arXiv subjects

Mohammad Omidyeganeh

Publications and source records attributed to Mohammad Omidyeganeh.

3 recordsLinked to original sources

arXiv2025

Filament inclination effect on turbulent canopy flows

Inspired by the spontaneous behaviour observed in filamentous layers -- where the balance between flow-induced drag and structural elasticity dictates the filaments' equilibrium streamlined posture -- we perform a series of large-eddy simulations to investigate how filament inclination affects turbulent shear flows developing both above and within a canopy of filaments. We examine six distinct filament inclination angles ranging from 0°to 90°. The in-plane solid fraction and filament length are chosen to achieve a fully dense canopy at zero inclination, and these parameters remain constant throughout our study. By setting a nominal bulk Reynolds number of 6000, we provide a detailed statistical characterisation of the turbulent flow. Our findings illustrate distinct changes in the flow regime with varying filament inclination. At lower angles, the canopy remains dense and significantly influences the flow, conforming to a classical canopy-flow regime. However, as the inclination approaches 90°, the intra-canopy region progressively becomes shielded from the outer flow. Remarkably, at 90°inclination, the flow drag reduces significantly, and the total drag becomes lower than that typically seen in an open, filament-free flow. We document this transition from a canopy-dominated regime to a scenario where the canopy becomes largely sheltered from the outer turbulent flow, highlighting key alterations in intra-canopy dynamics as filament inclination increases. Our observations are substantiated by an analysis of the velocity spectra, providing deeper insight into the interactions between the canopy and the developing turbulent boundary layer.

physics.flu-dyn↗

arXiv2022

On the solidity parameter in canopy flows

We have performed high-fidelity simulations of turbulent open-channel flows over submerged rigid canopies made of cylindrical filaments of fixed length $l=0.25H$ ($H$ being the domain depth) mounted on the wall with an angle of inclination $θ$. The inclination is the free parameter that sets the density of the canopy by varying its frontal area. The density of the canopy, based on the solidity parameter $λ$, is a widely accepted criterion defining the ongoing canopy flow regime, with low values ($λ\ll 0.1$) indicating the sparse regime, and higher values ($λ> 0.1$) the dense regime. All the numerical predictions have been obtained considering the same nominal bulk Reynolds number (i.e. $Re_b=U_b H/ν= 6000$). We consider nine configurations of canopies, with $θ$ varying symmetrically around $0°$ in the range $θ\in [\pm 78.5°]$, where positive angles define canopies inclined in the flow direction (with the grain) and $θ=0°$ corresponds to the wall-normally mounted canopy. The study compares canopies with identical solidity obtained inclining the filaments in opposite angles and assesses the efficacy of the solidity as a representative parameter. It is found that when the canopy is inclined, the actual flow regime differs substantially from the one of a straight canopy that shares the same solidity indicating that criteria solely based on this parameter are not robust. Finally, a new phenomenological model describing the interaction between the coherent structures populating the canopy region and the outer flow is given.

physics.flu-dyn↗

arXiv2011

Coherent Structures in Turbulent Flow over Two-Dimensional River Dunes

We performed large-eddy simulations of the flow over a typical two-dimensional dune geometry at laboratory scale (the Reynolds number based on the average channel height and mean velocity is 18,900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The flow separates at the dune crest and reattaches downstream on the bed (at x=5.7h). A favorable pressure gradient accelerates the flow over the stoss-side (the upward-sloping region for x > 8h) and an unfavorable gradient for x < 8h decelerates the flow over the lee-side of the dune. Due to the separation of the flow, a shear layer is generated after the crest that expands in the wake region towards the next dune. The outer-layer turbulence structures are visualized through isosurfaces of pressure fluctuations colored by distance to the surface. Spanwise vortices are generated in the shear layer separating from the crest due to the Kelvin-Helmholtz instability. They are convected downstream and either interact with the wall or rise to the surface, taking the form of large horseshoe-like structures. These structures may undergo an intense distortion, become one-legged, or be completely destroyed. As they grow to dimensions comparable to (or larger than) the flow depth, a strong ejection occurs between their legs. The interaction between two large coherent structures may result in merging, or in dissipation of the weaker one. Surviving eddies grow and rise along the shear layer emanating from the dune crest; they tilt downward and, eventually, their tips touch the surface. When the legs of the horseshoe are close to the surface, they create an upwelling, which expands and weakens. The legs of the vortex loop remain coherent for a longer time.

physics.flu-dyn↗