The physical mechanism behind the wake merging phenomena of pitching foils in side-by-side arrangement.

The unsteady hydrodynamics of two in-phase pitching foils in side-by-side arrangement is numerically investigated at Reynolds number of Re = 4000 and Strouhal numbers of St = 0.15 – 0.5. Separation distance between the foils varies between 0.5c (c being the chord length) and 2c. This study classifies the wake of the foils based on their deflected/horizontal and merged/separated characteristics in Strouhal number vs separation distance phase maps. The foils at higher Strouhal number initially produce two distinct vortex streets, which are deflected and parallel to each other. These streets merge with each other and form coherent vortical structures around the centerline after a particular number of pitching cycles that vary with Strouhal number and separation distance. The underlying physical mechanism for the merging phenomena is investigated, which reveals the presence of secondary structures shed from the main deflected vortex streets in the opposite direction. Interactions between secondary structures of the lower wake and the main vortex street of the upper wake triggers a rapprochement process. This eventually yields to the merging of two wakes.