Flowfield measurements in the wake of a robotic lamprey

Abstract

Experiments are reported on the hydrodynamics of a swimming robotic lamprey under conditions of steady swimming and where the thrust exceeds the drag. The motion of the robot was based on the swimming of live lampreys, which is described by an equation similar to that developed for the American eel by Tytell and Lauder (J Exp Biol 207:1825-1841, 2004). For steady swimming, the wake structure closely resembles that of the American eel, where two pairs of same sign vortices are shed each tail beat cycle, giving the wake a 2P structure. Force estimates suggest that the major part of the thrust is produced at or close to the end of the tail.

Fig. 1

Flow visualization of the wake produced by a flapping rectangular plate at a Strouhal number of 0.23. Flow is from left to right. The red and green dyes originate on opposite sides of the plate. From Buchholz and Smits (2007)

Fig. 2

The silver lamprey, Ichthyomyzon unicuspis

Fig. 3

The image shows the 13 servomotors comprising the robotic lamprey. Each servomotor is connected to its neighbours by rigid links

Fig. 4

Sketch of the water channel showing the laser sheet and camera arrangements. a Top view and b side view

Fig. 5

Comparison between lamprey (red line) and robot waveforms (blue line). The robotic lamprey motion is the approximation due to 13 connected segments

Fig. 6

Example of instantaneous velocity and vorticity fields in the wake of the steady swimming robot. The vectors represent the velocity field with the convective velocity subtracted, and the background color represents the out-of-plane vorticity. The flow is from top to bottom, and the tail of the robot is just out of the picture at the top

Fig. 7

Phase-averaged velocity and vorticity fields in the wake of the steady swimming robot. The vectors represent the velocity field with the convective velocity subtracted, and the background color represents the out-of-plane vorticity. The flow is from top to bottom, and the tail of the robot is indicated by the small black wedge near the top of each image

Fig. 8

Phase averaged velocity fields in the wake of a an American eel, taken from Tytell and Lauder (2004). b The robotic lamprey. Both represent the field at the same phase of motion

Fig. 9

An example of the phase-averaged velocity and vorticity fields in the wake of the robot when the thrust exceeds the drag

Fig. 10

Angle of force vector plotted against the Strouhal number. Positive values indicate that the resultant force acts to accelerate the robot

Fig. 11

Phase-averaged velocity and vorticity fields along the body and in the wake for a steadily swimming robot. The vectors represent the velocity field with the convective velocity subtracted, and the background color represents the out-of-plane vorticity. The flow is from top to bottom, and the body of the robot is indicated by the black shape

Fig. 12

The force produced from the wake to different x/L locations along the body (x/L = 0 is the tip of the tail). The blue curve is derived using one control volume only, and the red curve is derived from combining two control volumes to extend the domain