Mechanism of formation of wake patterns behind two staggered in-phase pitching foils

The mechanism of wake formation behind two staggered in-phase pitching foils is numerically investigated over a range of Strouhal numbers (0.15<St<0.4), Reynolds numbers (1000<Re<10000), and horizontal (1c<h<3c) and vertical (0.5c<v<2c) foil separation distances, wherecis the foil chord. First, two flow patterns are identified, constituting merged and separated wakes. These are characterized in wake maps to explain the role of geometric and kinematic parameters in the wake formation. Relevant geometric thresholds for the formation of different wake topologies are identified, and their correspondence to kinematics is explained. This provides a wake evolution mechanism that explains the formation of these patterns. This mechanism is triggered by the interaction of upper and lower wakes, which results in the rearrangement of vortex pairs downstream. The interaction and resultant flow patterns change with the offset position of the upper wake, which is given by the horizontal and vertical placement of the upper foil. A wide range of separation distances in staggered foils enables, for the first time, the study of interactions between an external vortex and a vortex street. This allows the mechanisms of wake pattern formation triggered by this interaction to be explored. A novel wake model is proposed to explain this interaction, which consistently holds for different Reynolds numbers.