Investigating the mechanism by which finite-size heavy particles are entrained in turbulent open channel flow over a smooth surface

TL;DR

This study uses particle-resolved DNS to unravel how finite-size heavy particles are entrained in turbulent open-channel flow over a smooth wall. By projecting hydrodynamic forces onto a locally defined relative velocity and decomposing into drag, lift, pressure, and viscous components, it demonstrates that lift, not rotation, governs initiation of lift-off, with both pressure and viscous effects contributing comparably. The analysis links lift-off to a preceding high wall-normal shear, generated by approaching quasi-streamwise vortices, and shows that particle spanwise mobility markedly changes entrainment statistics by altering exposure to coherent structures. These findings highlight the critical role of near-wall coherent structures in particle transport and provide a framework for isolating the mechanisms behind sediment resuspension in smooth-wall open-channel turbulence.

Abstract

The dynamics of entrainment of finite-size heavy particles in a turbulent open channel flow over a smooth surface are analyzed. Three types of simulations, namely with freely moving, rotation-constrained, and spanwise-motion-constrained particles, were conducted using particle-resolved direct numerical simulations. With the aid of a relative velocity suitably defined in the vicinity of the finite-size particle, we decompose the hydrodynamic force into drag and lift contributions and evaluate the local wall-normal shear rate around the particles. By means of coherent structure eduction techniques, we investigate flow structures before and during lift-off events. Rotation-constrained simulations revealed the insignificance of particle rotation in the entrainment mechanism. Spanwise-motion-constrained simulations revealed the importance of particle location with respect to flow structures with apparent changes in entrainment frequency, duration of the entrainment process, wall-normal shear around the particles, and distance to the nearest vortical structures during lift-off. The contribution of lift to the wall-normal force is found to be responsible for the initiation of particle entrainment, which is induced by a high-shear event associated with fast-moving fluid. The presence of quasi-streamwise vortices is shown to be an important ingredient for the entrainment of particles into the bulk flow. The results show that, at marginal Shields number values, a high wall-normal shear rate and the proximity of an intense quasi-streamwise vortex are essential elements of the entrainment mechanism.

Figures (11)

• Figure 1: Flow configuration. The streamwise and spanwise directions are periodic. Gravity is directed in the negative wall-normal direction. Particles are shown at their initial locations.
• Figure 2: (a) A spherical surface with radius $R_s$, trimmed by two wall-parallel planes at one particle radius $R_\mathrm{p}$, over which a fluid velocity is evaluated for the purpose of calculating the instantaneous relative velocity of the fluid in the vicinity of the particle. Black and red points on the spherical surface correspond to the sampling points $\mathbf{x}^{(i)}_l$ used for the fluid velocity, while the red points denote the subset of sampling points used for evaluating the local shear rate around the particle. (b) Schematic of the hydrodynamic forces acting on the particle. The relative velocity vector $\mathbf{u}_{\text{rel}}^{\mathrm{S}}$, total hydrodynamic force $\mathbf{F}^\mathrm{(H)}$, and its projections that define the drag and lift forces as well as the drag and lift forces contributions to $\mathbf{F}^\mathrm{(H)}$ in the wall-normal direction.
• Figure 3: (a) Time evolution of wall-normal hydrodynamic force $F_y^{(\mathrm{H})}$ (⁎), contact force $F_{\mathrm{c}}$ ($\circ$), particle submerged weight $F_g$ ($\hbox{$\triangle$}$), and wall-normal total force $F_y$ (-) of one entrained particle. Red dashed lines indicate $F_y^{(\mathrm{H})}=-F_g$ (horizontal) and $F_{\mathrm{c}} = 0$ (vertical). Their intersection marks the time when both are satisfied. (b) Time evolution of wall-normal velocity of the particle $v_{\mathrm{p}}$ ($\circ$) and wall-normal position of the particle center $y_{\mathrm{p,w}}$ (-) of one entrained particle. Here $y_\mathrm{p,w} = y_\mathrm{p} - y_\mathrm{w}$. Note that this data corresponds to the same entrainment event as in (a).
• Figure 4: The P.d.f. of the vortex volumes normalized by particle volume for the unconstrained case. The inset shows the same data on a log--log scale, highlighting power-law behavior ($V_{\boldsymbol{\omega}_{\mathrm{f}}}^{(-1.3)}$).
• Figure 5: Time evolution of hydrodynamic force in wall-normal direction $F_y^{(\mathrm{H})}$ (⁎) with lift $F_{y,\mathrm{lift}}^{(\mathrm{H})}$ ($\circ$) and drag $F_{y,\mathrm{drag}}^{(\mathrm{H})}$ ($\hbox{$\triangle$}$) contributions, The data is averaged over all corresponding entrainment events for UN (solid line), RC (dashed line), and ZC (dotted line) cases.