An instrument is undergoing development for use in measuring both the horizontal direction and the horizontal speed of wind in the speed range from 60 to 300 mph (about 27 to 134 m/s) at a rate of at least 50 samples per second. The speed range of this instrument greatly exceeds that of conventional anemometers, encompassing speeds observed in hurricanes and tornadoes. Unlike conventional anemometers, this instrument has a small exposure profile and contains no rotating mechanisms and, hence, is more rugged.
This instrument is based on measurement of the force exerted by wind on a stationary vertical rod. For the purpose of obtaining readings proportional to the wind force in two orthogonal horizontal directions, one end of the rod is attached to a cruciform carrier plate, the orthogonal arms of which are instrumented with strain gauges (see figure). The aerodynamic load on the rod causes the crossed arms to bend somewhat, and the bending is measured by the strain gauges.
The aerodynamic force on a rod in a cross flow is well understood and is a function of the Reynolds number. The force includes a steady component proportional to drag, which, in turn, is proportional to the square of the wind speed. In addition, over a certain range of the Reynolds number, vortices are periodically shed from the rod, giving rise to a side-to-side oscillating force, the frequency of which is linearly proportional to the wind speed. Hence, the strain-gauge outputs include steady drag components and oscillatory vortex-shedding components that are related to the wind velocity in known ways.
A signal-processing technique has been devised to extract, from the steady and oscillatory components of the strain-gauge outputs, two independent indications of the wind speed. These two indications can then be used to obtain an optimal estimate of the wind speed. When fully developed, the instrument will also include signal-processing electronic circuitry and data-storage circuitry. An internal battery power supply is an important element of the design inasmuch as commercial power would be unreliable in a major storm and should possibly make the instrument more vulnerable to damage by lightning.
This work was done by Jan Zysko of Kennedy Space Center and Stan Starr of I-NET, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Mechanics category. KSC-11886