UPDATE
Blowing in the Wind
An accurate, inexpensive sensor developed at Tech can measure average wind direction over long distances; the prototype offers a low-cost alternative to complex sensor arrays
By Lea McLees
A PROTOTYPE, non-Doppler optical sensor that makes inexpensive, accurate measurements of cross wind speeds over long distances holds promise for chemical manufacturing, aviation safety and meteorology.
Diagram of prototype, non-Doppler optical sensor. |
The single-ended, long-path laser wind sensor registers faint wind movements that an anemometer cannot measure. Test results show its measurements of higher wind speeds correlate with those of anemometers, says Dr. Mikhail Belen'kii of the Georgia Tech Research Institute (GTRI).
"The sensor is more sensitive and accurate than mechanical anemometers, and it may provide an advantage when monitoring winds over a wide area, by providing a low-cost alternative to complex arrays of traditional sensors," says Belen'kii, a principal research scientist in the Electro-optics, Environment and Materials Laboratory. "With some modification the sensor can measure both horizontal and vertical wind speeds."
The sensor, originally developed for chemical plants, is designed to work alongside other sensors that measure airborne chemical concentrations, says principal research scientist Dr. Gary Gimmestad.
"If you measure the concentration and the cross wind at the same time, you can get a good idea of the rate at which a pollutant is leaving a plant, " he explains.
But because the sensor measures average wind directions over long distances, it might have additional applications in aviation, meteorology or aerosol dispersion studies. It would be particularly useful in locations where erratic winds are the norm--tank farms, cities or widely varying landscapes. A provisional patent application has been filed.
The sensor's design is simple. Optics and electronics are mounted on a large telescope. A helium neon laser about two inches in diameter projects a beam of light from this unit onto a target approximately 100 feet away. The target is made of retroreflective materials.
The method is based on a laser beam degradation phenomenon known as the residual turbulent scintillation effect. The telescope collects laser light reflected by the target and sends it through the series of optics. Among these are two tiny detectors, each of which monitors a spot on the target inside the laser beam. The detectors pick up shadowy waves, or fringes, moving across the laser beam. The waves are visible on the target material, says researcher David Roberts.
"The fringes look a lot like the shadows of waves created on the bottom of a swimming pool on a sunny day," Roberts explains. "If you look at turbulent wind using a laser beam, you see something very similar to those waves traveling across the beam."
Each of the two detectors in the sensor registers the moment at which a dark fringe passes its view. By digitizing the points at which each detector picks up a single wave, a computer can measure time and separation. It then can compute the average velocity of a massive column of air crossing the laser beam. In this case, wind speed calculations were made every 10 seconds.
"Even though air may be flowing erratically--some going one direction at one end of the beam and some going exactly the opposite direction--you can get a net flow across the laser beam with this method," Roberts says.
The sensor correlated extremely well with anemometer readings in test results with 100 feet between the sensor and the target. A 5 percent discrepancy, within the limits of experimental error, was observed between wind speed measurements by the sensor and by the anemometer in laboratory tests.
Companies often rely on anemometers to check wind direction and velocity. But the anemometer measures wind in just one location--and in situations where winds blow erratically, that may not be representative of overall wind movement in a larger area. To truly duplicate the work the prototype sensor performs, a row of anemometers would have to be placed side by side in a line as long as the laser, Belen'kii says--a very expensive proposition.
"If you collected the same information using several meteorological towers, it would cost you much more," he says. "The cost of this sensor would be less than that of one meteorological tower."
The sensor is easier to use than Doppler systems, the researcher say. In addition, it measures wind across the beam of light instead of along the beam, as Doppler systems do. And unlike conventional LIDAR systems, this sensor can pick up turbulence.
"This might prove to be a better and more accurate way of measuring turbulence," Belen'kii said.
"It's really pretty robust, too," Gimmestad. said. "It operates well in sunlight and in darkness."
Like many optical systems, the sensor doesn't work as well in rain or fog, which obscures its target. And the sensor can only measure the component of wind that crosses the laser beam at right angles. However, one sensor could be made to rotate among several targets, checking air movements at a variety of angles.
Researchers next plan to test the sensor with technologies that measure airborne pollutant concentrations at a real refinery plant. Additional testing might include tracking cross wind speeds from the tops of city buildings, and modifying the sensor slightly to measure cross winds and wake vortices along airport runways. The sensor also could be configured to measure vertical winds, which would provide a three-dimensional capability.
Varying the laser wavelength, power and beam geometry, target type and range, receiver diameter and data processing algorithms could make the sensor useful in additional areas, as well.
A poster paper on this work was presented at the International Symposium on Optical Science, Engineering and Instrumentation in Denver during August 1996. The work was funded under project E-9004-104 in the GTRI Internal Research Program.
Further information is available from Dr. Mikhail
Belen'kii, Dr. Gary Gimmestad or David Roberts, Electro-Optics,
Environment and Materials Laboratory, Georgia Tech Research Institute,
Georgia Institute of Technology, Atlanta, GA 30332-0834. (Telephone:
404/ 894-0140, Belen'kii; 404/894- 3419, Gimmestad; 404/894-3493,
Roberts)
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Last updated: May 30, 1997