Simulation-based study of turbulent aquatic canopy flows with flexible stems

Abstract

Large-eddy simulation (LES) is performed to investigate the dynamics of flow and canopy motions and the energy transfer in turbulent canopy flows. Different from the traditional approach that models the canopy as a continuous medium with a drag coefficient prescribed a priori, an immersed boundary method together with a beam model is employed to explicitly capture the dynamics of individual stems and resolve monami. The simulation cases cover a broad range of stem flexibilities from rigid stems to oscillatory stems to stems yielding to the flow. For highly flexible canopies, the stem fluctuation is small such that the canopy behaves like a rigid canopy, which is used to explain the similarities of the flow features between rigid and highly flexible canopies. Analyses of the turbulent kinetic energy (TKE) budget show that, in the flexible canopy cases, the waving term associated with the canopy drag–flow velocity correlation can be as large as one-half of the shear production term near the canopy top. Spectral TKE budget analyses further reveal dominant effects at two characteristic scales: the monami scale associated with the coherent structures in the mixing layer and the wake scale associated with the interval between adjacent stems. For the TKE in flexible canopies, the waving term is found to play an important role in the interscale and wall-normal transport terms. Our LES data show that the spectral shortcut mechanism proposed by previous studies is caused by the waving term.

Steady Flow Over a Finite Patch of Submerged Flexible Vegetation

“…With hydrodynamic interactions, the collective behaviors of vegetation blades in a patch can be significantly different from that of a single vegetation blade. He et al (2022) also observed that there were no blade-to-blade interactions. They attributed it to the force spreading at the fluid-solid interface in the immersed boundary method, which is mediated by the fluid to prevent direct contact among vegetation blades.…”

“…In this study, we developed a novel multiphysics model framework that couples a soft‐body dynamics ANCF finite element model with a fluid solver. Compared to previous studies of flow over submerged flexible vegetation (He et al., 2022; Wang et al., 2022), one of the advantages of the newly developed model framework is its ability to capture both flow‐vegetation and blade‐to‐blade interactions, which allows the model to be applied to a wider range of vegetation density and Cauchy number compared to previous studies.…”

“…He et al. (2022) conducted large‐eddy simulations to study turbulent aquatic canopy flows with flexible stems and carried out analyses of turbulent energy budget. The authors showed that the wake production can be as large as one‐half of the shear production near the canopy top.…”

“…The authors showed that the wake production can be as large as one-half of the shear production near the canopy top. Both LES studies (He et al, 2022;Tschisgale et al, 2021) focused on an infinite vegetation canopy in a turbulent channel, which neglected the spatial heterogeneity at the canopy edge as in a finite vegetation canopy. Wang et al (2022) and Ni et al (2023) conducted high-fidelity simulations of flow over an array of flexible vegetation with finite patch sizes.…”

“…The high-fidelity large-eddy simulation model revealed the well-known coherent turbulent structures, including velocity streaks, Kelvin-Helmholtz vortices in the mixing layers as well as hairpin vortices in the outer flow region, and hence the turbulent generation mechanisms by vegetation. He et al (2022) conducted large-eddy simulations to study turbulent aquatic canopy flows with flexible stems and carried out analyses of turbulent energy budget. The authors showed that the wake production can be as large as one-half of the shear production near the canopy top.…”