Anemometry 4 The oldest known meteorological instrument about which there is any certain knowledge is the wind vane which was built in the first century BC and installed in the “Tower of the Winds” in Athens by Adronicos from Kyrrhos in Macedonia. It consisted of a bronze Triton driven around a vertical axis by the action of the wind and placed so that a rod held in the figure’s hand indicated the wind direction.

Systems for Measuring Wind 4 Eulerian System: Measures the character of air flowing past a fixed instrument. –Placement: WMO standard: Vertical distance should be 10 meters (33 feet) above open terrain, meaning the distance to any obstruction should be at least ten times the height of the obstruction.

4 If in open terrain and the anemometer cannot be exposed at the standard height, then an estimate of the speed at 10 meters can be made by: –Over Land –Over Sea surface

Anemometry 4 For the calculation of Wind Chill Temperature, the calculated wind at 5 feet will be used. 4 Included in the formula for determining Wind Chill is the determination of wind speed at 5 feet. Where, T = air Temp o F, V = wind speed, mph

Eulerian Wind Measuring Instruments Classification: 4 1. Aerodynamic: Utilize Kinetic Energy of wind. –Rotation Types Cup Anemometer Propeller Anemometer –Pressure Types Pressure Tube Anemometer Pressure Plate Anemometer Bridled Anemometer

4 2. Thermodynamic: Utilize heating/cooling power of air. –Hot-wire anemometer –Kata Thermometer –Crystal Anemometer

4 3. Sonic / Acoustic: Utilize effect of wind on sound. –Sonic Anemometer

Lagrangian System: changing position of a parcel is monitored as it moves with the air flow. 4 Lagrangian Wind Measuring Instruments Classification: –1. Optical: Tracking of a balloon. Satellite tracking of clouds. –2. Radio / Radar Radio Direction Finding Radar tracking of a balloon Doppler radar tracking of raindrops Wind Profilers

–3. Tracer Techniques: Tracking released material (e.g. smoke plume) Filter Samplers: Placing filter detectors at various radii about the origin and measuring the length of time for a tracer to arrive at a filter. Lidar: similar to radar which uses infrared, visible, or ultraviolet light in the form of a beam which is reflected from particles; e.g., smoke, dust, etc., Reflection from a moving particle causes a frequency shift between the incident radiation and the reflected radiation. The change in frequency is related to motion of the particle by:

. –Where,  v = change in frequence V s = frequency of source radiation c = velocity of light u = velocity of particle  = half-angle between incident and reflected rays

Rotation Wind Devices 4 Cup Anemometer: Consider one with 2 cups –  = Air densityc = Drag coefficient –A = Area of cupv = Wind speed –r = Arm lengths = Cup speed

4 Because the drag coefficient is different for the cups (c left > c right ) the cups will spin.

4 Research shows: –Maximum torque on a cup occurs when the cup is inclined 45 o to the wind direction.

–A 3-cup arrangement is best. –A semi-conical cup is better than a hemispherical cup. It is stronger. –A cup with a bead around the edge helps reduce turbulence about the cup. –The smaller and lighter a cup, the more sensitive it is to light winds and gusts. –Cup anemometers tend to have higher starting thresholds than propeller anemometers. –They tend to indicate higher speeds in gusty winds than propeller type anemometers.

–The wind speed, V, can be approximated by the following equation: where, V = wind speed v = cup tangential speed –When ratio of cup diameter, d, to the radius of the circle circumscribed by the cup centers, D, (i.e., ) is 0.5, then coefficients of higher power terms are negligible, so

–  = starting threshold –  = slope of input /output curve 4 The ratio is called the Anemometer Factor

4 Recall the general form of the time response equation: For the cup anemometer, this becomes: where: V f = forcing function, the final wind speed the cups are trying to get to and v is the linear speed of the cups.

4 To determine the time constant for the system, the anemometer is placed in a wind tunnel with the cups held motionless. The wind is increased to a particular speed, V f, and the cups are released. 4 The time it takes for the cups to increase in speed until they are registering 0.63 (63%) of the wind tunnel speed is the time constant.

 Doing this for several wind speeds might give: v f (m/s) (s) We see that the time 2 0.5response is not constant but varies with wind speed.

4 So, we define a new term which is constant.  The distance constant is defined as: the length of an air stream that will pass an instrument in a length of time equal to. The distance constant is constant for all wind speeds. Where V = actual wind speed.  As V increases,  decreases and L remains constant.

4 Solving the equation and integrating from v0 to v and from 0 to t, gives: 4 If we define: –Distance constant, L = V  the length of air flowing past anemometer in time,  –and, x = tv, then: where x is the horizontal displacement of the wind during time, t.

4 Typical distance constants run from 3.5 feet to 25 feet.

4 The distance constant, and time response, are functions of cup mass, air density and size of the cup. 4 To improve the distance constant and the time response, –Decrease the mass of the cup: (makes more fragile). –Increase the cross-sectional area of the cup.

4 Cup anemometers in gusty winds. –Cup anemometers tend to register too high in gusty winds due to two factors: Cosine Effect - From the changing angle at which winds strike the cups. Dynamic Effect - from the inertia of the cups tending to keep them turning faster as wind speed decreases.

4 The overspeeding is related to a non- dimensional parameter given by: where, –  = air density –R = radius of circle described by cups. –r = radius of cups –T = period of wind speed variation –V = wind speed –I = moment of inertia.