How does altering synchronization of pitching parallel foils change their wake dynamics?

TL;DR

This paper investigates how abrupt changes in the synchronization phase of pitching foils affect wake dynamics, revealing formation of secondary vortex streets and pressure equalization, with implications for swimming efficiency.

Contribution

It introduces a numerical simulation study of phase shifts in pitching foils, highlighting wake structure changes and pressure effects not previously explored.

Findings

Secondary vortex street formation due to phase change

Pressure levels on foils become similar after phase switching

Growth rate of secondary vortex street remains consistent

Abstract

This study was inspired from the swimming habits of red nose tetra fish that prefer side-by-side configurations and exhibit changes to their synchronization mid-swimming. Using numerical simulations, alterations to the unsteady wake dynamics imposed by abrupt changes in the phase angle between two pitching side-by-side foils were examined at Reynolds number of 4;000 and Strouhal number of 0:50. Four hybrid modes were considered in thisstudy with two modes representing an abrupt phase change by pi during the 20th cycle. The other two modes represented a simplified case of burst-and-coast swimming, in which there was a brief (2 oscillation periods) suspension of oscillations before imposing a change in phase angle. In all cases, the foils initially performed out-of-phase pitching, and then they started their in-phase motion by either performing the upstroke or down-stroke first. This kinematic change resulted in the formation and growth of a secondary vortex street in between two primary streets, which enabled and maintained a split wake configuration. Furthermore, the phase switching altered the pressure levels on the top and bottom surfaces of both foils to almost similar levels, which attributed to a reduction in the side-force. The growth rate of the secondary vortex street remained consistent for all four hybrid modes.