Anemometry - Texas A&M University
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Transcript Anemometry - Texas A&M University
Hand-Held Anemometer
Uses a Savonius rotor
Easily portable - often used by military
Threshold of 0.75 knots
Accurate to 3% of full scale
Twin-tail vane. Accurate to ±2o
Operator error probably greatest cause of
error
Propeller Anemometers
Flat Blade - Airmeters
Helicoidal - Aerovane
Bivanes
Airmeters
Blades at 45o to wind
– Then: v V tan 45o and
v = V.
Blades must point into the
wind.
Responds well to light winds.
Accuracy of properly oriented
airmeter ~ ±0.2-3% of reading.
Aerovane
Gives both wind speed and
direction.
Vane keeps propeller
pointed into wind.
Usually have a lower
starting threshold than cup
anemometers.
Accuracy: 1-3% reading
Vane: ±2o
Threshold: Impeller 2 mph
Vane: 5 mph
Propeller blade tip moves
about 4 times faster than a
cup anemometer.
Tend to underestimate low
wind speeds
Tend to overestimate gusty
wind speeds.
– Less a problem than with cup
anemometers.
Bi-Vanes
UVW anemometer
– Measures 3-dimensional winds:
horizontal and vertical.
– Threshold for best is about 0.5
mph.
– Range: 0.5 to 112 mph
Starting and Stopping Threshold
Pressure Type Anemometer
Pressure-Plate
Anemometers
– Mayan Anemometer
(sometime between
1200 and 1400 AD)
– Small, light pith balls
dropped from a basket
and were blown
downwind.
– Leon Battista Alberti ~1450
constructed a swinging plate
anemometer.
– Leonardo DaVinci
b. 1452
– Probably drawn
1480’s - 1500’s
Robert Hooke - 1667
– Swinging Pressure
Plate Anemometers all
had a problem of
Resonance
Magnification.
Normal-Plate Anemometer
Resistance in AC
circuit varies with the
position of armature and
pressure plate. Current
required to keep armature
and
pressure plate in
positon becomes a
measure of the forc (wind
speed) acting on pressure
plate.
– Not affected by Resonance Magnification.
– Can measure gusty winds well.
2
c
Av
– Force on plate given by:
F
2
Micro-machined Force-
balance Anemometer
Built by NASA’s Jet
Propulsion Laboratory
2 cm X 2 cm.
Proof Mass hangs between
two capacitor plates. As air
moves the device, the proof
mass attempts to remain
constant. This changes the
capacitance.
Measures very small air
motions.
Pressure - Tube (Pitot Tube)
Dines Anemometer:
Pdy namic Pat mos.
Ptransverse
v
2
2
v2
Pat mos.
2
P Pdy namic Ptransverse
P Pat mos.
v 2
Pat mos.
v 2
2
2
v 2
v 2
P C
2
2
T hen,
2P
v
C
However, density is difficult to measure. If
we use the General Gas Law equation and
substitute for density, we get:
2 RTv P
v
where,
Pat mos.
P = Pdynamic - Ptransverse, (a small value, about
1 - 2 mb for a 20 mph wind)
Patmos = Atmospheric Pressure.
R = gas constant for dry air.
Tv = Virtual temperature of air. T is usually
used with negligable error.
Possible Errors of Pitot Tube
Anemometers
Tubing must be tight and not clogged (ice).
Changes in pressure must be accurately
measured.
Must be directed into wind.
Low sensitivity at low wind speeds.
Compressibility of air may cause errors at
high speeds (aircraft).
Density and humidity changes can affect
results.