Particle–fluid interactions in a plane near-wall turbulent flow

Abstract

The role of particles heavier than the fluid (glass spheres in water) in a turbulent open channel flow over a smooth bed is examined at volume concentration about $10^{-3}$. The present work focuses on the dynamical interaction between the solid (particles) and the fluid phases in the near-wall region. Experimental measurements have been performed by means of phase Doppler anemometry to acquire two velocity components, particle size and concentration data simultaneously; the Reynolds number of the flow was close to 15 000. It is observed that in the particle-laden flow, the vertical profiles of the streamwise mean velocity (for both fluid and solid phases) are reduced in the outer layer ($y^{ + }\,{ >}\, 20$), but increased in the viscous sublayer ($y^{ + }\,{<}\,5$) in comparison to the clear-water conditions, leading to an apparent slip kinematic boundary condition close to the wall ($y^{ + } \,{\approx}\,2$). Moreover, in the presence of solid particles, the flow exhibits a velocity close to the wall ($y^{ + }\,{ <}\, 15$) which is smaller than that of the particles, while in the outer layer the opposite takes place. In particle-laden flow, turbulence intensities of the streamwise and especially of the vertical velocity are damped for $y^{ + }\,{>}\,10$–20 (depending on particle inertia) but enhanced in the very near-wall region ($y^{ + }\,{ <}\, 5$), as is the Reynolds stress. These findings can be explained if they are referred to the mechanism of particle entrainment and deposition, which takes place close to the wall. This mechanism is related to particle inertia and to the dynamic of the structure of near-wall turbulence, which connects the buffer and outer regions with the very near-wall region. A significant momentum exchange between the two phases, which is particularly effective in the buffer region, is revealed by the quadrant analysis of the Reynolds stresses.

Entropy-Based Modeling of Velocity Lag in Sediment-Laden Open Channel Turbulent Flow

“…For convenience, the velocity lag is non-dimensionalized asû l (=u l /u * ), where u * is the shear velocity. From the experimental results, Rashidi et al [4] and Righetti and Romano [26] proposed a monotonic variation of velocity-lag along the vertical direction in open channel flow. Based on this proposition, it is assumed that the velocity lag increases monotonically from the water surface to the channel bed, having a zero value at the water surface and u l max at the reference level (above the bed) y = a, where a is the vertical distance from the bed to the reference level.…”

“…To test the validity of this model, i.e., Equation (13) with a wide range of sediment-laden flow conditions and different types of particles, experimental data from Rashidi et al [4] and Kaftori et al [31] for polystyrene particles, Best et al [3] and Righetti and Romano [26] for glass particles and Muste and Patel [2] and Muste et al [32] for natural sand particles were selected. A summary of these data with flow conditions and other flow characteristics is given in Table 1.…”

“…To test the proposed model for glass particles, the computed values of velocity lag were compared with the observed data of Best et al [3] and Righetti and Romano [26]. In all cases, the model parameters were computed as mentioned earlier.…”

“…In all cases, the model parameters were computed as mentioned earlier. Figure 9 compares the Best et al [3] data, and Figure 10 shows the Righetti and Romano [26] data. From Figure 9, it can be seen that all theoretical models gave almost similar results.…”

Flow analysis in a channel with flexible vegetation using double-averaging method

“…The features of these coherent structures in the dense and sparse configurations can presumably differ, as suggested by the third order moments analysis, but after all they are responsible for the vertical transport of momentum and substances, such as nutrients, oxygen and sediment, between the vegetation layer and the outer flow (Ikeda andKanazawa 1996, Ghisalberti andNepf 2005). Focusing our attention on the transport by suspension of fine sediments, it is indeed well known that ejections are responsible for the maintaining into suspension of particles heavier than fluid, supplying the required net turbulent upward momentum flux (see, e.g., Sumer and Deigaard 1981, Kaftori et al 1995, Lu and Willmart 1973, Righetti and Romano 2004. The impact of plants on the suspended sediment transport capacity can be therefore investigated generalizing the approach proposed by Lu and Willmart (1973) …”