Experimental Characterization of Quadrotor Flow Topology Near the Ground Using Event-Based Stereo-PIV

We experimentally investigate a fixed quadrotor drone approaching a ground surface through velocity and force measurements. For constant rotor speed, we identify a distinct regime when the distance between the drone and the ground ℎ is smaller than six times the rotor radius R, leading to up to 15% thrust increase. This regime is further explored using a novel particle image velocimetry (PIV) methodology based on event-based cameras. Two-dimensional three-component (2D-3C, stereoscopic) velocity fields measured in the wake of the quadrotor reveal that the observed effects result from the combined variation in helical swirl and hyperbolic radial expansion in the mean wake structure. Based on this insight, we propose a simple three-dimensional streamline model incorporating the measured wake features, which shows good agreement with thrust measurements. The present work extends our understanding of the ground effect in rotorcraft. In particular, the event-based PIV method demonstrates strong potential for rotorcraft aerodynamics, offering high data-compression efficiency, virtually unlimited acquisition duration without loss of temporal resolution, and significantly accelerated data processing.