Unveiling Spatio-temporal Turbulence Structures with In Situ Measurements from a Multicopter UAS Fleet

Kurzfassung

Small uncrewed aerial systems (UAS) have become a popular tool in recent years, driven by their affordability, accessibility, and ease of use. Beyond their widespread use in aerial photography and cinematography, UAS have found valuable applications in scientific research, particularly in Earth observation. In atmospheric sciences, fixed-wing UAS were initially employed to collect in situ measurements, primarily within the atmospheric boundary layer (ABL). These early efforts drew upon established methodologies from piloted aircraft research, utilizing flow probes and fast-response sensors to measure thermodynamic variables and derive turbulent fluxes. This study focuses on small multicopter UAS, commonly referred to as "drones," which offer enhanced maneuverability due to their vertical take-off and landing capabilities and advanced control systems. While multicopter UAS are frequently used to collect vertical profiles of wind, temperature, and humidity, resembling radiosonde measurements, resolving turbulent fluctuations is often not the primary objective. However, deploying multiple systems at strategically chosen locations within the ABL to observe thermodynamic features at high resolution unlocks a wealth of new research possibilities. We demonstrate that the DLR SWUF-3D (Simultaneous Wind Measurement with a UAS Fleet in 3D) quadrotor fleet, comprising 50 UAS of varying types, enables the resolution of turbulence eddies with frequencies up to 2 Hz by individual drones. This capability extends to both 3D wind measurements and temperature measurements utilizing a newly developed fine-wire platinum resistance thermometer (FWPRT). In addition to field measurements, the wind algorithm was rigorously calibrated and verified in a wind tunnel environment. Within the limitations of resolving the smallest scales, the collected data can be effectively used to calculate fluxes of momentum and sensible heat with acceptable uncertainties across a range of atmospheric conditions. Furthermore, we investigate how data from the SWUF-3D fleet can be used to calculate spatial correlations and coherence within ABL flows. Flight patterns specifically designed for this purpose include box patterns with equidistant drone separations in three dimensions and line patterns with linear or logarithmic spacing between drones. Each pattern exhibits distinct advantages and disadvantages, providing unique insights into the flow characteristics. While the box pattern offers three-dimensional correlation information, it requires a larger number of drones to achieve multiple separation distances. Conversely, the line pattern, by combining data from all possible drone distances, enables a multi-point estimation of the spatial structure function. The SWUF-3D fleet has been primarily operated at two distinct sites: the WiValdi research wind farm in Northern Germany, which boasts a unique meteorological mast array for validating spatially separated measurements, and an Alpine site selected as part of the TEAMx observational campaign. These two locations provide ideal environments for investigating the unique characteristics of turbulence in challenging conditions, which are often difficult to probe with conventional instrumentation. We compare the results of the two sites for specific atmospheric conditions showing a path forward for more systematic studies in upcoming experiments. The flexible deployment of a UAS fleet presents a promising new approach for acquiring valuable observations in such environments