Transcript Anemometry - Texas A&M University

Sonic / Ultrasonic Anemometers
 The sonic anemometer
measures the change in the
speed of sound due to
motion of the air.
 The time between
transmission and reception
of a sound wave pulse
traveling a known distance
is measured from which the
wind speed is determined.
 C = local speed of sound.
 a = an angle of wind with
respect to sound wave.
 u = component of wind.
speed parallel to axis.
 v = component of wind
speed perpendicular to axis.
 t1 = time to travel from transmitter to receiver
with wind V.
 B = distance sound travels in time t1 with no
wind
d  B  ut1
B  Ct1 cosa
d  Ct1 cosa  ut1
 Then,
d
t1 
Ccosa  u
Similarly,
d
t2 
Ccosa  u
and, 1 1 2u
 
t1 t2
d
so,


1
1
d
the     u
 2t 1t 2
component
along one axis.
which gives us
wind
 A pulse along an axis 90o horizontally to the
previous one gives the component along its
axis.
 Combining the two components gives the
horizontal wind speed and direction.
 Consider:
– t1 = 305 ms
– t2 = 295 ms
– d = 100 mm
 1

1

u  100mm



305m s 295ms 
u  5.5571m s
Thermal Wind Devices
 Hot-Wire or Hot-Film Anemometer
– Type 1:
• Current applied to small wire.
• Wire warms due to resistance of the wire.
• Airflow across wire removes heat changing the
temperaure of the wire.
• Changing the temperature causes a change in the
resistance of the wire.
• Changing the resistance causes a change in the
current in the wire.
• Changing current causes a voltage imbalance in a
bridge circuit.
• The voltage imbalance becomes a measure of the
wind speed.
– Type 2:
• Everything is the same except the voltage imbalance
is used to increase the current flow in the sensor
wire until its temperature (resistance) is at a
specified amount (Rs) greater than the resistance at
ambient temperature (Ra), The overheat ratio
(Rs/Ra) is usually expressed as a percentage,
typically 50%.
 The relationship between current and the
wind speed is given by: I 2  A  B V
where A and B are usually determined
during calibration of the instrument and are
related to heat losses due to convection,
radiation, wire support conduction. “A” is the
current in the wire when V = 0.
 Sometimes the equation is written in terms of
temperature as:
I Rs  (Tw  Ta )a   V 
2
where Tw = wire temp. and Ta = ambient
temp.
 Wire used is typically platinum about .01 to
.1 mm thick and about 1 mm long.
 Tungsten is also used.
 Advantages:
– High accuracy
– Very sensitive
– Rapid time response
~1 sec.
– Can detect low speeds
~0.1 m/s
Disadvantages
Not good in rain.
Radio interference
Orientation
Fragile
Expensive
Requires frequent
maintenance
High power
consumption
 Hot-film anemometers use a thin film of
platinum.
 Usually more
durable.
 More inaccurate at
low wind speeds
than hot-wire
anemometer.
Kata Thermometer
 A spirit-in-glass thermometer with a large bulb.
 Two marks on the stem at 35oC and 38oC.
 The thermometer is warmed to over 40oC, placed in
the wind and the time taken for the spirit column to
fall from 38oC to 35oC is measured. The wind

F
speed can be derived from: B2V  

36.5  T  t  A 



where F is oF and t is in seconds. A and B are
constants determined from calibration.
Quartz Crystal Anemometer
 Can measure wind speed, direction,
temperature, humidity, solar radiation,
particulate deposition.
 The resonance frequency of oscillation of
the current in a circuit in which the quartz
crystal is mounted will change as the
temperature of the quartz crystal changes.
 Resonance frequency
is determined with
heater off. Crystals
at air temp.
 Heater turned on and
raised to temp. above
air temp.
Each crystal’s frequency changes
dependent on its temperature. (warmed by
heater, cooled by air blowing across them.)
 Wind speed is determined by amount of
frequency change for each crystal.
 For wind direction:
 Wind flow blows heat from heater across
crystals.
 Amount of frequency shift of each crystal is
a measure of amount of heat received.
 Comparison is made between crystals to
determine wind direction.
Vortex Anemometer
 Frequency of vortex formation is a function
of wind speed.
 Ultrasonic signal (150 KHz) is beamed
through region of vortex formation.
 Vortices scatter some of ultrasonic signal
resulting in modulation of amplitude of
ultrasonic signal with a frequency equal to
the frequency of vortex formation.
 150 KHz stripped from signal at receiver
leaving modulation signal.
 Wind speed related to frequency of
modulation by: f  SV
v
d
where,
–
–
–
–
fv = Frequency of Modulation
S = Strouhal number, typically about 0.2
d = Diameter of obstruction
V = Wind velocity
Comparison of several
anemometers
Type
Rotating
Speed of
Response
Slow
Required
Electronics
None
Dynamic Pressure
Fast
Aerodyna mically
Cooled
Sonic
Very f ast
Linearizing
(AC or DC)
Linearizing
(AC or DC)
Complex
Very f ast
digital
Cost
Very modest
Modest
High
Very h igh