Stay grounded: hydrodynamic interactions between river stingrays and the ground

Potamotrygon motoro is a negatively buoyant and dorsoventrally flattened stingray that swims along the ground in South American river basins. Whether the form of this stingray generates lift to facilitate vertical equilibrium in swimming, and how anticipated lift and drag forces vary with height from the ground have not been explored. In this study, we measured lift and drag forces, and the posterior flow field using particle image velocimetry (PIV), to characterize the hydrodynamic performance of P. motoro as a function of flow speed and distance from the ground. The experiments were conducted in a recirculating flume where a deceased ray (14 cm disc width, DW, and 10.2 cm disc length, DL) was attached to load cell at an angle of attack of 0°, to measure forces and flow at different heights from the substrate (0.001-0.85DW) at a range of speeds (0-19 cm/s; 0-1.4 DL/s). The ray unexpectedly generated negative lift when positioned furthest away from the ground (≥ 9 cm). However, lift changed to positive and increased in value as the ray was positioned closer to the ground. Furthermore, as expected, drag decreased as the ray was positioned closer to the ground. From the lift-drag ratios (L/D) we observed three distinctive regions: weak ground interaction with negative L/D due to negative lift (> 0.5 DW), intermediate ground interaction with slightly positive and constant L/D (0.07-0.5 DW), and strong ground interaction with high L/D (< 0.07 DW). Hence, we hypothesize that P. motoro may benefit from hydrodynamic interactions with the substrate that provide inherent stability when swimming close to the ground, without the need to adjust the angle of attack of the body or fins. This vertical stabilization mechanism, if valid, has potential applications for benthic robotics by enabling robots to stay at a desired distance from the ground without increasing their weight or possessing a separate active control system.