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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 • • • • 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