
(Fig.A-8), which is known as continuum mechanics. A typical feature of these
fields of mechanics is that they neglect the actual physical composition of the studied
matter (- i.e. the fact that fluid consists of individual molecules) and operates with
a model of homogeneous ideal object continously distributed in space. In place of the
actual forces between the molecules it works with an assumption of stresses
continously and evenly distributed in virtual cuts delimiting the investigated volumes.
This approach leads to advatageous simplification of the mathematical description
(the limiting approach towards infinitesimally small objects, upon which the definition
of derivatives is based, would not actually work in a particulate medium). The
agreement with reality is, nevertheless, almost perfect because real particles are
incomparably smaller than the volumes under study in technical problems:
one cubic millimetre of ait contains, unde common atmospheric conditions, around
molecules. An idea about distances between molecules may be based upon the
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Fig.A-8
Position of fluid mechanics among other fields of
mechanics - and its main subdivision.
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Vaclav TESAR : "BASIC FLUID MECHANICS"
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