| |
Fig.B-5
Basic types of liquid-filled manometers.
|
|
| |
- one arm is replaced by a vessel, in which the surface level decrease
is negligible (as documented by the example of diameters
shown in Fig. B-5, it may be up to three decimal orders
of magnitude smaller than the read difference
).
In aerodynamic laboratories, there are in common use
arrangements according to C,
Fig. B-5, where the inclination (usually adjustable) of the arm with the scale
makes possible to increase sensitivity.
These micromanometers are commonly manufacured with inclinations up to
1:50, where on a millimetre scale
one scale division corresponds to height difference
= 0,02 mm,
- so that with alcohol the corresponding pressure difference is
= 0,157 Pa.
In other commonly used variants, the accuracy of reading
the column height is increased by optical means, e.g. in the
u Betz micromanometer the displacement is read on a scale carried by a float, etc..
Most often, colculations of
pressure
in fluid is performed in order to evaluate some force effect on some object, e.g.
on submersed body
or some cover covering a hole in the vessel wall.
Pressure was actually introduced
(in chap. [A]) as the derivative
; force is therefore found by the inverse mathematical
operation,
| |
| |
|  |
 | Fig.
B-6
Force acting on
a piston
located
in the out- put cross section, generated by pressure
which it is pos- sible to assume to be constant
across the whole cross- -sectional area. |
integration of pressure across the investigated surface:
 |
 |
- because in hydrostatic problems pressure increases with
depth, the integration in case of complex surfaces may represent
mathematically interesting and demanding problem.
Let us, however, turn our attention first
to importand cases in which the positional dependence
may be neglected - either because the fluid is gas
or because the pressure is so high that its variations
between the highest and lowest position
of the surface represents an unimportand contribution to
the overall effect:
|
 |
Piston actually represents
a movable wall supported from outside.
Pressure action on this wall may be thus converted
into mechanical force, which is then transmitted by the supporting component
- the piston rod
in Fig.B-8,
where there is also some indication how the force is calculated.
This conversion is usually described as
= fluido / mechanical transformation.
The same device may, on the other hand, also perform the
(= mechano / fluidic )
conversion of a mechanical action
into a fluidic one, realised by moving fluid.
The term "fluidics" used in this context is a description
involving both hydraulics
(systems using liquids as working fluids) as well as pneumatics
(working with gas, usually air). It originated from latin word "fluo"
= to flow. Piston travel is limited to finite lengths, and
that is why its motion as well as the fluid flow have to be alternating ones.
This limitation may be removed
on the fluid side by valves (usually placed in the cylinder head)
- or motion of the whole device reative to inlet and outlet
openings - which make possible to admit (or to generate)
continuous one-direction flow.
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This is page Nr. B02 from textbook
Vaclav TESAR : "BASIC FLUID MECHANICS"
Any comments and suggestions concerning this text may be mailed to the author
to his address
tesar@fsid.cvut.cz
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