Hydrodynamic characteristics of the ocellate river stingray Potamotrygon motoro

The ocellate river stingray (Potamotrygon motoro) is a negatively buoyant and dorsoventrally flattened fish, resembling a hydrofoil, that swims extremely close to the substrate in South American river basins. Previous research, utilizing force transducers and particle image velocimetry (PIV), demonstrated that P. motoro exhibited positive lift-drag ratios when swimming near the ground. Conversely, negative lift-drag ratios were observed when the specimen was positioned further from the substrate, attributed to the unexpected negative lift inherently generated by the ray's form. This finding suggests that the form of P. motoro might intrinsically stabilize the ray's vertical position close to the ground. However, the hydrodynamic mechanism inducing negative lift away from the substrate remains unclear. This study employed a 3D-printed model, based on a scanned specimen of P. motoro, to confirm similar lift-drag patterns relative to the model's height from the substrate in a recirculating flume for different body size. Computational fluid dynamics is also used to test whether the high contoured slope between the lip and head of P. motoro induces this negative lift and to determine if a vertical stabilizing mechanism is indeed generated by the ray's form. Not only does this advance our understanding of locomotion in stingrays, but it also have implications for the design of bioinspired underwater vehicles moving along the ground.