Performance of ray fins in fish locomotion
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Performance of Ray Fins in Fish Locomotion. Qiang Zhu and Kourosh Shoele Department of Structural Engineering University of California San Diego. dorsal fin. caudal fin. anal fin. pectoral fin. pelvic fin. tendons. rays. Cartilage pad. base. ligament. membrane. branches.

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Performance of ray fins in fish locomotion

Performance of Ray Fins in Fish Locomotion

Qiang Zhu and Kourosh Shoele

Department of Structural Engineering

University of California San Diego

Characteristics of ray fins

dorsal fin

caudal fin

anal fin

pectoral fin

pelvic fin



Cartilage pad





Characteristics of Ray Fins


  • Structural lightness

  • Deployability

  • Multi-dimensional controllability

  • 3D anisotropic flexibilty

Lauder Lab Videos

Ray fins of bluegill sunfish (Alben et al. 2007)

Tangorra et al. 2007

Fluid structure interaction model
Fluid-Structure Interaction Model

  • Potential flow model with boundary element method

  • Wakes are modeled as shear layers and mathematically represented by distributions of dipoles

  • Valid in high Reynolds numbers

  • Nonlinear Euler-Bernoulli beam model for embedded rays

  • Fully-coupled fluid-structure interactions

Zhu et al. JFM 2002, Zhu & Kourosh JEB 2008

Caudal fin dynamics
Caudal Fin Dynamics

  • Advantages of a fin with flexible rays:

  • Reduced lateral force

  • Increased propulsion efficiency

  • Reduced dependence on kinematic parameters

  • Achievement of different locomotion modes (homocercal, heterocercal)

Zhu & Kourosh JEB 2008

Pectoral fins in labriform swimming


Shape and structure

Flapping+rowing+baseline rotation

Thorsen et al. 2005

Pectoral Fins in Labriform Swimming

Rigid Rays:All the rays are rigid (red lines)

Fin A:All the rays have the same stiffness (green lines)

Fin B:The ray at the leading edge is strengthened (blue lines)

Kourosh & Zhu JEB 2009, 2010


  • The ray-fin structure enables detailed control of fin motion (since each ray can be controlled individually), and is essential to the performance of the fin in locomotion.

  • Flexibility of the rays may enhance fin performance. It, however, depends upon detailed distribution of the stiffness (e.g. reinforcement at the leading edge).

Broader view other skeleton reinforced membranes in nature
Broader ViewOther Skeleton-Reinforced Membranes in Nature

Insect Wings

Mollusk Nacre

Cell Membrane

Venation structure in the wing of a Manduca sexta (from Combes & Daniel 2003).

Cell membrane contains a lipid bilayer supported by a protein scaffold (the cytoskeleton) (T. Wittmann, Science Photo Library).

Mollusk (e.g. abalone) nacre contains a chitin network that controls the growth and mechanical properties of the shell (Asaro & Harley 2004).

Zhu et al. Biophy. J 2007; Zhu & Asaro Biophy. J. 2008; Peng et al. Phys. Review E. 2010


This study is supported by NSF.