Swimming near substrates: Stingray self-propelled undulatory simulations

Abstract

Fish movement investigations typically assume motion through unbound fluids. However, benthic organisms experience ground effects when moving near substrates. To identify the internal ground-effect mechanisms on self-propelled stingrays, simulations applying an in-house developed fluid-structure interaction algorithm were performed. A geometric model was developed from three-dimensional (3D) laser-scan data of a live freshwater stingray. The ratios of distances between the stingray and solid boundary to its disc width and unbounded fluid were investigated at two swimming frequencies: 2.385 and 3.384 Hz. Velocity fluctuation amplitude during one cycle near a substrate was approximately 30% less than that in unbounded fluid. The two peaks of the force coefficient in one cycle decreased and increased, respectively, with decreased power loss near the substrate; this is related to the pressure and wake distributions on the lower stingray surface at two typical instances. Pressure regions on the front and rear lower wing-crest surfaces decreased when approaching the substrate at the first typical instance, whereas that of the wing trough increased for the second typical instance. The vortex size on the lower wing surface decreased near the substrate for both instances.