Numerical study of the feeding current around a copepod

Three-dimensional, numerical simulations of the feeding current around a tethered copepod were performed using a finite-volume code. The copepod's body shape was modeled to resemble Euchaeta norvegica, and was represented by a curvilinear body-fitted coordinate system. In the simulations, the appendages that generate the feeding current were replaced by a distribution of forces acting on the water adjacent and ventrally to the body. First, the accuracy of the code was verified by simulating two viscous, zero-Reynolds-number flows for which analytical solutions are available. Then, simulations with realistic body shape and Reynolds numbers were carried out. The main features of the computed feeding current were compared with observations from Yen and Strickler (Invert. Biol., 115, 191-205, 1996), and good agreement was obtained. The entrainment region, as visualized by tracking particles in the feeding current and by plotting the resulting stream- tube, is quite large. The result can be used to quantify how the copepod takes advantage of the feeding current to trap the algal particles in its capture area. The configuration of the feeding current near to the body surface of the copepod is controlled by how the copepod forces the feeding current and by the copepod's morphology. These parameters were varied and their effects studied in a systematic manner. Specifically, by comparing various spatial distributions of the same amount of total force, it was shown that a distributed force dissipates less energy (and increases the entrainment rate) than a concentrated force, it is thus energetically more desirable. Variations of the copepod's body shape and of the distribution of forces showed little effect on the far field of the feeding current, and therefore do not appear to affect the detectability by other mechano-receptional organisms. The length scale of the influence field of the feeding current was shown to be anisotropic in three direc- tions, extending 5-7 mm above or ventrally to the copepod, 1 cm down from the abdomen. The results also suggest that the net reaction force on the copepod from the feeding current is of the same order of magnitude as the excess weight of the copepod, but is not sufficient to balance the excess weight completely.