Longitudinal ridges on a leatherback turtle: hydrodynamic role and application to concept-car design

Abstract

Leatherback sea turtles (Dermochelys coriacea) are known to have a superior diving ability and be highly adapted to pelagic swimming. They have five longitudinal ridges on their carapace and these ridges are one of remarkable morphological features distinguished from other marine turtles. Although it was conjectured that these ridges might be an adaptation for flow control, no rigorous study has been performed to understand their hydrodynamic roles. Therefore, to elucidate the hydrodynamic role of these ridges in the leatherback turtle swimming, in Part 1, I model a carapace with and without the ridges by using three dimensional surface data of a stuffed leatherback turtle in the National Science Museum, Korea. The experiment is conducted in a wind tunnel in the ranges of the real leatherback turtle's Reynolds number and angle of attack. The ridges are slightly misaligned to the streamlines around the body to generate streamwise vortices, and suppress flow separation on the carapace, resulting in enhanced hydrodynamic performances during different modes of swimming. At high negative angles of attack and relatively low swimming speed corresponding to a vigorous swimming condition of hatchlings, the ridges significantly decrease the drag and increase the lift. At high positive angles of attack and relatively high swimming speed that represents the conditions of ascending swimming of adults, the ridges enhance the lift and lift-to-drag ratio while increasing the drag. This study is the first experimental demonstration that the longitudinal ridges on the carapace of leatherback sea turtle, which are locally misaligned to the streamlines around the body, suppress flow separation on the carapace by generating streamwise vortices. These results suggest that shapes of some morphological features of living creatures, like the longitudinal ridges of the leatherback turtles, need not be streamlined for excellent hydro- or aerodynamic performances, contrary to our common physical intuition. From this conceptual approach, in Part 2, I develop a newly-designed concept car model which has the longitudinal ridges on the surface and investigate the aerodynamic performance of the concept model. At zero yaw angle, the drag coefficient of the concept model is about 5% lower than that of the base model (Hyundai motors). To understand the effect of side wind on the aerodynamic characteristics of the model, I also consider non-zero yaw angles and measure the drag and side forces. At non-zero yaw angles, the drag coefficient on the concept model is lower by upto 13% than that of the base model, and the side force coefficient on the concept model is lower by upto 20% compared to that of the base model. These results support that, unless the yaw angle is very large, the aerodynamic effects of the concept design in terms of drag and side forces are still similar to those of zero yaw angle. Flow-field analysis shows that the ridges on the concept model generate streamwise vortices, and suppress flow separation on the rear slanted surface, resulting in the drag reduction of the concept model.