Two-phase flow insights into open-channel flows with suspended particles of different densities

Abstract

The effect of particle density on the turbulent open-channel flow carrying dilute particle suspensions is investigated using two specific gravities and three concentrations of solid particles. The particles, identical in size and similar in shape, were natural sand and a neutrally buoyant plastic. The particles were fully suspended, and formed no particle streaks on the channel's bed. Accordingly, the changes in the flow are attributed to the interactions between suspended particles and flow turbulence structures. Measurements were obtained by means of image velocimetry enabling simultaneous, but distinct, measurement of liquid and particle velocities. The experimental results show that, irrespective of particle specific gravity, particle suspension influences bulk velocity of flow and the Kármán coefficient, while friction velocity essentially remains constant. The results also show that particles in suspension modify local water turbulence over the flow depth, but in ways not accurately predicted using the customary parameters for characterizing turbulence modification.

Theoretical/numerical model for the transport of non-uniform suspended sediment in open channels

“…We observe that the sediment particles with larger size move slower than the smaller particles, as expected. The mean velocity of the sediment with smaller diameter is close to the mean velocity of the carrier phase, also as expected, in agreement with findings of Muste et al [70]. The simulation results for other test cases also possess the same trend.…”

Particle–turbulence interaction and local particle concentration in sediment-laden open-channel flows

“…They explained then that the result of U p > U f very near the wall might be caused by the following two factors, i.e., one is the deceleration of fluid velocity due to viscous effect and the other is due to the highNotation a reference point for Rouse equation (h0.05 h p ) C volume-averaged sediment (particle) concentratioñ cðtÞ instantaneous particle concentration C H threshold-value-averaged particle concentration in ejections and sweeps cv sediment flux d c critical particle diameter of turbulence modulation d p particle diameter H threshold value in instantaneous Reynolds stress h p maximum elevation of the lifted-up particles L x integral scale or length scale of the energy-containing eddies N e , N s counted number of ejections and sweeps, respectively S u ( f ) energy spectrum of u(t) T e , T s ejection period and sweep period, respectively T H duration time between the neighboring threshold range values H U cw streamwise mean velocity of clear-water flow U f fluid mean velocity of sediment-laden flow U p particle mean velocity of sediment-laden flow U *0 friction velocity of clear-water flow u 0 f , v 0 f streamwise and vertical turbulence intensities of fluid phase, respectively u 0 cw ,v 0 cw streamwise and vertical turbulence intensities of clear-water flow, respectively x, y streamwise and vertical coordinates, respectively y þ hy U Ã0 =n normalized by the inner variables b R Rouse-fitted value of diffusivity to eddy viscosity b Exp measured value of diffusivity to eddy viscosity d boundary-layer thickness k von Karman constant h Kolmogoroff microscale r p particle specific density speed particles which inrush into the viscous sublayer. These surprising results of Kaftori et al (1995) that the particle velocity might become larger than the carrier water velocity very near the wall have recently been verified using innovative PIV/PTV measurements by Nezu and Azuma (2004a), Muste et al (2009) and others, who all pointed out the importance of simultaneous measurements of particles and fluid in openchannel flows. Such simultaneous measurements of particles and fluid have first been conducted by LDA and PDA as mentioned above, although these measurement systems have some difficult limitations.…”

Velocity and concentration profiles of saline and turbidity currents flowing in a straight channel under quasi-uniform conditions

“…Indeed, in a classical paper of open channel flows, Vanoni (1946) documented experimentally that an increase in the mean concentration of suspended sediment was associated with an increasing velocity gradient at the wall. It was first hypothesized and then confirmed by theoretical investigations (Villaret and Trowbridge, 1991;Herrmann and Madsen, 2007;Bolla Pittaluga, 2011), experimental observations (Muste et al, 2009) and numerical simulations (Cantero et al, 2009) that the latter effect might originate from suspended sediments damping turbulence and decreasing turbulent mixing. The second reason is related to sediment entrainment from the bed.…”