A simplified computational model of possible hydrodynamic interactions between respiratory and swimming-related water flows in labriform-swimming fishes.

Hydrodynamic interactions in bony fishes between respiratory fluid flows leaving the opercular openings and simultaneous flows generated by movements of downstream pectoral fins are both poorly understood and likely to be complex. Labriform-swimming fishes that swim primarily by moving only their pectoral fins are good subjects for such studies. We performed a computational fluid dynamics (CFD) investigation of a simplified two-dimensional model of these interactions based on previously published experimental observations of both respiratory and pectoral fin movements under both resting and slow steady swimming conditions in two similar labriform swimmers: the bluegill sunfish (Lepomis macrochirus) and the largemouth bass (Micropterus salmoides). We carried out a parametric study investigating the effects that swimming speed, strength of opercular flow, and phase difference between the pectoral fin motion and the opercular opening and closing have on the thrust and side slip forces generated by the pectoral fins during both the abduction and adduction portions of the fin movement cycle. We analyzed pressure distributions on the fin surface to determine physical differences in flows with and without opercular jets. The modeling indicates that complex flow structures emerge from the coupling between the opercular jets and vortex shedding from pectoral fins. The jets from the opercular openings appear to exert significant influence on the forces generated by the fins; they are potentially significant in the maneuverability of at least some labriform swimmers. The numerical simulations and the analysis establish a framework for study of these interactions in various labriform swimmers in a variety of flow regimes. Similar situations in groups of fishes using other swimming modes should also be investigated.