Transcript Barometry

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