Pressure energy
In Fig.A-23 there is, quite analogically to Fig.A-21, an element of pipeline system in which work is done at the price of pressure energy. While at the input there is pressure , in the element output there is a lower pressure . The work required to
Fig.A-23
get the marked fluid column of length into the element is again calculated as a product of force and distance - this time the force in question (it is expressed in Fig. A-14) and the distance is equal to the lenth of fluid column . If the decrease of pressure energy of fluid is evaluated as the difference between the input work and output work, the result ... where is the pressure drop across the element. As a result we obtain the expression for the pressure energy . For specific value - again, the transition to specific values does not apply to the intensity factor, which in this case is pressure . An important fact is the formal similarity of expressions for the both kinds of energy, pressure and position one. Similarly, it is possible to find at least certain similarity also in other kinds of energy. Here in both cases we find invariance (which need not be the case for the other cases) of the specific value of extensity factor: we work here with the gravity acceleration as if it were a constant (which, strictly speaking, it is not - but its variations, e.g. with increasing height above sea level, are negligible for technical calculations) and since we limit here our attention to the model of incompressible fluid, there is also

Compressibility effect: are worth at least a brief note, even though we shall not handle them further in this textbook : in compressible case,


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This is page Nr. A11 from textbook Vaclav TESAR : "BASIC FLUID MECHANICS"
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