An open source ultrasonic anemometer for the spatially distributed and time synchronized measurement of large scale...

The quantification of flow speed and -direction is key to many aerodynamic investigations. In case of large scale flow fields and outdoor environments optical techniques such as particle image velocimetry or Shake-the-Box are difficult to implement. Instead, a sufficient amount of point-wise measurements can be employed. Ultrasonic anemometers are a compelling solution due to their high accuracy and low drift even at low to moderate flow velocities. Commercial ultrasonic anemometers often lack a synchronization method and an analog output and most of the available products are closed source and therefore the algorithms employed have unknown characteristics such as time delay, error handling and filtering. The goal of this paper is to develop an ultrasonic anemometer that can measure flow velocities up to 35 m s^-1 and address the above issues while being low cost. The sensor is based on off-the-shelf electronic components, two custom printed circuit boards, and uses an STM32F4 microcontroller as its main processor. A semi-automated calibration and validation process was performed, achieving a mean flow speed magnitude accuracy of $\pm 0.3\mathrm{m/s}$ and an angle error of less than 1%.