Measuring Agricultural Spray Droplet Distribution in Propeller Wake

Date of Award


Degree Name

M.S. in Aerospace Engineering


Department of Mechanical and Aerospace Engineering


Sidaard Gunasekaran


In recent years there has been a rapid expansion of UAVs being used for agricultural spraying. UAVs have many advantages, and their use has the potential to greatly benefit the agricultural industry through reduction in cost, labor, and potentially, off target contamination. There are currently conflicting claims within the literature regarding various factors involved in spraying with UAVs and the potential for drift from sprays emitted in their wake. This inhibits implementation of rules and regulations and can misinform operators of the best practices to ensure minimal drift of chemicals being applied. This is in part due to limitations in the robustness of testing practices being implemented, whether in field or laboratory settings. To explore this problem several experimental methods were employed to examine the effect propeller wake, nozzle location, and number of propellers have on agricultural spray droplet distributions. Experiments were conducted using standard agricultural nozzles sprayed at varying transverse and downwind distances from one of two APC 17x7 propellers at different RPMs. Traditional droplet size testing was performed by traversing the nozzle and propeller across a laser diffraction instrument to determine droplet size distributions. Results indicated a shift to larger droplet sizes with the introduction of propeller wake. However, flow visualization, volume collection, and shadowgraph images coupled with an extinction based statistical algorithm reveal a plume widening effect and significant oscillations induced by the propeller wake leading to a loss in total volume and significant reduction in probability of droplets existing downwind of the propeller due to finer droplets being carried away by the wake before reaching the measurement 4 plane of the laser diffraction instrument. The results clearly show that placing a nozzle directly beneath the propeller hub for both 1 and 2 propellers is the worst-case scenario for the nozzle locations examined. For the configurations explored, distance between the nozzle and propeller, and propeller RPM were not shown to have a significant impact on the spray dynamics. Ultimately these results show that conventional laser diffraction measurements results are inherently biased when taken in propeller wake. In general, a single measurement technique is not robust enough to fully quantify the effect propeller wake has on sprays, and various methodologies need to be implemented to capture the full wake effect on the spray.


Aerospace Engineering, Agriculture, UAV: Agricultural Sprays: Spray Drift: Laser Diffraction:

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