IWM 2104 Windspeed measurement

• Wind


  A three-dimensional (two horizontal and vertical components) vector quantity, which is fully determined when knowing both its components: direction and speed

  
• For most operational meteorological purposes, the vertical component is ignored, surface wind is practically considered as a two-dimensional vector

  
• Wind


  • Plays an important role in crop evapotranspiration and thus determines crop water use
  • Measurement of wind is necessary for studying the crop growth as well as for other purposes
• Wind measurement sites


  Should be in an open, level location, where the distance between the anemometer and any obstruction is at least 10 times the height of the obstruction

  
• Wind sensors


  Located usually on an open mast or tower (usually at 10 metres height above the ground surface), to minimize frictional effects near the ground

  
• Wind measurement for agricultural purposes

  Wind is measured at 2 m height from ground

  
• Units of wind speed

  • meters per second (m/s)
  • kilometers per hour (km/h)
  • miles per hour (mph)
  • feet per second (ft./s)
  • knots (kt)

  
• Anemometer


  A device for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind.

  
• Types of anemometers


  • Cup Anemometers
  • Propeller Anemometers
  • Pressure-tube Anemometer
  • Sonic Anemometer
  • Thermal Anemometer
• Cup anemometer


  Has three or four cups mounted symmetrically around a freewheeling vertical axis or a shaft
• Dr. John Thomas Romney Robinson, an astronomer in Northern Ireland, invented the first cup anemometer


  1846
• Three-cup anemometer


  Developed by the Canadian John Patterson in 1926, it can respond to the gusty winds more quickly and produce higher aerodynamic torque than the four-cup anemometer
• The three-cup anemometer has been found to have an error of less than 3% up to 60 mph (97 km/h)
• Cup anemometer material


  The cups are conventionally made of brass, but in recent years cups are made of light aluminum alloy or carbon fiber thermo-plastic have become the common, allowing significant reductions in weight
• Working principle of the cup anemometer

  1. The wind pressure (also called drag force) of the open face of the cup (e.g., concave side of the cup) is greater than that of the smooth conical or hemispherical opposite face (e.g., convex side)

  2. The difference in the wind pressure between these two sides causes the shaft to rotate as the cups spin in the direction from the convex side to the concave side of next cup

  3. Rotor assembly responds to ambient winds by increasing or decreasing rotation rate to balance forces on the cup surfaces

  4. The revolution speed is proportional to the wind speed irrespective of wind direction
• Output of a cup or propeller anemometer

  • The raw output is the mechanical rotation rate of the cup wheel (and supporting shaft)
  • The shaft is coupled to an electrical transducer (usually generator) which produces an electrical output signal, typically a DC voltage proportional to shaft rotation rate and therefore to wind speed
  • An AC transducer may be used, which produces an AC voltage with amplitude and frequency proportional to rotation rate
  • Another option is an optical transducer that generates pulse rate which is proportional to rotation rate
• Propeller or windmill anemometer


  • The propeller element is kept facing into the wind by being mounted on a wind vane. This makes the sensing head directionally sensitive.
  • As the wind blows through the rotor, differential drag forces across the blades, together with lift from the blade aerofoil itself, causes the blades to spin.

  
• Cup anemometer


  • It has vertical axis of rotation
  • Due to inertia the anemometer will respond faster in response to an increase in wind speed than to a decrease in speed. This means the cups accelerate faster than they lose speed
  • Therefore, in a turbulent flow (e.g., vertical currents) the cup anemometer will overestimate the horizontal wind speed
  • Most cup anemometers have threshold wind speed (e.g., the wind speed that first moves the cup) of 0.2 to 1 m/s
  • It's omnidirectional, requiring no orientation into the wind
• Propeller anemometer


  • It has horizontal axis of rotation
  • Lack of omni-directional response to the wind. That's why propeller arrays have to be mounted on vanes to keep them oriented into the wind.
  • The propeller is automatically positioned into the oncoming wind and records the wind speed and the direction at the same time.
  • The propeller anemometer often freezes in the associated part of the blade under cold weather and can not work normally

  
• Pitot-tube static anemometer


  • Operates on pitot tube principle
  • The pitot-static tube is actually a pair of concentric tubes with two ports (e.g., pitot and static)
  • The stagnation or pitot port, at the end of the tube, is a blunt obstacle to airflow
  • The static port is located at a point far enough back along the tube and at right angles to the direction of wind flow; hence this ports have no dynamic flow effects, so the pressure observed there is just the ambient atmospheric pressure
  • The pitot-static tube must be oriented into the airflow. A typical tube will tolerate misalignment errors up to ±20°.
• Sonic anemometer


  • Consists of pairs of transmitters and receivers
  • Operate on the principle that the time between transmission and reception of a sound pulse is a function of the speed of sound plus the wind-speed component along the transmitter-receiver axis.
  • A time differential is found by subtracting the time taken for travel in one direction from time of travel in the other direction.
  • The time differential is assumed to be a component of wind along the transmitter-receiver axis.
  • With three sets of emitters and receivers oriented in the x, y, and z directions it is possible to determine u, v, and w simultaneously.

  
• Thermal anemometry (hot-wire anemometer)


  • Operates on the principle that the rate of convective cooling of a heated body is a function of the rate of fluid flow past that body
  • In this anemometer, an electrically heated wire is placed in the wind
  • The amount of power needed to keep wire hot at a constant temperature is used to calculate the wind speed
  • It is because the higher the wind speed, the more power is required to keep the wire at a constant temperature
  • These types are sensitive and mainly used in low wind speed measurements.
• Components of wind vector

  • The wind vector can be expressed either in terms of three orthogonal velocity components (e.g., u, v, and w) or as a wind speed and direction.
  • u is the ZONAL VELOCITY, i.e. the component of the horizontal wind in west-east direction (e.g., latitudinal lines) +ve for a horizontal velocity towards EAST, -ve for a horizontal velocity towards WEST
  • v is the MERIDIONAL VELOCITY, i.e. the component of the horizontal wind in south-north direction (longitudinal lines) +ve for a horizontal velocity towards NORTH, -ve for a horizontal velocity towards SOUTH
  • w is used for the VERTICAL VELOCITY, which is typically +ve for an upward velocity and -ve for an downward velocity

  
• Three ways to present horizontal wind direction

  • φVECT is the WIND VECTOR AZIMUTH, i.e. the direction TOWARDS which the wind is blowing
  • φMET is the METEOROLOGICAL WIND DIRECTION, i.e. the direction FROM which the wind is blowing
  • φPOLAR which is the WIND VECTOR POLAR ANGLE in two-dimensions

  
• Wind direction (meteorological) is defined as the direction from which the wind is blowing, relative to true north, not magnetic north. Thus a southwesterly wind blows from the south-west to the north-east.

  
• Ways to measure wind direction

  • By direction (Sixteen point of a compass)
  • By degree (measured in clockwise as N, E, S, W means 0°, 90°, 180° and 270°, respectively)

  
• Wind vane


  • An easy indicator in the form of a shaft, mounted on a vertical axis that rotates freely around its axis
  • Consists of a brass-arm, mounted on ball bearing to a vertical axis, which is supported by means of an iron-stand
  • To one side of the brass arm (indicator) there is an arrowhead and on another side there are two flat vanes forming an acute angle (about 20°) or a vertical plate, which looks like the vertical tail part of an airplane and serves as a "rudder"

  
• Working principle of wind vane

  1. Below the wind vane there are four direction arms fixed to the vertical axis by means of a brass boss. In between the direction arms there are corner indicators.

  2. The direction arms and corner indicators are tightened to the stand. The direction arms are labeled with N, S, E and W.

  3. The indicator rotates freely around its axis and the arrowhead is faced to the direction from which wind is blowing

  4. To enable remote indication of the vane's angle of rotation, a potentiometer or selsyn motor is mounted on the rotation axis.

  
• Beaufort wind scale


  • An empirical measure that relates wind speed to observed conditions at sea or on land
  • Created by Irish hydrographer Admiral Francis Beaufort in 1805, to help sailors estimate the winds via visual observations
  • If a measuring instrument becomes faulty or is not available, wind can be estimated by observing smoke as a guide and using the Beaufort Scale, which starts with 0 and goes to a force of 12
  • It emphasizes more on the observed effect of the wind, rather than the actual wind speed

  
• Wind rose


  • A graphic tool used by meteorologists to give a succinct view of how wind speed and direction are typically distributed at a particular location during a defined period
  • Using a polar coordinate system of gridding, the average wind direction is shown as one of the sixteen compass points, each separated by 22.5° measured from true north
  • The width of the bar represents the magnitude of wind speed and colour bands show wind speed ranges

  
• The length of the bar for a direction indicates the percent of time the wind came from that direction
• The wind rose shows that during this particular sampling period the wind blew from the west 30% of the time, and from the north and the northeast 12% of the time, etc.

  
• The longest spoke shows the wind blew from the west at speeds between 1-4 knots (light blue) about 4% of the time, 4-7 knots (dark green) about 18% of the time and 7-11 knots (dark blue) about 8% of the time.