A Simple Method to Determine Drag Coefficients in Aquatic Animals

Abstract

The drag coefficient CD= 2.D/(Aρν2) (D drag, A characteristic area of the object, ρ density of medium, ν relative speed between object and medium) is important in determining energetic requirements of a biological or technical ‘construction’ that is moving in a fluid. A small value indicates low energetic requirements. Before World War II drag coefficients of cares were usually determined by deceleration measurements. We have modified the measuring system and developed a simple method of analysis to determine CD values without influencing the swimming animal in any way. Gentoo penguins (Pygoscelis papua) were filmed during decelerating underwater gliding (‘Ausgleiten’) without any movements of extremities, parallel to a large underwater window of a penguinarium. From these films distance-time functions were plotted (Fig. 1 a). Using distance-differences Ax and time differences Δt between two points, momentary velocity ν = Δx/Δt can be determined. The reciprocal velocity ν-1 as a function of time t gives a straight line in the case of undisturbed deceleration as explained by the following.