Anemometry
PPT Slide
Systems for Measuring Wind
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:
Eulerian Wind Measuring Instruments Classification:
2. Thermodynamic: Utilize heating/cooling power of air.
3. Sonic / Acoustic: Utilize effect of wind on sound.
Lagrangian System: changing position of a parcel is monitored as it moves with the air flow.
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Rotation Wind Devices
Because the drag coefficient is different for the cups (cleft > cright) the cups will spin.
Research shows:
A 3-cup arrangement is best.
The wind speed, V, can be approximated by the following equation: where, V = wind speed v = cup tangential speed
a = starting threshold
Recall the general form of the time response equation: For the cup anemometer, this becomes: where: Vf = forcing function, the final wind speed the cups are trying to get to and v is the linear speed of the cups.
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, Vf, and the cups are released.
Doing this for several wind speeds might give: vf (m/s) l (s) We see that the time 2 0.5 response is not constant 4 0.2 but varies with wind 10 0.1 speed.
So, we define a new term which is constant.
Solving the equation and integrating from v0 to v and from 0 to t, gives:
The distance constant, and time response, are functions of cup mass, air density and size of the cup.
Cup anemometers in gusty winds.
The overspeeding is related to a non-dimensional parameter given by: where,
Email: alcorn@ariel.met.tamu.edu
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