Effect of pitching phase difference on three-dimensional wake features behind parallel foils*

The flow around parallel oscillating foils (side-by-side configuration) is numerically simulated at Reynolds number of 4000 using overset grid assembly technique in OpenFOAM. The foils are separated by 1c-2c, where c is the chord length, for a range of Strouhal numbers (St=0.25-0.5) and phase offsets (φ=0-π). It is revealed that both Strouhal number and phase difference have great impact on the three-dimensionality of the wake. In-phase pitching foils produce a two-dimensional wake at St=0.25 when their separation distance is 1c, whereas three-dimensionality is observed in the wake of out-of-phase pitching foils for the same configuration. Vortex streets of the upper and lower foils merge in the mid-wake for the in-phase case, while the upper and lower wakes of out-of-phase pitching cases exhibit no merging phenomenon. For out-of-phase pitching, vortex dipoles interact immediately after shedding from the foils, which results in the wake divergence from one another. This suggests that wake interactions impact formation of three-dimensional flow features in close vicinity of the foils. We will now expand our parameter space to form general arguments about the wake three-dimensionality and dynamics.