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,
2P
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.