title: Efficiency of fish propulsion.
abstract: The system efficiency of a self-propelled flexible body is ill-defined, hence we introduce the concept of quasi-propulsive efficiency, defined as the ratio of the power needed to tow a body in rigid-straight condition over the power it requires for self-propulsion, both measured for the same speed. Through examples we show that the quasi-propulsive efficiency is a rational non-dimensional metric of the propulsive fitness of fish and fish-like mechanisms, consistent with the goal to minimize fuel consumption under size and velocity constraints. We perform two-dimensional viscous simulations and apply the concept of quasi-propulsive efficiency to illustrate and discuss the efficiency of two-dimensional undulating foils employing first carangiform and then anguilliform kinematics. We show that low efficiency may be due to adverse body-propulsor hydrodynamic interactions, which cannot be accounted for by an increase in friction drag, as done previously, since at the Reynolds number Re = 5 000 considered in the simulations, pressure is a major contributor to both thrust and drag.
authors.forename: A P
authors.initials: AP
authors.lastname: Maertens
authors.forename: M S
authors.initials: MS
authors.lastname: Triantafyllou
authors.forename: D K P
authors.initials: DK
authors.lastname: Yue
mesh_terms: D000818:Animals; D003198:Computer Simulation; D004734:Energy Metabolism / Q000502:physiology*; D004735:Energy Transfer / Q000502:physiology*; D005399:Fishes / Q000502:physiology*; D017276:Friction; D008954:Models, Biological; D012212:Rheology / Q000379:methods*; D033081:Shear Strength / Q000502:physiology; D013314:Stress, Mechanical; D014783:Viscosity