The flow field around a freely swimming copepod in steady motion. Part II: Numerical simulation

Three-dimensional, numerical simulations of the flow field arounda freely swimming model-copepod were performed using a finite-volumecode. The model copepod had a realistic body shape representedby a curvilinear body-fitted coordinate system. The beatingmovement of the cephalic appendages was replaced by a distributedforce field acting on the water ventrally adjacent to the copepod'sbody. In the simulations, we took into account that freely swimmingcopepods are self-propelled bodies through properly couplingthe Navier–Stokes equations with the dynamic equationfor the copepod's body. Flow fields were calculated for fivesteady motions: (1) hovering, (2) sinking, (3) upwards swimming,(4) backwards swimming and (5) forwards swimming. The numericalresults confirm the conclusions drawn from the theoretical analysisusing Stokes flow models by Jiang et al. [in a companion paper(Jiang et al., 2002a)] for a spherical copepod shape and showthat the geometry of the flow field around a freely swimmingcopepod varies significantly with the different swimming behaviours.When a copepod hovers in the water, or swims very slowly, itgenerates a cone-shaped and wide flow field. In contrast, whena copepod sinks, or swims fast, the flow geometry is not cone-shaped,but cylindrical, narrow and long. The relationships betweencopepods' swimming behaviour and body orientation, hydrodynamicconspicuousness, energetics as well as feeding efficiency werediscussed, based on the simulation data. It is shown that thebehaviour of hovering or swimming slowly is more energeticallyefficient in terms of relative capture volume per energy expendedthan the behaviour of swimming fast, i.e. for a same amountof energy expended a hovering or slow-swimming copepod is ableto scan more water than a fast-swimming one. The numerical resultsalso suggest that the flow field generated by a fast-swimmingcopepod enables the copepod to use mechanoreception to perceivethe food/prey and therefore increases the food concentrationin the swept volume and that the flow field around a free-sinkingcopepod favours the copepod's mechanoreception while minimizingthe energy expense, so that the energy budget can still be maintainedfor both cases.