Measurement of Wind Speed

1
Measurement of Wind Speed

• Why do we want to measure wind speed?
• Calculate surface conductances
• Study
• Transport of dust and pests
• Heat and vapor loss
• Wind loading of buildings, crops
• Weather prediction
• Surface-atmosphere exchange of gases (CO2, H2O,
  etc.)
• Turbulence Studies

2
What makes a good Anemometer?

• Depends on what you are interested in finding
  out.
• Characteristics of a good anemometer
• Wide range of wind speeds for operation
• Some anemometers have threshold below which they
  dont work
• Some are much more accurate at one end of the
  scale compared with the other
• Linearity of the response
• Convenient if output of anemometer is simple
  linear output with wind speed
• Not as important now as we have more capable
  dataloggers

3
Characteristics of Good Anemometer

• Response time
• Quick response is essential in measuring eddy
  movement
• Not so important for ½ hour measurements
• Size of instrument
• Larger instruments are difficult to put into
  confined space
• laboratory experiments
• In-canopy measurements
• Large instruments may change flow of wind in the
  immediate area

4
Characteristics of Good Anemometer

• Directional Response
• Wind is a vector quantity
• Sometimes need both speed and direction
• Requires anemometer to point into wind

5
Methods for Wind Measurement Dynamic Pressure of
Wind

• Pitot Tube Anemometer
• Relies on Bernoullis Eq.
• Compares static pressure (air pressure) with
  dynamic pressure (pressure of wind)
• Where u is wind speed and ? is density
• Used on airplanes
• Used in wind tunnels
• Not in Environmental measurement
• No directional component

6
Force or Drag Anemometer

• Pressure Plate Anemometer
• Simplest kind
• Hinged plate
• Marked with calibrated wind speeds
• Problems
• Not particularly accurate
• Not very precise
• No measurement at low wind speeds
• Uses
• Fertilizer application
• Rough wind speed measurement

7
Force or Drag Anemometers

• Cup anemometer
• Most familiar to general public
• 3 or 6 cups turning about axis
• Low friction bearings
• Wind causes higher drag on front side of cups
• Measure shaft rotation
• Reed switch
• Light chopper
• Tachometer Generator

8
Parameters for Cup Anemometers

• Threshold
• Wind speed at which cups begin to rotate gt stop
  rotating
• Results from friction of the shaft (Cf, static gt
  Cf, dynamic)
• Typically 0.1 to 0.4 m s-1
• Linearity
• Helpful in data analysis but not nearly as
  important these days
• Response time/distance
• Time constant of anemometer
• Distance wind at new speed goes past anemometer
  until it reaches the same speed
• How do you get response distance?

9
Propeller Anemometer

• Similar principle to cup anemometer drag force
• Differs in
• Light weight propellers
• Unidirectional
• Must find a way to point to the wind

10
Propeller Anemometer

• Much better capability for low wind speeds
• Somewhat more expensive
• Must be oriented into mean wind direction
• On a swiveling base
• Using three perpendicular anemometers

11
Hot Wire Anemometer

• Ts Ta inversely proportional to wind speed
• Why?
• Two types
• Constant temperature
• Heat to constant temperature
• Look at energy required to maintain temperature
• Constant Current
• Measure temperature change with constant heating
  current

12
Hot Wire Anemometers

• Both types - high sensitivity at low wind speed
• Constant current anemometer - too sensitive at
  low wind speed
• and not very sensitive at very high wind
  speed
• Constant temperature anemometer complex
  temperature control circuitry

13
Sonic Anemometer

• Measures difference in travel time of sound
  between transducers
• Sound travels faster in direction of wind flow
• Lowers speed against wind
• Advantages
• High speed measurements
• Less than 100 ms
• No threshold
• Linear output
• Disadvantages
• Expensive, can be delicate

14
Todays Lab

• Use propeller anemometer to calibrate wind tunnel
  wind speed settings

15
Todays Lab

• Calibrate cup and plate anemometers
• Measure the starting and stopping thresholds of
  the cup anemometer
• Observe and discuss sonic anemometers

16
Todays Lab

• Check theoretical agreement of a heated needle
  anemometer
• cp (air) 29.3 J mol-1 K-1
• diameterneedle 1.27mm
• Lneedle 60mm
• Rheater 69.3 ohms
• Vheater 2.4V
• Tnon-heated needle read from datalogger
• Theated needle read from datalogger

17
Todays Lab

• d characteristic dimension
• Cylinder with fluid flow along axis, d is length
  of cylinder
• Cylinder with fluid flow normal to axis, d is
  diameter of cylinder