Hydrodynamic interactions between shape and substrate could provide a stabilizing effect in the vertical position of river stingrays

Potamotrygon motoro is a foil-shaped stingray that swims along the ground in South American river basins. 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 dead ray (14cm width, W) was attached to load cell at an angle of attack of 0°, to measure forces and flow at different heights from the substrate (0.001-0.85W) at a range of speeds (0-1.33W/s). The ray generated negative lift when positioned furthest away from the ground (>0.5W). 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 (>0.5W), intermediate ground interaction with slightly positive and constant L/D (0.07-0.5W), and strong ground interaction with high L/D (<0.07W). Hence, P. motoro may benefit from hydrodynamic interactions with the substrate that inherently stabilizes the fish near the ground when swimming.