Development of an omni-directional weather-monitoring anemometer



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This work presents the design, fabrication, calibration and testing of a pressure-based three-component anemometer capable of measuring accurate wind speeds in extreme weather conditions. The groundwork, at the outset, covers the development of a 12-hole omni-directional flow-velocity measurement probe capable of measuring flows up to 155? from the probe axis. The new 12-hole design is optimal in the sense that the calculation of the four unknown flow quantities, i.e., two flow angles, flow speed and static pressure, is achieved with the minimum necessary number of holes/ports on the probe tip. The fact that this design has 33% lesser number of holes compared to an earlier design, has significant implications in the instrument?s spatial resolution, frequency response as well as cost of interfacing and usage. A prototype 12-hole probe with a spherical tip diameter of 3/8 inches was fabricated and tested. Good flow prediction accuracy was obtained. Further groundwork on multi-hole probe technology was carried out, developing new methods for correcting and refining the calibration and reduction procedures. When calibrating multi-hole velocity probes in a wind-tunnel, offset (or bias) errors often exist in the recorded flow angles due to errors in aligning the traverse system exactly with the flow direction and due to the angularity of the tunnel flow itself. These offset angles are hard to quantify from direct measurements with any degree of accuracy. Although usually small (less than 0.5? in most good calibrations), these errors still need to be corrected to increase the flow measurement accuracy of the probe. In this work, a method is developed that computes offset errors in all types of multi-hole probes ? from the traditional 5- and 7-hole probes to the omni-directional 18-hole probe and the nextgeneration 12-hole probe ? using simply the pressure data obtained during their calibration. The algorithm doubled the measurement accuracy for most probes. Other issues related to post-processing of the pressure data from flow studies, when the multihole probe encountered unsteady and reversed flow conditions, were also examined. The design of the anemometer (herein called a Weatherprobe) builds on that of the 12- hole probe and is capable of measuring wind velocities up to ?45? to the horizontal plane and 360? around the horizontal plane. Due to the non-conventional arrangement of its pressure ports, newly developed calibration and data-reduction algorithms were used. The probe was calibrated and its measurement accuracy assessed in a calibration facility. All associated instrumentation was assembled from the ground up and ruggedized for harsh-weather applications. Field tests performed over many days next to a 3-D sonic anemometer showed good agreement in measured flow properties, thus validating the entire Weatherprobe system. This probe has widespread applications in weather monitoring, wind energy potential estimations and structural wind load evaluations.