Abstract
We designed a robotic fish caudal fin with six individually moveable fin rays based on the tail of the bluegill sunfish, Lepomis macrochirus. Previous fish robotic tail designs have loosely resembled the caudal fin of fishes, but have not incorporated key biomechanical components such as fin rays that can be controlled to generate complex tail conformations and motion programs similar to those seen in the locomotor repertoire of live fishes. We used this robotic caudal fin to test for the effects of fin ray stiffness, frequency and motion program on the generation of thrust and lift forces. Five different sets of fin rays were constructed to be from 150 to 2000 times the stiffness of biological fin rays, appropriately scaled for the robotic caudal fin, which had linear dimensions approximately four times larger than those of adult bluegill sunfish. Five caudal fin motion programs were identified as kinematic features of swimming behaviors in live bluegill sunfish, and were used to program the kinematic repertoire: flat movement of the entire fin, cupping of the fin, W-shaped fin motion, fin undulation and rolling movements. The robotic fin was flapped at frequencies ranging from 0.5 to 2.4 Hz. All fin motions produced force in the thrust direction, and the cupping motion produced the most thrust in almost all cases. Only the undulatory motion produced lift force of similar magnitude to the thrust force. More compliant fin rays produced lower peak magnitude forces than the stiffer fin rays at the same frequency. Thrust and lift forces increased with increasing flapping frequency; thrust was maximized by the 500× stiffness fin rays and lift was maximized by the 1000× stiffness fin rays.
Highlights
The caudal fin of fishes has often been viewed by engineers as a relatively simple propulsive surface that is flapped by fish in a twodimensional motion that is an extension of the undulatory wave generated by myotomal body musculature (Barrett et al, 1999; Alvarado and Youcef-Toumi, 2006; Anton et al, 2009)
These shape changes and kinematic patterns are produced by fin rays within the caudal fin that are moved by intrinsic caudal musculature that is distinct from the segmented body muscles (Lauder, 2006; Flammang and Lauder, 2008; Flammang and Lauder, 2009)
As the fin reversed direction, the gradual decrease in thrust turned into a net drag force. These results were consistent with previous results from studies that used computational fluid dynamics (CFD) to predict the forces generated by flexible, trapezoidal foils (Akhtar et al, 2007; Zhu and Shoele, 2008)
Summary
The caudal fin of fishes has often been viewed by engineers as a relatively simple propulsive surface that is flapped by fish in a twodimensional motion that is an extension of the undulatory wave generated by myotomal body musculature (Barrett et al, 1999; Alvarado and Youcef-Toumi, 2006; Anton et al, 2009). In many fishes the caudal fin is used not exclusively for thrust production, and to create forces and moments that control the orientation of the fish via complex conformational changes and motion programs (Lauder, 1982; Drucker and Lauder, 1999; Lauder, 2000; Lauder and Drucker, 2004; Flammang and Lauder, 2008; Tytell et al, 2008; Flammang and Lauder, 2009) These shape changes and kinematic patterns are produced by fin rays within the caudal fin that are moved by intrinsic caudal musculature that is distinct from the segmented body muscles (Lauder, 2006; Flammang and Lauder, 2008; Flammang and Lauder, 2009). Fish are capable of altering the relative magnitudes of these forces and to vector water momentum in appropriate directions to execute maneuvers; this ability is a function of the design of the caudal fin with its individually controllable fin rays
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