The second form of anemometers used in ecological studies are hot-wire anemometers, in which an electrically heated wire element is cooled by the wind, and the wind speed calculated by the rate of heat loss. Unlike mechanical anemometers, these devices have a rapid response time and the lack of moving parts allows them to be installed close to the ground or within vegetation, so they have the potential for measuring small-scale eddies and microclimatic effects. However, the relative expense of the units is prohibitive for many ecological applications.
More complex ultrasonic anemometers measure wind speed in three directions based on the time of flight of sonic pulses between pairs of transducers. While expensive, such sonic anemometers are suitable for measuring turbulent air flow with a very high temporal resolution and are typically used in conjunction with infrared or laser-based gas analysers to measure ecosystem fluxes using the eddy covariance method (Burba and Anderson, 2007). The eddy covariance micrometeorological technique, which involves high-speed measurements of fluxes of water, gas, heat, and momentum within the atmospheric boundary layer, is widely used by micrometeorologists across the globe.General introduction of forward-curved squirrel-cage fan 1.6.1 Intrusive measurement techniques
Pitot tube, hot-wire and hot-film anemometers are common measurement methods that extract point-wise velocity by direct contact between the anemometer probe and the flow stream. These methods affect the near-probe flow and are considered as intrusive measurement techniques.
The pitot tube extracts velocity after measuring total (stagnation) and static pressures. The difference between total and static pressures is the dynamic pressure (ρV2/2) which is a function of velocity at the measurement point. Although a pitot tube interferes with the near-probe flow field, its construction, setup and utilization are easy, and consequently it is widely used in performance tests of turbomachines to extract velocity, flow rate and pressure components. Hot-wire and hot-film anemometers work on the basis of electrical resistance of their sensor (probe) that depends on heat transfer and therefore on the local flow velocity. The sensor is made of a thin metal wire (materials such as tungsten and platinum) or film so that its resistance is very sensitive to temperature variation. The probe is heated to a temperature that is considerably higher than the fluid temperature. The final temperature and resistance of the probe depends on the rate of heat loss from the probe into the fluid that strictly depends on the fluid velocity. Accordingly, measurement of voltage drop across the probe wire/film leads to extraction of local velocity magnitude. In comparison with the pitot tube, the hot-wire and hot-film methods are more accurate and have considerably shorter response time (significantly higher measurement frequency). Consequently, when the purpose is to analyse temporal evolution of highly unstable and intermittent flow fields (such as turbomachinery flows), hot-wire and hot-film anemometers are superior to the pitot tube. PHYSICAL MEASUREMENTS | Other Measurements Types of Anemometers Air velocity sensors in common use include pitot tubes, mechanical anemometers, thermal anemometers, ultrasonic anenometers and laser Doppler anemometers. Pitot tubes (Figure 5) measure the pressure difference due to the kinetic energy of the moving gas. The pitot tube is pointed directly against the air flow so that air impinges on a hole at the tip of the tube. Other holes on the side of the tube measure the static pressure. Velocity, v, is related to the pressure difference ΔP according to eqn , where ρ is the gas density in kg m−3, ΔP is measured in pascal (Pa) and C is a constant ranging from 0.98 to 1.0. The pressure difference is very small at low air velocities. At v = 1 m s−1 it is only 0.6 Pa or about 0.06 mm of water. The resolution of a pitot tube depends therefore on what is used to measure the temperature difference. An inclined U-tube can at best be read to a few tenth of a mm (1 Pa), while micromanometers have resolutions down to 0.1 Pa or less. Pitot tubes are therefore not useful at low air velocities. Pitot tubes are cheap and do not need calibration. They are highly directional instruments (they should be aligned to within 5°) and can be used only when the air flow direction is known, as in pipes and ducts. Mechanical anemometers measure the rotational speed of vanes or cups driven by the wind. Because of static friction, they only operate beyond around 0.2–0.3 m s−1. Vane anemometers (Figure 6) are highly directional. As with all mechanical devices, they should not be subject to shock, dust, etc. that may affect bearing friction. The cup anemometer (Figure 7) consists of three or more cups mounted symmetrically about a vertical axis. Such instruments are insensitive to air flow direction as long as it is horizontal, and are therefore widely used in meteorological measurements. Thermal anemometers measure the cooling effect of air flow over an electrically heated sensor. There is a vast difference in price and performance between laboratory thermal anemometers and industrial models; this article is only concerned with the latter. Thermal sensors come in several shapes: hot wire, hot film or hot bead (Figure 8). A hot wire sensor consists of a very thin wire made of a noble metal such as platinum or tungsten tensioned between two metal prongs. A hot film sensor consists of a thin film of a noble metal usually deposited near the tip of a thin ceramic probe. A hot bead sensor usually consists of a thermistor because of the requirement for a high resistance in spherical shape. Thermal anemometers are better at low velocities than vane anemometers. Good instruments will provide temperature compensation to ensure that the calibration is independent of air temperature. Hot wire and hot film sensor readings are dependent on air flow direction, while hot beads are almost omnidirectional, except when a probe component is in the way. All thermal anemometers are affected by turbulence in the air. Ultrasonic anemometers measure the effect of wind on the speed of sound travelling between a pair of transducers. By using three orthogonal pair of transducers, they can measure all components of velocity. Some models can measure air turbulence as well as mean velocity. These anemometers are robust, do not suffer calibration shift, have no moving part, and can measure quite low velocities (0.01 m s−1). They are still fairly expensive but are gaining popularity, especially in meteorological applications. Laser Doppler anemometers use two laser beams that intersect to create a fringe pattern. As a micrometre-sized particle passes through these fringes, the scattered light from it fluctuates in intensity. The frequency of this fluctuation is measured with a photodetector and converted to particle velocity. Laser Doppler systems are very accurate and can measure very low velocities, down to a few mm s−1, but they are very expensive and require expertise, as laser beams are dangerous to handle. BOUNDARY LAYERS | Observational Techniques In Situ Wind Speed and Direction For average wind speed and/or direction over some time period, cup (or propeller) anemometers and wind vanes are usually the most convenient. Operational designs must withstand continuous exposure to stormy conditions, but there are also ‘sensitive’ instruments intended for research work. Apart from mechanical strength, the difference is reflected in their starting speed and distance constant (response time converted to run of wind). A sensitive cup anemometer will start from rest in a breeze of 0.3 m s−1 and have a distance constant less than 1 m. For best accuracy (typically 1%) cups must be calibrated individually, although calibration in the steady horizontal flow of a wind tunnel can lead to uncertainty. In a gusty wind, cup anemometers overestimate for two reasons: the rotor responds more quickly to an increasing wind than to the reverse, and, in a wind gust with a vertical component, shielding by the upwind cup is reduced. A propeller has poor ‘cosine’ response (to off-axis wind direction), but the error is usually minimized by mounting it on the front of a wind vane. A cup-anemometer–wind-vane pair are often mounted at opposite ends of a horizontal bar (B and C in Figure 1). KINEMATICS Measurement of Winds A variety of methods are used to measure the horizontal wind flow. Some operate from fixed positions such as surface weather station anemometers and weather vanes that give measurements in a fixed (Eulerian) reference system. Others such as weather balloons, clouds tracked from satellites, and raindrops tracked with Doppler radar provide estimates for tracers presumed to be following the wind flow (a Lagrangian system reference). Cloud tracking techniques for this purpose have the additional challenge of determining the height of the cloud and sorting out situations where the clouds do not move with the horizontal wind such as for the stationary wave clouds seen around mountains. Combining such observations into a single description of the wind flow requires taking into account the distinction between trajectories and streamlines and the differing impacts of space and time averaging. Surface Layer Measurements of Turbulence Fast-Response Wind Sensors For measurements of the vertical wind component a number of devices have been developed, including vertically aligned light propellers and various types of drag anemometer. The latter is based on the relationship between the force on a body, e.g., a golf ball, and wind velocity. The sensing element is usually based on the bending of the supporting beam detected by a strain gauge (a filament in which electrical resistance changes with strain). Both devices have frequency response problems. These occur in the propeller because of finite starting and stopping velocities and inertia, and in the drag anemometer because of eigenmode vibrations (Kármán vortices). In addition, most propeller and drag anemometers distort the wind flow with their bulk. Another instrument that has been used to measure wind speed in the atmosphere is the hot-wire anemometer. This instrument is optimized for laboratory-type flows (very small size and very rapid frequency response). The principle is that a metal wire is heated by an electric current. The more it is cooled by the (perpendicular) wind, the more current is needed to maintain the temperature of the wire at the set level. Since it is a fine-scale instrument it is only suited to very detailed studies of the atmospheric turbulence – like the eddy cascade towards smaller scales and final dissipation. Few studies have used this instrument in micrometeorological research, and it has not been developed into more rugged designs. The instrument that has won the most widespread use in micrometeorology is the ultrasonic anemometer (Figure 1). This is because it has no fragile or moving parts and does not significantly interfere with the flow, if properly designed. It builds on the principle of the propagation of sound. A transmitter sounds a pulse (typically of 100 kHz, i.e., ultrasonic) and a receiver some distance l away detects it some time t later. This time depends on the speed of sound, c, as well as the local instantaneous wind velocity. If this is done in the opposite direction as well the wind velocity can be derived. The precise relationship is given by eqn : where va is the wind velocity component along the transmitter–receiver axis. Figure 2 provides an explanation of the principle. Here vn denotes the wind velocity normal to the transmitter–receiver axis