In Fluid Mechanics of pipeline flows, which is one of the main objects od study in the present textbook, the interest is not limited only to individual elements or components and their hydrodynamic properties. Of equal importance are the laws which govern behaviour of multi-component systems. After all, it is only in a system where an element can perform a useful role. The system approach, which will be discussed in the present chapter [G], is quite new and perhaps even unusual. Some basic tasks it solves were previously approached from a different point of view. As an example, time dependence of dicharging liquid from a vessel is a traditional problem solved already in 17th century (B. Pascal). Here we shall tackle it as a transition process in a system consisting of two elements: the vessel with its capacitance and the discharge orifice, characterised by its dissipance. This approach presents a number of advantages - if for no other reason then because it provides a unifying platform for treating in an analogous manner various different systems (even outside the field of Fluid Mechanics) which, at the first sight, were previously not considered as possessing any mutual connection. A similar viewpoint is common e.g. in solution of electric circuits and it is possible to find certain analogy between processes in hydraulic and pneumatic systems on one hand and in electric circuits on the other hand. There are not unusual attempts, especially among electrical engineers facing the problem of solving a hydraulic system, to construct an electrical model (perhaps as a real laboratory model - but more often just a mathematical model) and then transfer results obtained on the model to the investigated fluid flow system. This, however, cannot be recommended. Not only this makes easy losing from one's sight an important physical factor or aspect, but there is also the danger of the analogy being only partial one and following it blindly may lead to fundamentally wrong results. A warning against such an approach may be the essential nonlinearity of fluidic (=hydraulic, pneumatic) devices when compared with more or less linear electric elements (Ohm's law). And as far as accumulation elements go, there is no electrical analogy to the one-terminal property of fluid-containing vessels. Finally, let us mention of there being no electric analogy to state parameters.

State parameters:

Especially in more extensive systems, with a large number of elements connected in a complex manner, it is apparent that it would not be acceptable to work with detailed (perhaps two- or three-dimensional) description of the pressure and velocity field inconversions of energy components so that one does not find there any corresponding elements to such a basic device as confusers and diffusers. individual terminals of an element. We need as little numerical values for the description as possible.
For system dynamics solutions, a typical approach (sometimes called a "zero-dimensional" one) uses just state parameters without consideration of spatial relations.
Fig.G-1 (Above) Meaning of state parameters used
to desrcibe state of hydraulic and pneumatic elements
Fig.G-2 (Right) State of a two-terminal element is determined
by a set of two state parameters. A desirable property of the para-
meters forming the set is that their product represents directly
power in proper units, watts.


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This is page Nr. G01 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
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