Anemometry

The art or science of wind observation

Anemometer

• An instrument used to measure wind speed• Sometimes accompanied by a vane

Anemometer Function

• To measure some or all components of the wind velocity vector

• Vector can be written as orthogonal components

• Can also be written as a speed and direction

2-D Anemometer Measurements

• What direction is wind measurement usually taken?

3-D Anemometer Measurements

• Wind Vector is expressed in three dimensions– Speed– Direction– Elevation angle

Standard Units of Measurement

• m/s• Knots (nautical miles per hour)

Measurement Conversions

• 1 m/s = ____ knots = ____ mph• 1 mph = ____ m/s = ____ knots

Measuring Wind Speed

• Wind velocity is turbulent• Subject to variations in speed, direction and

period• Wind vector can be described in terms of

mean flow and gustiness (variation) about the mean

• What is the mean?

Methods of Measurement

• Ideal wind-measuring instrument would be able to measure winds over all speeds

• Able to respond to rapid changes (turbulent fluctuations)

• Have a linear output• Exhibit simple dynamic performance

characteristics• Very difficult to design a sensor to meet all

these specs

Wind Force

• The wind force (or drag force) on an object can be used as one method to measure wind speed

• Drag force refers to forces that oppose the motion of a solid object

• Drag forces act in a direction opposite to the instantaneous velocity and are velocity dependant

Cup Anemometer

• Invented by Dr. John Thomas Romney Robinson (1846)

• Typically composed of three or four cups mounted on one end of three or four horizontal arms, which in turn were mounted at equal angles to each other on a vertical shaft

• Raw output is the mechanical rotation rate of the cup wheel

3-Cup Anemometer

• Developed by the Canadian John Patterson in 1926

• Led to a cup-wheel design which was linear and had an error of less than 3% up to 60 mph

• Each cup produced maximum torque when it was at ___ degrees to the wind flow

• The three cup anemometer also had a more constant torque and responded more quickly to gusts than the four cup anemometer

Propeller Anemometers

• The axis of rotation must be parallel to the direction of the wind and therefore horizontal

• Since the wind varies in direction and the axis has to follow its changes, a wind vane must be employed

• An aerovane combines a propeller and a tail on the same axis to obtain accurate and precise wind speed and direction measurements from the same instrument

Cup and Propeller Anemometers

• Linear measurement over most of their range (except at the low end)

• Each anemometer has a threshold value at the low end

• The starting threshold is higher than the stopping threshold

• Lower limit commonly referred to as zero, upper limit is maximum speed the anemometer can sustain without damage

Cup Anemometer Thresholds

Cup and Propeller Anemometer Errors

• Usually react quicker to speed-ups than slow-downs

• Susceptible to overestimation of wind speeds• Air Density has an effect on threshold speed

Wind Vanes

• Flat plate or airfoil that can rotate about a vertical shaft

• Orients itself along the wind vector• Counter-weight to balance it on the vertical

shaft• Azimuth angle is converted to a voltage

Ideal Wind Vane

Maintenance Requirements

• Maintain the bearings• Mechanical integrity (damage)• Alignment• Transducer operation

Drag Sphere Anemometer

• Measures wind velocity by measuring the drag force of an object in the flow

• No moving parts• Response is determined by the spring torque

of the supporting members • Supports are generally very stiff to make

anemometer rugged and increase frequency response

Pitot-Static Tube

• Invented by Henri Pitot in the early 1700’s• Measures wind speed by the change in

pressure exerted by the wind• Must be oriented into the wind• Not ideal for typical measurements

Heat Dissipation

• Hot-wire and Hot-film anemometers• Wind speed is inferred from the cooling of a

heated wire or film• Dependent on speed and density of flow

(mass flow) past the sensing element• Response speed is a function of the thermal

mass of the element

Hot-Wire Anemometers

• Fastest conventional wind sensors available• Constructed of very fine platinum wires (5

micrometers thick)• Well-suited for atmospheric turbulence and

aircraft measurements

Hot-Film Anemometers

• Made by depositing a thin film of platinum on a cylindrical quartz or glass core

• Then insulated with a very thin quartz or ceramic coating

• Rod thickness is usually ≥ 50 micrometers which can inhibit frequency response

Heat Dissipation Anemometer Characteristics

• Probe configurations are available to sense 3D wind vector

• Susceptible to atmospheric contamination which affects calibration

• Larger probes are more rugged than smaller hot-wire probes

• Rain produces spikes in data• Expensive and power-hungry• Susceptible to drift, problems with low winds

Sonic Anemometers

• Measures the time required to transmit an acoustic signal across a fixed path to determine wind velocity

• Responds linearly to wind speed• Uses speed of sound to determine wind

vector

Calibration

• What is the easiest way to calibrate all wind sensors?

Exposure

• What is the standard height above surface for wind measurement?

• Must have good exposure in all directions• Distances to obstructions should be ___ times

the height of the obstruction• Building tops are bad sites for anemometers• Seasonal effects

Exposure Problems

Data Processing

• WMO standard averaging time is ____• Maximum 3-second wind speed and direction• Maximum 1-minute average speed and

direction• What is the problem with averaging direction?

Gust definitions

• Gust Peak Speed• Gust Duration• Gust Magnitude• Gust Frequency• Gust Amplitude• Gust Lull Speed