Flexible structures reconfigure and manipulate vortex rings to reduce drag*

Extreme weather conditions have become increasingly frequent in recent years, damaging rigid infrastructure like buildings and bridges. Plants resist uprooting in such weather conditions by streamlining themselves and reducing their projected area in the flow. Integrating flexible components into infrastructural design could allow them to withstand adverse weather better. We study the drag reduction and deformation of flexible radially cut disks that are translated vertically through water. We use temporally and spatially resolved velocity field measurements to quantify the properties of the vortex ring that forms on the leeward side of the flexible disks. The net entrained fluid volume vortex and the core vortical volume are reduced for deforming disks due to streamlining. The vortex consequently achieves a more uniform vorticity distribution, given by a lower non-dimensional energy. The more uniform vorticity distribution is related to a smaller pressure drop in the core, resulting in drag reduction. Using the vortex panel method we also model the drag reduction and deformation of the flexible disks. The model is validated experimentally and predicts the reduced drag of flexible structures based on the deformed shape and the incoming flow conditions.