Performance Evaluation of Three-Component Ultrasonic Anemometers with Orthogonal and Non-Orthogonal Transducer Arrays

Ultrasonic anemometers (UA) are frequently employed to measure wind, air temperature, and turbulent exchange of energy and matter in the atmospheric boundary layer. They are fast-response, linear, accurate, first-principle instruments.  Their accuracy is determined by the lengths of the acoustic paths, the direction cosines of the path geometry, and the time of flight of the acoustic signals. A fundamental limitation of UA is the self-shadowing wake effect caused by the ultrasonic transducers and support structures interfering with the flow field, leading to underestimation of the wind measurement along the acoustic paths. To minimize the transducer wake effects, numerous UA designs with different geometry, orientation, and length of the ultrasonic paths have been proposed, but there is no consensus on optimal transducer arrangement. In a widely used non-orthogonal UA design each of the three acoustic paths is tilted 60 degrees from the horizontal plane and equally spaced 120 degrees around the vertical axes. The advantage of the non-orthogonal UA is that the transducers are taken out of the horizontal plane and the three sensing paths intersect forming a small measurement volume preserving the correlation between the components of the wind vector. Alternatively, in a less common orthogonal UA design, the acoustic paths are arranged perpendicular to each other and parallel to the axes of a Cartesian coordinate system, allowing the measurement of the vertical wind component by a single pair of transducers. A disadvantage of the orthogonal UA is the large separation between the wind components and the self-shadowing effects of the transducers in the horizontal plane. To compare the performance of the orthogonal and non-orthogonal UAs we designed a unique integrated twelve-transducer probe, combining both designs in one structure with all six acoustic paths referenced to a common coordinate system. Such an arrangement reduces the uncertainty of the combined wind measurements by eliminating the need for coordinate rotation to align each UA coordinate system to the mean flow field. This study is unique because the two UAs use the same ultrasonic transducers, have equal path length to transducer diameter ratios, utilize the same time-of-flight signal processing algorithm, sample rate and measurement bandwidth. The primary difference between the two UAs is the orientation of the six acoustic paths. We demonstrate the details of the design of the combined probe and present results from a field experiment.