The flow field around a freely swimming copepod in steady motion. Part II: Numerical simulation
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Three-dimensional, numerical simulations of the flow field around a freely swimming model-copepod were performed using a finite-volume code. The model copepod had a realistic body shape represented by a curvilinear body-fitted coordinate 9,stem. The beating movement of the cephalic appendages was replaced by a distributed force field acting on the water ventrally, adjacent to the copepod’s body. In the simulations, we took into account that freely swimming copepods are self-propelled bodies through properly coupling the A vier-Stokes equations with the dynamic equation for the copepod’s body. Row fields were calculated for five steady motions: (1) hovering, (2) sinking, (3) upwards swimming, (4) backwards swimming and (5)forwards swimming. The numerical results confirm the conclusions drawn from the theoretical analysis using Stokes flow models by Fiang et al. fin a companion paper Fiang et al., 2002a)] for a spherical copepod shape and show that the geometry of the flow field around a freely swimming copepod varies significantly with the different swimming behaviours. When a copepod hovers in the water, or swims very slowly, it generates a cone-shaped and wide flow field. In contrast, when a copepod sinks, or swims fast, the flow geometry is not cone-shaped, but cylindrical, narrow and long. The relationships between copepods’ swimming behaviour and body orientation hydrodynamic conspicuousness, energetics as well as feeding efficiency, were discussed, based on the simulation data. It is shown that the behaviour of hovering or swimming slowly is more energetically efficient in terms of relative capture volume per energy expended than the behaviour of swimming fast, i.e. for a same amount of energy expended a hovering or slow-swimming copepod is able to scan more water than a fast-swimming one. The numerical results also suggest that the flow field generated by a fast-swimming copepod enables the copepod to use mechanoreception to perceive the food/prey and therefore increases the food concentration in the swept volume and that the flow field around a free-sinking copepod favours the copepod’s mechanoreception while minimizing the energy expense, so that the energy budget can still be maintained for both cases.