Water Circulation Beneath a Hovering sUAS for Water Quality Monitoring Applications

Water quality has traditionally been measured via in situ sensors and satellites. The latter has limited applicability for smaller inland water bodies, while the former requires significant logistics, labor, and expense for routine sampling, and reactive/spurious sampling is often not feasible as a result (e.g., sampling pre-/post-storm). Consequently, small uncrewed aircraft system-based (sUAS-based) sampling has emerged as a potential solution to bridge these sampling gaps and challenges. But sampling from an sUAS is complicated by the need to pump water from depth, rather than suspending a sensor from the sUAS, due to concern over sampling sUAS-impacted waters. Here, we measure the water flow below a hovering sUAS in a laboratory by applying the particle image velocimetry flow measurement technique. Observations suggest the development of two counter-rotating vortices under the sUAS, where, in the center of the vortex pair, water is upwelled to the surface, which would, therefore, be a sampling location relatively free of contamination by the sUAS. This location coincides with the still spot on the water surface underneath the sUAS; thus, if one wanted to sample water by suspending a sensor underneath an sUAS, then the optimal sampling location would be within this still spot.