Form, function, and divergence of a generic fin shape in small cetaceans

Tail flukes as well as the dorsal fin are the apomorphic traits of cetaceans appeared during evolutionary process of adaptation to the aquatic life. Both appendages present a generic wing-like shape associated with lift generation and low drag. Variability of the form of appendages was studied in seven species of cetaceans having different body size, external morphology, and specialization. Hydrodynamic performance of the fin cross-sections was examined with the CFD software and compared with similar engineered airfoils. Affinity of hydrodynamic design of both appendages was found in a wing-like planform and cross-sectional design optimized for lift generation. Distinctions in the planform and cross-sections were found related with the fin specialization in thrust production or swimming stability control. Cross-sectional design of the dorsal fin was found to be optimized for the narrow range of small angles of attack. Cross-sections of tail flukes were found to be more stable for higher angles of attack and had gradual stall characteristics that is associated with their propulsive efficiency as oscillating foils. The results obtained are the evidence of divergent evolutionary pathways of a generic wing-like shape of the fins of cetaceans under specific demands of thrust production and swimming stability control.