How does ground effect impact the formation of three-dimensional instabilities in the wake of parallel oscillating foils?*

Wake dynamics and flow instabilities around two pitching foils placed in side-by-side (parallel) configurations are numerically examined for Strouhal numbers of St = 0.3 and 0.5<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.5 at Reynolds number of 8000. Two phase offsets, φ = 0<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>ϕ=0 and π<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>π, representing in-phase and out-of-phase oscillations, are examined for a range of separation distances between 0.5c<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.5c and 1.5c<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>1.5c with increments of 0.25c<!--[if gte msEquation 12]> style='mso-bidi-font-style:normal'>0.25c, where c denotes the chord length. Ground effect, i.e., proximity to the adjacent foil, is revealed to be key in the formation of fine scale instabilities for all kinematic settings. While trailing edge vortices are shed from the closely spaced foils, braid region detaches from the rollers and subsequently convects downstream. They interact with the vortex dipoles in the mid-wake, leading to the formation of three-dimensional structures that enwrap the dipoles. These observations remain consistent for both Strouhal numbers, despite the wake at the higher Strouhal number demonstrating more pronounced instabilities. The detachment process differs significantly between in-phase and out-of-phase motions, possibly because of the contrasting growth of leading-edge vortices associated with these particular phase offsets. This study provides valuable insights into the underlying flow instability mechanisms of fish schools. We aim to expand on this study by developing a more comprehensive model of the wake across a broader parameter space.