Representing submerged flexible vegetation with buoyancy-dominated, non-uniform surrogates

Submerged flexible vegetation significantly influences aquatic flow dynamics, as well as mass and energy transport. While simplified plant surrogates have facilitated theoretical, numerical, and experimental studies, many existing models rely on uniform blades, beams, cylinders, or other simplified flexible structures, and therefore, only partly represent the distributed leafy clusters and thin tensile stems of many submerged plants. In this study, we introduce a buoyancy-dominated, non-uniform plant surrogate composed of pellets connected by a thin flexible filament to represent a class of aquatic plants with compliant stems and distributed leaf clusters, such as Cabomba caroliniana and Ceratophyllum demersum. We develop a mechanistic model that links hydrodynamic forces to plant posture, where posture denotes the time-averaged inclination and bending shape of the surrogate in the streamwise–vertical plane. The analysis yields non-dimensional parameters that predict surrogate behavior and clarify how drag coefficient CD, density ratio ρ*, pellet Froude number Fr*, and submergence ratio H* affect the force balance. Virtual stiffness is introduced as an equivalent, flow-condition-dependent measure of bending produced by spatially varying pellet properties. Laboratory experiments with an isolated physical surrogate test the theoretical model for the surrogate itself; they do not by themselves establish full biological equivalence to natural vegetation. The comparisons show that most posture predictions fall within a ±15% visual error band under the tested moderate-flow conditions, while larger deviations occur when pellet interaction, filament drag and unsteady motion become more important. The results provide an efficient framework for three-dimensional physical and mathematical modeling of fluid–vegetation coupling.