leads to acceptable resuts
if the investigated flow takes place in cavities with rather simple
geometry, with gradual and smooth changes of cross sections (there should be no sharp edges
that cause separation of flow and generate two-dimensional effects such as backflow and
vortical motions. The cavities (pipes) should not be very long, because of the tendency do develop complex
velocity profiles by friction on long walls.
Fig.C-3 An illustrative example for explanation of the concept: Evaluation of efflux (otflow) velocity of gas leaving a vessel under the influence of pressure difference. To avoid problems with unsteadiness, it is assumed that pressure in the vessel is kept constant by the blower. Because of very large corss section F inside the vessel, the gas velocity at position X is negligibly small. Working with gas here means the positional changes are negligible, too. As a result, there is a one-to-one correspondence between pressure and velocity. Note that we neglect losses in this chapter - the real efflux velocity will be lower. | ![]() |
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The unequivocal, one-to-one dependence between velocity and pressure changes
may be used for experimental determination of fluid flow velocity. If we invert the problem
of Fig.C-3, it is possible to find the pressure increase caused by stopping fluid to halt.
In the inlet of the Pitot tube (Fig.C-4 ... note that fluid cannot pass through the pipe
because its other end is blocked by a manometer to which the tube is connected)
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Vaclav TESAR : "BASIC FLUID MECHANICS"