Spectral response of the flow in V-formations of cylinders

Group motion is ubiquitous in nature and migratory birds use formation flight to reduce their energy expenditure compared to solo flying. Similar formation flight patterns have shown to reduce the drag force in fixed-winged aircraft as well. Flows past arrays of bodies are also common in environmental settings where vegetation patches can help with preventing soil erosion in flood plains. Tight bio-inspired V-formations of cylinders, with frontal overlaps between members, not only show the ability to reduce the drag force but also reduce the magnitude of the Reynolds stress levels in the wakes of the bodies. In this talk, we focus on the impact of the geometry of V-formations on the turbulence statistics of the wakes of each of the members and the vortex shedding patterns. We perform water tunnel experiments with 2D-2C high-resolution and time-resolved particle image velocimetry (PIV) and to access the velocity field around and in the interior of the array with a single laser, we use a multi-light-sheet strategy employing multiple beam splitters. We consider a range of V-formations with and without frontal overlaps and discuss the variations in the frequency response of the flow in the wake of every member of the formation and how the spectral energy density of turbulent wakes is altered as a function of the geometry of the formation and compared to a single-body scenario. Lastly, we explore how this change impacts the forces experienced by each member of the formation.