Drag reduction by reconfiguration of a poroelastic system

Because of their flexibility, trees and other plants deform with great amplitude (reconfigure) when subjected to fluid flow. Hence the drag they encounter does not grow with the square of the flow velocity as it would on a classical bluff body, but rather in a less pronounced way. The reconfiguration of actual plants has been studied abundantly in wind tunnels and hydraulic canals, and recently a theoretical understanding of reconfiguration has been brought by combining modelling and experimentation on simple systems such as filaments and flat plates. These simple systems have a significant difference with actual plants in the fact that they are not porous: fluid only flows around them, not through them. We present experimentation and modelling of the reconfiguration of a poroelastic system. Proper scaling of the drag and the fluid loading allows comparing the reconfiguration regimes of porous systems to those of geometrically simple systems. Through theoretical modelling, it is found that porosity affects the scaling of the drag with flow velocity. For high porosity systems, the scaling is the same as for isolated filaments while at low porosity, the scaling is constant for a large range of porosity values. The scalings for the extreme values of porosity are also obtained through dimensional analysis.