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Swimming in density-stratified fluid: study on a flapping foil.

Prabal Kandel1, Jian Deng1

  • 1Department of Mechanics, Zhejiang University, Hangzhou 310027, People's Republic of China.

Bioinspiration & Biomimetics
|July 8, 2022
PubMed
Summary

Fluid stratification significantly impacts swimmer performance. While high stratification levels have minimal effect, moderate levels (Froude number = 2) can boost propulsive efficiency by up to 18.3%.

Keywords:
flapping foilfluid dynamicsstratified fluidswimming

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Area of Science:

  • Fluid dynamics
  • Bio-inspired locomotion
  • Hydrodynamics

Background:

  • Understanding aquatic animal locomotion in varying water densities is crucial.
  • Density stratification in fluids can alter flow dynamics and affect swimming efficiency.

Purpose of the Study:

  • To investigate the propulsive performance of a simplified swimmer (pitching NACA0015 airfoil) in both homogeneous and density-stratified fluids.
  • To analyze how fluid stratification influences wake structures, drag/thrust, and overall propulsive efficiency.

Main Methods:

  • Two-dimensional numerical simulations were employed to model the pitching airfoil.
  • Direct comparisons were made between performance in homogeneous and stratified fluid conditions.

Main Results:

  • Fluid stratification alters wake structures and modifies drag/thrust forces.
  • At low Froude numbers (Fr=1), propulsive efficiency significantly decreases due to internal waves.
  • At a moderate Froude number (Fr=2), propulsive efficiency increases by 18.3% compared to homogeneous fluids, peaking at A_D=2.50.

Conclusions:

  • The effect of stratification on swimmer performance is dependent on the Froude number.
  • Moderate stratification can enhance propulsive efficiency, offering potential advantages for aquatic swimmers.