Momentum fluxes across multiple mixing interfaces subject to partially-distributed submerged canopy flows

Abstract

Partially-distributed submerged canopy flows (PSCFs), characterized by the presence of multiple mixing interfaces, are a common feature in riverine and wetland systems. These flows are subject to shear-induced instabilities at the mixing interfaces, which tend to generate multi-dimensional large-scale vortices (LSVs). Yan et al. (2022a) have highlighted the presence of both expected canopy shear layers and unexpected three-dimensional hydrodynamic features in PSCFs. In this paper, we focus on quantitatively analyzing the contribution of different transport mechanisms to individual momentum fluxes, and on identifying the underlying similarity in these fluxes. The results indicate that advection momentum fluxes, generated by secondary circulations (SCs), are equally important as LSVs-driven diffusion momentum fluxes near the mixing interfaces. LSVs transfer momentum from the outer free waters through turbulent diffusion to the canopy interior, while momentum is consumed by the SCs in the area across the interfaces. Consequently, the coexistence of SCs-driven advection and LSVs-driven diffusion leads to particular hydrodynamic features such as near-bed velocity deflection in the junction, outward shift of horizontal mixing layer, significant transverse variation of energy slope and so on. In addition, the observations confirm that the value of the momentum fluxes highly depends on canopy geometric and hydraulic factors, enabling the achievement of the dependency of several momentum fluxes on canopy geometric and hydraulic metrics with the self-similarity analysis. This research offers new insights into the essential physical interpretation of three-dimensional flow behavior subject to multiple mixing interfaces in PSCFs.