How to measure the swimming efficiency of a robotic fish? 袁涛 SA

Definition1 when moving at constant forward velocity U, The system propulsive efficiency is defined as where P E = U T A is the useful power, T A denotes the time-averaged thrust T, and P p is the time-averaged power expended by the motors to drive the propulsor Different measurement methods may have different definitions of propulsive efficiency

Definition 2 Conventionally, the thrust efficiency based on the thrust force for a constant swimming speed is defined by the following U denotes the self-propelled speed of the fish T f denotes The time-averaged thrust force : where f denotes the flapping frequency of the robotic fish and I denotes the impulse of the vortex ring. P fluid denotes the time-averaged power: where PT denotes the measured total average power and Pm denotes the mechanical transmission power

Apparatus 1 The robotic mechanism has length L = 1:25m and consists of a streamlined mainbody capable of flexing and a propulsive tail n. The outer shape of the vehicle in its position at rest is an exact replica of the shape of a bluen tuna (thunnus hynnus),including the caudal and two smaller attached ns. The Ocean Engineering Testing Tank Facility at MIT in a water tank with dimensions 35m long by 2:5m wide by 1:25m deep. The robot is connected to an overhead carriage through a streamlined strut so it can be towed at constant speed. It was towed at mid-depth to avoid interference eects with the free surface and the bottom.

Apparatus 1 Strain-gauge-based miniature load cells (Entran elf-tc500) are mounted on each of the twelve actuating strings (two strings per motor), while displacement sensors (ETI servomount precision potentiometers) provide motion measurements The drag or thrust on the overall mechanism is measured by two force transducers, a load cell (Entran elf-tc1000) and a Kistler quartz force transducer, for redundancy.

Apparatus 2 Active towing method: P denotes the robotic fish center of mass; U denotes the flow speed; The passive towing method, where the robotic fish is free to move upstream and downstream on a low-friction air-earing system The self-propelled method. The x-axis is along the fish axial length, the y-axis is in the lateral direction and the z-axis is selected to be orthogonal to the horizontal plane

Apparatus 2 the box on the carriage represents the shelf for the robotic fish’s power supply, motion control, amplifier and data acquisition system DPIV: digital particle image velocimetry

Advantages 1.It proved to be reliable and repeatable; The results were repeatable within 4% and consistent 2. based on a force-feedback control technique which combines the advantages of passive towing method and active towing method

Limitations 1.It did not correct for: (a) interference drag which was expected to be more important for the mast{body and body{tail interactions; and (b) the presence of the two smaller ns. 2. It cannot conclusively determine whether the differences we found in thrust performance are due to body undulation or caudal fin movement.

Thank you!