Gymnarchus niloticus, a typical freshwater fish, swims by undulations of a long-based dorsal fin aided by the two pectoral fins, while commonly it holds its body rigid and straight. The long flexible dorsal fin is the main propulsor of G. niloticus; it has also considerable influence on the streamline profile. This paper proposes a CFD approach to validate that the natural arrange- ment of the propulsive dorsal fin is optimal. Using morphological dataand a smoothness-keeping algorithm, the dorsal fin is ‘virtually’ moved forward and backward with several displacements from the natural location. For each case, we reconstruct geometry, generate CFD grids, and calculate the pressure, viscous and total drag coefficients respectively. The results show that the pressure and total drag coefficients increase whether the dorsal fin is displaced forward or backward, and that greater dis- placement from its original position leads to greater pressure and total drag coefficients. This suggests that the natural position of the dorsal fin is significant for maintaining the fish’s streamline profile and reducing drag.
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