Low-Cost Sensor Systems for Atmospheric Boundary Layer and Air-Water Interaction Investigations

An evaluation of low-cost gas sensors multi-hole probes in conjunction with unmanned aerial systems (UAS) is investigated for conducting fundamental research in atmospheric boundary layer studies. Combination of gas sensing, particularly CO2, and velocity measurements helps illuminate the role between gas exchanges between land, water, and the atmosphere. As an alternative to traditional expensive and ground-based sensor networks to monitor CO₂ levels for leaks, unmanned aircraft offer the potential to perform in-situ atmospheric leak detection over large areas for a fraction of the cost. This project developed a proof-of-concept sensor system to map relative carbon dioxide levels to detect potential leaks. The sensor system included a Sensair K-30 FR CO₂ sensor, GPS, and altimeter connected an Arduino microcontroller which logged data to an onboard SD card. Ground tests were performed to verify and calibrate the system including wind tunnel tests to determine the optimal configuration of the system for the quickest response time (4-8 seconds based upon flowrate). Tests were then conducted over a controlled release of CO₂ in addition to over controlled rangeland fires which released carbon dioxide over a large area as would be expected from a carbon sequestration source. 3D maps of carbon dioxide were developed from the system telemetry that clearly illustrated increased CO₂ levels from the fires. These tests demonstrated the system’s ability to detect increased carbon dioxide concentrations in the atmosphere.

For the multi-hole probes, the principal behind these probes is fundamentally the same as standard Pitot-static probes that are used on both ships and aircraft today measuring water velocity and airspeed. The application of multi-hole probes has been around since Admiral Taylor developed a five-hole probe in 1915 for extracting three-dimensional velocity vector measurements on a ship. These multi-dimensional mean-velocity devices measure pressure along a set of ports on the probe tip. The measurements taken are used to calculate dynamic pressure and then back out flow angles. Multiple head geometry types are studied and optimized for desirable performance characteristics for increasing fast-response operable angular ranges. Each probe and sensor package have been designed to take pitch, yaw, and pitot-static data. This data is validated through extensive calibration in a low turbulence subsonic wind tunnel before in-field testing is done for comparisons. Probe tip geometry and internal tube dimensions give each probe different performance characteristics as no probe can be perfectly manufactured identically and must all, therefore, be calibrated once in their lifespan in known testing flow regimes. Utilizing five-hole probes in this research is focused on the development and implementation of unmanned aircraft systems and their integration with sensors for atmospheric measurements on earth with the emphasis on meteorology and atmospheric physics.

Robust methods have been used for computing wind speeds with combined gas sensor and multi-hole probes by using fixed-wing UAS in the convective boundary layer and show good potential for extracting both large-scale and small-scale turbulence measurements. Turbulence is a crucial element for atmospheric boundary layer physics, but the complexity of its dynamics and internal interactions has limited the fundamental understanding of central transport processes that occur within fluids and the atmosphere. To understand these complex phenomena, it is a primary interest to obtain a spatial description of the structure of turbulence. Thus, it has shown to be advantageous to use UAS to spatially sample the flow field using fast temporal response sensors that collect more data than a fixed-point measuring device. The accurate detection of sideslip angle in wind gusting is a desirable characteristic for wind turbulence studies. This is done and validated by using in flight comparisons to ground towers and with additional onboard sensors such as ultrasonic anemometers. Oklahoma State University’s Unmanned Systems Research Institute (USRI) has a great advantage in utilizing Oklahoma’s Mesonet, DOE ARM SGP, and Lake Carl Backwell sites for flight testing, and each site is heavily instrumented with ground towers and hardware for measurement validation. Additionally, this approach can similarly be applied and studied to extract three-dimensional velocity data from turbulent water wave measurements making gas and multi-hole probes a more versatile and robust option for atmospheric boundary layer, CO₂ exchange, and air-water interaction research.