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Fig.D-21 |
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Fig.D-22 Characteristics
in logarithmic co-ordinates: as is shown here on the example of characteristic of the orifice from Fig.D-20, in logarithmic co-ordinates typical characteristics of hydraulic and pneumatic devices transform into straight lines with slope +2. Note the broken lines of constant dissipated power. With dif- ferent magnitudes of dissipance Q, these straight lines are placed at different posi- tions (so that there is different specific-energy drop and different dissipated power at the same mass flow rate) but remain parallel. |
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The difference between the two approaches: The fact that value of the coefficient
is equal for geometrically similar devices may be used with advantage
in experimental determination of the
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Fig.D-23
An experimental verification of the fact that dissipance value of an element (here an orifice with rounded upstream inlet) is sufficiently constant to be useful as a characterisation scalar quantity which determines in a unique manner the element behaviour. Very exact measurements would reveal that at different states of fluid (in this case air at different temperatures and magnitudes of barometric pressure) there are different values of dissipance - and that small but unavoidable friction loss component decreases the value with increasing flow rate. As documented here, this is all indistinguishable in the usual scatter caused by common experimental errors. |
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Vaclav TESAR : "BASIC FLUID MECHANICS"