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Undulating fins produce off-axis thrust and flow structures.

Izaak D Neveln1, Rahul Bale, Amneet Pal Singh Bhalla

  • 1Department of Biomedical Engineering, Northwestern University, Evanston, IL, USA.

The Journal of Experimental Biology
|September 28, 2013
PubMed
Summary

Researchers analyzed the wake of undulating fins, revealing linked vortex tubes shed from the fin

Keywords:
Bio-mimeticBio-roboticsDigital particle image velocimetry (DPIV)Fish locomotionKnifefishUndulating fin propulsion

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Area of Science:

  • Fluid dynamics
  • Biomechanics
  • Robotics

Background:

  • The wake structures of swimming and flying organisms are well-studied, but the wake generated by freely swimming undulating fins remains unanalyzed.
  • Elongated undulating fins, like those in knifefish, enable remarkable agility and offer potential for advanced underwater vehicle design.
  • Understanding these wakes is crucial for both biological insights and engineering applications.

Purpose of the Study:

  • To characterize the flow structure and hydrodynamics of a freely swimming undulating fin.
  • To investigate the vortex dynamics responsible for thrust generation in undulating fins.
  • To compare the wake of a robotic model with that of a freely swimming knifefish.

Main Methods:

  • Utilized particle image velocimetry (PIV) to measure fluid velocity fields around a robotic undulating fin model.
  • Employed high-fidelity computational fluid dynamics (CFD) simulations for both a virtual knifefish and the robotic model.
  • Measured fin kinematics and body morphology from a freely swimming knifefish for accurate CFD simulation.

Main Results:

  • A series of linked vortex tubes are shed from the long edge of the fin as an undulatory wave propagates along its length.
  • Successive vortex tubes generate an oblique jet, providing forward propulsion.
  • The vortex structure shares similarities with linked vortex rings from oscillating caudal fins but is distorted by undulatory kinematics.

Conclusions:

  • The study elucidates the complex wake structure of undulating fins, characterized by shed vortex tubes and oblique jets.
  • This research provides a foundation for understanding the biomechanics of undulatory propulsion and informs the design of bio-inspired underwater vehicles.
  • The findings highlight the unique hydrodynamics associated with elongated, undulating fins compared to more traditional caudal fin propulsion.