Evolution of the wake and performance of oscillating foils in multi-degree of freedom motion

• Author / Creator

  Verma, Suyash

• This dissertation numerically evaluates the wake dynamics of oscillating foils with combined heaving and pitching motion. Spatio-temporal evolution of vortical structures and associated instabilities contributed to transitions in the wake topology. This also coincided with changes in the foil's propulsive performance. A fundamental association of these transitions and foil kinematics is investigated at a range of Reynolds numbers and motion parameters, to ultimately improve understanding of efficient biological swimming practices. The spatial arrangement of two-dimensional primary rollers changed with transitioning of the foil kinematics from heave- to pitch-dominated motion, or vice-versa. A novel mathematical relationship is modeled to accurately predict these changes at low Reynolds number, which involved relative circulation of coherent paired rollers, and governing foil kinematic settings. Apparent similarities in roller arrangement also coincided with either mean momentum deficit or excess in the wake. Thus, it is demonstrated that transitions of the wake topology may not always possess a direct one-to-one association with propulsive performance. The three-dimensional wake topology at $Re =$ 8000 highlighted spanwise undulations of primary rollers characterized by elliptic instability, and a consequent growth of secondary coherent structures. The latter resembled hairpin and horseshoe formations on a large spatial scale, and spanwise corrugations (valleys and bulges) on a finer scale. The changes in kinematics further reflected a fundamental association with the spanwise instability, and alteration in mechanisms that governed the growth of secondary structures. For the heave dominated kinematics, a paired primary and secondary leading edge vortex triggered elliptic instability, which inherently contributed to the growth of secondary hairpins, and their eventual transformation to ribs. As heave domination weakened, secondary hairpin growth occurred through a deforming trailing edge vortex rather than a secondary leading edge vortex. The onset of pitch domination eventually led to an absence of hairpin structures owing to the decreased circulation of primary rollers. The evolution of secondary structures thus presents a fundamental route to three-dimensional transition in wake topology while showcasing a crucial association with multi-degree of freedom foil kinematics.

• Subjects / Keywords

  • Unsteady Wakes
  • Vortex Shedding
  • Oscillating foil
  • Swimming

• Graduation date

  Spring 2023

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-8ech-k135

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.