Summary http://edugen.wiley.com/edugen/courses/crs2436/crowe9771/crowe9771...
Summary
There are many methods and instruments for measuring velocity, pressure, and flow rate:
· For velocity measurement: stagnation tube, Pitot tube, yaw meter, vane and cup anemometers, hot-wire and
hot-film anemometers, laser-Doppler anemometer, and particle image velocimetry
· For pressure measurement: static tube, piezometer, differential manometer, Bourdon-tube gage, and several
types of pressure transducers
· For flow rate measurement: direct volume or weight measurement, velocity-area integration, orifice meter,
flow nozzle, venturi meter, electromagnetic flow meter, ultrasonic flow meter, turbine flow meter, vortex
flow meter, rotameter, and weir
Flow rate or discharge for a flow meter that uses a restricted opening (i.e., an orifice, flow nozzle, or venturi) is
calculated using
where K is a flow coefficient that depends on Reynolds number and the type of flow meter, Ao is the area of the
opening, "h is the change in piezometric head across the flowmeter, and Dpz is drop in piezometric pressure
across the flowmeter.
Discharge for a rectangular weir of length L is given by
where K is the flow coefficient that depends on H/P. The term H is the height of the water above the crest of the
weir, as measured upstream of the weir, and P is the height of the weir. Discharge for a 60° triangular weir with
H > 2 cm is given by
When flow is compressible, instruments such as the stagnation tube, hot-wire anemometer, Pitot tube, and flow
nozzle may be used. However, equations correlating velocity and discharge need to be altered to account for the
effects of compressibility. To observe shock waves in compressible flow, a schlieren technique or an
interferometer may be used.
Uncertainty analysis provides a way to quantify the accuracy of a measurement. When a parameter of interest x is
evaluated using an equation of the form x = f(y1, y2, & , yn), where y1 through yn are the independent variables,
the uncertainty in x is given by
where is the uncertainty in variable yi. This equation, known as the uncertainty equation, is very useful for
estimating uncertainty and for planning experiments.
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