Numerical Investigation of Regime Transition in Canopy Flows

Abstract

We have carried out highly resolved simulations of turbulent open channel flows. The channel wall is covered with different filamentous layers sharing the same thickness (h=0.1H, where H is the open channel height) and bulk Reynolds number (i.e., Re_b=U_bH/nu , , U_b is the bulk velocity and nu the kinematic fluid viscosity). The layers are composed of rigid, slender cylindrical filaments mounted perpendicular to the bottom wall. We have selected two layer configurations characterised by filament spacing ratios of Delta S/H=pi /24simeq 0.13 and Delta S/H=pi /32simeq 0.098. The geometrical features of the two layers, allow to classify them as transitional canopies (lambda =dh/Delta S^2simeq 0.15, where d is the filament diameter, i.e. dh is the filament frontal area) (Monti et al. 2020), which is defined as the separation between the sparse-dense asymptotic regimes, proposed by Nepf (2012). While the physical characterisation of the two asymptotic regimes is fairly understood, the transitional conditions remain an open question since the physical characteristics unique to the sparse and dense scenarios coexist in the transitional regime. By resolving every single filament with the aid of an immersed boundary technique in the framework of a Large Eddy formulation, we report the physical mechanisms that emerge at the onset of different regimes (chosen values of lambda fall on the verge between a dense and a sparse condition) and verify the criterion associated with the inception of the transition regime.