Investigating the nature and dominance of feedback mechanisms within vegetated channel flows: a high-resolution numerical modelling approach

The flow and plant dynamics of vegetated channel flows are governed by a variety of processes and feedback mechanisms that interact, across a range of scales, to form a complex inter-connected system. It is well documented that vegetation exerts a significant drag force on the flow, creating a drag discontinuity between the canopy layer and the flow above. This has been shown to control the mean flow and turbulent structure through the development of a canopy shear layer, which leads to the generation of coherent roller vortices at the canopy top. In turn, the canopy reacts to the flow forcing through reconfiguration to minimize drag, and responds to the passage of vortices through exhibiting coherent monami. It has been hypothesized that the vegetation consequently acts to modulate the turbulence structure through the vibrational response of the natural frequency of the vegetation. Hence the interaction of processes is complex and nonlinear. Here we report on a series of high resolution numerical experiments designed to investigate the exact nature and role of these feedback mechanisms within the flow-vegetation system. Two biomechanical models are developed and applied within a computational fluid dynamics framework to investigate the nature of the time-dependent flow dynamics. The first model, for semi-flexible vegetation uses the Euler Beam equation to drive plant motion, whilst the second model uses an n-pendula approach to represent cases of highly flexible vegetation. Both models were validated through a series of laboratory experiments using particle image velocimetry that employed both real and prototype vegetation. The high-resolution numerical models enable detailed analysis of both the plant motion and corresponding flow field. The results clearly show the presence of a strong drag discontinuity, coherent canopy motion and large scale turbulent structures formed at the canopy top. Time series and spectral analysis reveals a clear, time-dependent, process linkage between the flow and plant motion, illustrating that both flow and plant characteristics interact to drive the overall system.