Experimental optimization of bird-inspired V-formations

Group motion and flow past arrays are commonly observed in nature and engineering applications. For example, large birds like geese and pelicans use formation flight patterns to reduce energy expenditure, and the arrangement of vegetation patches in flood plains has significant effects in preventing soil erosion. Therefore, understanding the mechanics of flow around arrangements of objects can be used as an optimization tool to achieve desired dynamic responses, like minimizing energy expenditure in a swarm of drones. In this talk, we will focus on the dynamics of flow past V-formations of circular cylinders inspired by flight patterns observed in migratory birds and aim to find the optimal angle of V for the lowest possible drag experienced by all the members. Our experiments are conducted in a water tunnel using 2D-2C high resolution and high-speed particle image velocimetry (PIV). To access the velocity field around and in the interior of the array with a single laser, we use a quadruple-light sheet strategy employing multiple beam splitters. We present results from flow past different V-shaped arrangements of cylinders, looking at the complex interactions between the array members, mean and fluctuating quantities, as well as the dynamics of the wake and the vortex shedding. We observe that close packing of cylinders (with their projected views partially covered by the upstream/downstream members) reduces the turbulent kinetic energy in the array interior. We use the PIV data to obtain pressure and calculate the drag force on each array member. When the cylinders are closely packed, all the array members experience reduced drag in comparison to the drag on an isolated cylinder. Lastly, we attempt to create a phase map relating the angle of the V-formation and the drag response of all the members and identify the optimal formation designs for lowest total energy expenditure.