RIASSUNTO
For many years, the maximum swimming speeds of dolphins have been reported almost entirely through observations. The objective of this paper is to estimate the swimming speed of dolphins using a theoretical analysis and numerical method. The FLUENT software solver and User Defined Function (UDF) dynamic mesh method were used to simulate the dolphin kicking during its swimming. Three peak-to-peak tail motion amplitudes and frequencies were chosen to study. The simulation results of the resistance of dolphin were compared with experimental results. The thrusts generated by dolphin fluke motion were compared with available data from the references. In conclusion, the dolphin can reach a very high speed because of its large thrust generated by its fluke motion and high propulsive efficiency.
INTRODUCTION
For a long time, dolphins have been considered good swimmers with extremely high thrust efficiency and minimum resistance. Swimming encompasses the transfer of kinetic energy and momentum from the animal’s propulsive movements to the water. High speeds allow increased foraging and active pursuit but require large energy expenditures because thrust power is directly related to the cube of velocity. Low swimming speeds have been observed for cetaceans while foraging and migrating (Lang, 1975; Webb, 1975; Fish, 1998). A dolphin swimming at a constant speed balances forces and moments acting on it by the principle of momentum conservation. The total thrust produced by the action of the caudal flukes balances the total resistance (i.e., drag) that the animal’s body encounters moving forward (Fish and Rohr, 1999). It is uncertain whether special properties of the dolphin’s skin itself contribute to the drag reduction or whether it is simply due to the maintaining of an attached turbulent boundary layer (Fish, 1993).
In the wild, dolphins swim over a wide range of speeds. The highest swimming speeds recorded were those of captive dolphins, ranging from 8.0 to 8.2 m/s and typically lasting for a few seconds (Rohr et al., 1998). Estimations of thrust based on the motion of the flukes can be used to assess independently the drag due to body form and swimming motions (Triantafyllou et al., 1993). Many studies of dolphin swimming have used the rigid model; this model assumes that the thrust generated by a swimming dolphin is equal to the estimation of drag from a gliding dolphin. Measurements of hydrodynamic force generated by swimming dolphins have been made using bubble digital particle image velocimetry (DPIV), where the movement of the bubbles was tracked with a high-speed video camera. Dolphins swam at speeds of 0.7 to 3.4 m/s within the bubble sheet oriented along the midsagittal plane of the animal (Fish et al., 2014).