Download presentation source

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 cos a
d  Ct1 cos a  ut1
 Then,
d
t1 
Ccos a  u
Similarly,
d
t2 
Ccos a  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 fast
Linearizing
(AC or DC)
Linearizing
(AC or DC)
Complex
Very fast
digital
Cost
Very modest
Modest
High
Very high