Search

We establish through numerical simulation conditions for optimal undulatory propulsion for a single fish, and for a pair of hydrodynamically interacting fish, accounting for linear and angular recoil. We first employ systematic two-dimensional (2-D) simulations to identify conditions for minimal propulsive power of a self-propelled fish, and continue with targeted 3-D simulations for a danio-like fish; all at Reynolds number 5000. We find that the Strouhal number, phase angle between heave and pitch at the trailing edge, and angle of attack are principal parameters. For 2-D simulations, imposing a deformation based on measured displacement for carangiform swimming provides, at best, efficiency of 35 %, which increases to 50 % for an optimized motion; for a 3-D fish, the efficiency increases from 22 % to 34 %. Indeed, angular recoil has significant impact on efficiency, and optimized body bending requires maximum bending amplitude upstream of the trailing edge. Next, we turn to 2-D simulation of two hydrodynamically interacting fish. We find that the upstream fish benefits energetically only for small distances. In contrast, the downstream fish can benefit at any position that allows interaction with the upstream wake, provided its body motion is timed appropriately with respect to the oncoming vortices. For an in-line configuration, one body length apart, the efficiency of the downstream fish can increase from 50 % to 60 %; for an offset arrangement it can reach 80 %. This proves that in groups of fish, energy savings can be achieved for downstream fish through interaction with oncoming vortices, even when the downstream fish lies directly inside the jet-like flow of an upstream fish.

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.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011., Cataloged from PDF version of thesis., Includes bibliographical references (p. 109-112)., While the vast majority of underwater vehicles rely exclusively on sonar and vision to detect obstacles and maneuver, live fish also use their lateral line organ. The role played by the canal lateral line system is particularly important for hypogean fish, such as blind Mexican cave fish, who use it to avoid obstacles and navigate dexterously in complex environments. Similarly, pressure sensors could be used on underwater vehicles to expand their range of operability by filling the gap left by sonar and vision systems in turbid cluttered environments. To understand how much information can be extracted from the artificial lateral line of an underwater vehicle exploring an unknown environment, the case of a foil passing a static object in still water is analyzed. A two-dimensional potential flow approach based on a source panels method is used to characterize the spatio-temporal pressure signature of the object as sensed by the vehicle. Simulations are used to estimate the sensing range of an artificial lateral line and the appropriate density of pressure sensors. To emulate the object-detection and shape-recognition capabilities of the lateral line, an adapted unscented Kalman filter is combined with the hydrodynamic model. The method developed is experimentally tested in a water tank, using a hydrofoil instrumented with pressure sensors passing a static cylinder. The results show that location and shape informations of an elliptical cylinder can be successfully inferred from experimental pressure measurements. Performance of the proposed method for object identification using pressure sensors are discussed and ways to improve it are suggested., by Audrey Maertens., S.M.