Transcript Anemometry - Texas A&M University

Anemometry
 The oldest known meteorological instrument
about which there is any certain knowledge is
the wind vane which was built in the first
century BC and installed in the “Tower of the
Winds” in Athens by Adronicos from
Kyrrhos in Macedonia. It consisted of a
bronze Triton driven around a vertical axis by
the action of the wind and placed so that a
rod held in the figure’s hand indicated the
wind direction.
Systems for Measuring Wind
 Eulerian System: Measures the character
of air flowing past a fixed instrument.
– Placement: WMO standard: Vertical distance
should be 10 meters (33 feet) above open
terrain, meaning the distance to any
obstruction should be at least ten times the
height of the obstruction.
 If in open terrain and the anemometer
cannot be exposed at the standard height,
then an estimate of the speed at 10 meters
can be made by:
– Over Land V h  V10 0.233 0.656log10 h  4.75
Vh
or,V10 
0.233 0.656log10 h  4.75
– Over Sea surface
100.13
V10  Vh  
h 
Anemometry
 For the calculation of Wind Chill
Temperature, the calculated wind at 5 feet
will be used.
 Included in the formula for determining Wind
Chill is the determination of wind speed at 5
feet.
Wind Chill ( o F) = 35.74 0.6215T  35.75(V 0.16 )  0.4275T(V0.16 )
Where, T = air Temp oF,
V = wind speed, mph
Eulerian Wind Measuring
Instruments Classification:
 1. Aerodynamic: Utilize Kinetic Energy of
wind.
– Rotation Types
• Cup Anemometer
• Propeller Anemometer
– Pressure Types
• Pressure Tube Anemometer
• Pressure Plate Anemometer
• Bridled Anemometer
 2. Thermodynamic: Utilize heating/cooling
power of air.
– Hot-wire anemometer
– Kata Thermometer
– Crystal Anemometer
 3. Sonic / Acoustic: Utilize effect of wind
on sound.
– Sonic Anemometer
Lagrangian System: changing position of a parcel is
monitored as it moves with the air flow.
 Lagrangian Wind Measuring Instruments
Classification:
– 1. Optical:
• Tracking of a balloon.
• Satellite tracking of clouds.
– 2. Radio / Radar
•
•
•
•
Radio Direction Finding
Radar tracking of a balloon
Doppler radar tracking of raindrops
Wind Profilers
– 3. Tracer Techniques:
• Tracking released material (e.g. smoke plume)
• Filter Samplers: Placing filter detectors at various radii about
the origin and measuring the length of time for a tracer to
arrive at a filter.
• Lidar: similar to radar which uses infrared, visible, or
ultraviolet light in the form of a beam which is reflected from
particles; e.g., smoke, dust, etc., Reflection from a moving
particle causes a frequency shift between the incident
radiation and the reflected radiation. The change in
frequency is related to motion of the particle by:
vs 
v  2 u sin 
 c 
.
– Where,
•
•
•
•
•
vs 
v  2 u sin 
 c 
v = change in frequence
Vs = frequency of source radiation
c = velocity of light
u = velocity of particle
 = half-angle between incident and reflected rays
Rotation Wind Devices
 Cup Anemometer: Consider one with 2 cups
1
1
2
2
 rArcri ght v  s
Torque
Air
rArc
– r
density
= Drag
ri ghtcoefficient
left =
left v  s cTorque
2
2
– A = Area of cup
– r = Arm length
v = Wind speed
s = Cup speed
 Because the drag coefficient is different for
the cups (cleft > cright) the cups will spin.
 Research shows:
– Maximum torque on a
cup occurs when the
cup is inclined 45o to
the wind direction.
– A 3-cup arrangement is best.
– A semi-conical cup is better than a
hemispherical cup. It is stronger.
– A cup with a bead around the edge helps
reduce turbulence about the cup.
– The smaller and lighter a cup, the more
sensitive it is to light winds and gusts.
– Cup anemometers tend to have higher starting
thresholds than propeller anemometers.
– They tend to indicate higher speeds in gusty
winds than propeller type anemometers.
– The wind speed, V, can be approximated by
the following equation:
2
3
V




v


v


v
....
where,
V = wind speed
v = cup tangential speed
– When ratio of cup diameter, d, to the radius of
the circle circumscribed by the cup centers, D,
d
(i.e.,
) is 0.5, then coefficients of higher
powerDterms are negligible, so
V    v
V    v
–  = starting threshold
–  = slope of input /output
curve
d
 The ratio is called the
D
Anemometer Factor
 Recall the general form of the time response
equation: dx(t)  x(t)  x
For the cup
I
dt
anemometer,
this becomes:
dv
  v  Vf
dt
where: Vf = forcing function, the final
wind speed the cups are trying to get to and
v is the linear speed of the cups.
 To determine the time constant for the
system, the anemometer is placed in a wind
tunnel with the cups held motionless. The
wind is increased to a particular speed, Vf,
and the cups are released.
 The time it takes for the cups to increase in
speed until they are registering 0.63 (63%)
of the wind tunnel speed is the time
constant.

Doing this for several wind speeds
might give:
vf (m/s)  (s) We see that the
time
2
0.5 response is not constant
4
0.2 but varies with wind
10
0.1 speed.
 So, we define a new term which is constant.
 The distance constant is defined as: the
length of an air stream that will pass an
instrument in a length of time equal to .
The distance constant is constant for all
wind speeds. L  V
Where V = actual wind speed.
 As V increases,  decreases and L remains
constant.
dv

 v  Vf
integrating from v0 to v
dt
 Solving the equation and
and from 0 to t, gives:
t
v  V f  (v0  V f )e 
 If we define:
– Distance constant, L = V,the length of air
x
flowing past anemometer in time, ,
– and, x = tv, then: v  V f  (v0  V f )e L
where x is the horizontal displacement of the
wind during time, t.
 Typical distance constants run from 3.5 feet
to 25 feet.
 The distance constant, and time response,
are functions of cup mass, air density and
size of the cup.
mcup
mcup
L  Vf  
and  
Cg rA
V f Cg rA
 To improve the distance constant and the
time response,
– Decrease the mass of the cup: (makes more
fragile).
– Increase the cross-sectional area of the cup.
 Cup anemometers in gusty winds.
– Cup anemometers tend to register too high in
gusty winds due to two factors:
• Cosine Effect - From the changing angle at which
winds strike the cups.
• Dynamic Effect - from the inertia of the cups
tending to keep them turning faster as wind speed
decreases.
 The overspeeding is related to a non-
dimensional parameter given by:
0.55r R 2 r 2TV
where, K 
I
– r = air density
– R = radius of circle described by cups.
– r = radius of cups
– T = period of wind speed variation
– V = wind speed
– I = moment of inertia.