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Updated : 16/10/2016

GHowSAW Wind Speed

Introduction

There are various types of anemometer (wind speed measuring device) on the market. Some are highly responsive and can be used for defining the detailed turbulent structure of the wind, whilst others are better suited to measuring the underlying mean wind speed. For many applications such as wind resource assessment, power performance testing and characterisation of acoustic emission, it is the mean speed (usually averaged over 10 minutes) at a particular height above ground that is of interest. The most appropriate type of instrument for such applications is undoubtedly the three-cup anemometer.

Other types of instrument are available, and can be more appropriate in other applications. Such instruments include propeller-vane, fixed propeller, sonic, thermal, laser Doppler (LDA) and SODAR anemometers.

Selection

Although cup anemometers from different manufacturers may seem superficially similar, there are subtle design differences that can have a significant influence on behaviour and accuracy. The principal design parameters are noted in the table below with an indication of their importance in relation to key behavioural limitations.

Cup Anemometer Design Parameters and Their Qualitative Influence on Operational Characteristics

Qualitative influence on ...

Design Parameter .. sensitivity to vertical components of wind ... linearity of calibration ... dynamic responsiveness ... calibration sensitivity to horizontal wind direction
Rotor geometry (shape of cups, cup to rotor size ratio) Very important, although a detailed understanding sufficient for optimisation does not yet exist. Some influence from edge profile. Responsiveness depends upon balance between aerodynamic and inertial forces. Short arms (high cup to rotor size ratio) will give better response. Not influential.
Size of rotor Not important. Bigger rotors will have better linearity since mechanical friction will become relatively unimportant. Bigger rotors will have greater inertia and will be less responsive. Not influential.
Shaft length Important - the longer the shaft the less the body distorts the flow over the rotor. No influence. Not important. Helps minimise effects of body.
Body geometry Affects differences in sensitivity between upward and downward components in vertical winds. Shape and size affects magnitude of flow disturbance over the rotor. No influence. Not important. If body is not of uniform profile, then calibration will have a directional dependency.
Miscellaneous protrusions (e.g. cable entries, external shaft heaters etc) Could be of slight influence. No effect. No effect. Major influence of unexpected significance.
Type of bearings Not significant. Major influence, the magnitude of which may vary with temperature Second order effect.  May have an effect
Type of signal generation device No effect. May have an effect if the rotor is ‘loaded’ by the signal generator Second order effect is possible. No effect.

Alternative types of Anemometer

The cup anemometer is the preferred instrument for long term recording of mean wind speeds. Nevertheless cup anemometers are not a meteorological panacea, and should additional measurements of wind structure be required, then other types of instrument may well be preferable.

‘Alternative’ anemometry can be classified as mechanical, acoustic or thermal depending upon the working principle. In the mechanical category, there are propeller type devices, including helicoid propeller, propeller-vane, propeller-bivane and three-axis propeller devices. Within the acoustic category, there are sonic and SODAR anemometers. Thermal devices are generally restricted to indoor or laboratory applications and not suitable for meteorological applications.

Propellor type anemometers

Originally invented in the 1880's the propeller-vane and propeller-bivane anemometers based on the helicoid concept were developed for turbulence measurements in the 1960's
. The design of the helicoid propeller is such that the rate of rotation (above the effects of bearing friction) is linearly proportional to the wind speed. For making actual measurements of horizontal wind speed, and unlike the cup anemometer, a tail vane is needed to keep the propeller facing into the wind (the propeller vane anemometer).

Advantages
The helicoid propeller anemometer, when used in conjunction with an orienting vane, provides both wind speed and direction information in a single unit. In theory the propeller anemometer should not require wind tunnel calibration. The very low starting speed (particularly with photo-electronic signal generation) is an advantage over more expensive cup anemometers exhibiting the same characteristics. The propeller anemometer is a reasonable sensor for measurement of turbulence.

Disadvantages
For the hobbyist the price, in excess of R10 000, is prohibitive.
A particular disadvantage of the propeller vane is related to its inability, in a real turbulent dynamic wind, to track changing wind directions perfectly. This inertial effect can result in directional overshoots that can place the rotor off the wind axis resulting in a lower wind speed reading. This is particularly true in low wind speeds under unstable flow conditions.

Acoustic (Sonic) Anemometers

Being non-mechanical in operation, it overcomes many of the problems associated with cup and propeller anemometers and flow direction vanes particularly with regard to dynamic response characteristics which are so important in turbulence research. These advantages come at a high price in terms of complexity and therefore cost.

The sonic anemometer operates on the principle of precisely measuring the time it takes an ultrahigh frequency acoustic pulse (typically 100 kHz) to traverse a known path length in the direction of the wind and opposed to it.

Advantages
The sonic anemometer, like the helicoid propeller, is a fundamental principle instrument. In its purest form it does not exhibit the non-linearities and other errors associated with its mechanical brethren. Its chief attributes are its resolution and precision (and not necessarily the accuracy) with which it can measure the total wind vector. It has the capability of providing excellent measurements of incident flow angles when properly utilised. The sonic anemometer is ideally suited to measurement of turbulent structure.

Disadvantages
For the hobbyist the price, in excess of R25 000, is prohibitive.
Sonic anemometers are not well suited to definition of mean wind speed. The most obvious disadvantage of using sonic anemometry is inherent cost, but there are also technical reasons. Firstly, sonic anemometer accuracy is not always particularly good, although dynamic response is excellent.

Acoustic (SODAR) Anemometers

Although they both rely upon acoustic principles, SODAR anemometers differ greatly from sonic anemometers in the spatial scale of their measurement. Sonic anemometers study wind structure by employing acoustic principles between closely spaced transmitters and receivers, whereas SODAR instruments look at larger scale structures using a combined transmitter/receiver and remote back-scattering.
To cover an altitude range of 20 up to 150 metres, as is of interest for large wind energy applications, so called mini-SODARs can be used. These have an operating frequency of 4 to 6 kHz (i.e. are in the audible range) and can provide continuous profile information with moderate resolution in space (between five and ten metres) and time (every second).

Advantages
The main advantage of SODAR relate to its ability to define wind profiles and to look at higher elevation wind speeds.

Disadvantages
Prohibitively expensive for a hobbyist.

Sensing

One of the resons that the cup-anomometer is so popular amongst hobbyist using one-wire systems is that it very simple to count the output pulses using a DS2423 counter. However, with the discontinuation of this device by Maxim an alternative method is required.
However there are some posible DS2423 Alternatives

The schematic of this arrangement with a DS2423 one-wire slave is shown below.


Full Size

Installation

To obtain the best results from your anemometer careful siting is essential.
  • For best results, place the anemometer above local objects that obstruct wind flow.

  • If mounting on a roof, mount the anemometer at least 1.2m and ideally 3m above the roof line.

  • The standard for meteorological and aviation applications is to place the anemometer 10m above the ground. Most home users don't meet this standard but can still enjoy basic wind readings.

  • The standard for agricultural application is to place the anemometer 2m above the ground. This is important for evapotranspiration (ET) calculations.

  • For best results use a spirit level to make sure that the cups rotate in the horizontal plane.

Because the preferred location of an anemometer is not readily accessible it is important to thoroughly check the anemometer before installation. The following points should be attended to prior to final installation:-

  • If you can access them readily then oil the main bearings

  • Balance the rotor for longer life.

    • Hold the unit horizontally either by clamping it to a table or using a bench vice making sure that the anemometer cups can rotate freely.

    • Gently rotate the cups and note which cup is heavier and falls to the bottom. Apply a small amount of metal tape to the back side of the uppermost cup.

    • Gently rotate the cups again. Continue adding tape to the top cup(s) until the rotor seems balanced. You don't need to balance it perfectly but the rotor should spin slowly and stop smoothly without swinging back and forth.

  • Because the anemometer will be exposed to the elements, especially rain, make sure that any joints in the housing and cable entry points are sealed properly. The cable should be tied to the mast with a drip loop to prevent water collecting at the cable gland. Regardless of how well the unit itself may be sealed it is always advisable to protect any internal circuit boards with PCB lacquer spray

Lightning Protection

It is normally essential to protect the meteorological mast installation from lightning strike. Due to their nature, masts cannot avoid being struck by lightning and the challenge therefore is to ensure that a strike does not damage the test equipment. A number of key precautions can be taken:

  • a lightning finial (attractor) should be mounted at the top of the mast, in such a position that it affords the tower top anemometer with protection (normally a 60º protection umbrella can be assumed) - it is normally adequate to use the tower as the path to ground, but added protection can be afforded by running a separate cable

  • an adequately sized earth connection (earthing rod) should be strapped to the tower base

  • the instrumentation system should be designed so that it does not provide a low resistance path to earth, the aim being to encourage the strike to pass via the structure

  • lightning surge arrestors should be used should the data system not have in-built protection.

Sometimes the probability of lightning strike will be very low, and it may be decided not to use a lightning protection finial so as not to disturb the mast top anemometer.

Sources

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