![]() | Fig.C-7
This is the standard probe for measuring velocity.
Commonly used on airplanes to measure flight speed (its construction
is then complicated by heating coil necessary to prevent blockage by
ice). For use in aerodynamic laboratories
these probes are usually made in small dimensions - both because the investigated
flow is sometimes restricted (e.g. between the compressor blades)
and because we need minimum influencing of the flow by probe presence.
Probes of this sort are commonly available with outer diameter d less than 1 millimetre |
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In cases where fluid is not brought to total rest and there is just
change of velocity magnitude due to
variation of cross-sectional area, we shall find two
unknown quantities in the integrated Bernoulli's
equation. To obtain unique solution, another condition is required and this is available
from the Castelli Theorem: velocity changes depend in a unique manner upon the changes of cross-sectional area
(of course, under the assumption of one-dimensionality, i.e. assuming rectangular shape of velocity profiles).
Fig.C-8 Typical problem solved by integration of the Castelli Theorem: T... Task: to evaluate (mean) velocity at a cross section, G ... Given velocity at another position and the area ratio. | ![]() |
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Vaclav TESAR : "BASIC FLUID MECHANICS"