Effect of Phase Difference on Wake Characteristics and Propulsive Performance of Pitching Foils in Side-by-Side Configurations

Abstract Numerical investigations of unsteady flows around two pitching foils arranged in side-by-side (parallel) configurations, as a simplified models of fish schools are conducted in OpenFOAM for a range of Strouhal number from 0.15 to 0.5 and at a fixed Reynolds number of 4000. Effects of the phase difference between motions of the two foils is examined in the range of ϕ = 0 – π of increments of π/6. Phase differences of ϕ = π/6, π/2, and 5π/6 with Strouhal numbers of St = 0.25 and 0.4 are selected for analyzing transitions and evolution in the wake. The phase difference has a major impact on characteristics of the wake. Smaller phase angles, such as ϕ = π/6, lead to the development of a merged wake, resembling the wake of two foils undergoing in-phase pitching motion. On the other hand, a split wake is observed when the phase difference is set to higher values, e.g., ϕ = 5π/6, which resemble the wake of those performing out-of-phase pitching motion. Also, ϕ = π/2 yields maximum disparity between the propulsive performance of the foils, which drops to zero as the phase angle approaches to in-phase or out-of-phase.