Turbulent velocity profiles in sediment-laden flows

Abstract

A theoretical analysis shows that velocity profiles in sediment-laden flows are similar to those in clear water. The modified log-wake law, which is developed for clear water by Guo, is also valid in sediment-laden flows. The analysis of the effects of sediment suspension on turbulent kinetic energy and turbulent diffusion shows that: (1) sediment suspension increases mean flow energy loss; (2) sediment suspension weakens turbulent diffusion in the vertical direction and then increases velocity gradient; and (3) sediment suspension affects velocity profile in two ways: average concentration and density gradient. The comparison with narrow-channel laboratory data confirms the theoretical analysis and shows that: (1) the modified log-wake law agrees well with experimental data for sediment-laden flows; (2) both average concentration and density gradient reduce the von Karman constant; and (3) for a given width-depth ratio, sediment concen tration slightly increases the wake strength while density gradient has little effect on it. In addition, the modified log-wake law can reproduce experimental data where the maximum velocity occurs below the water surface. RÉSUMÉUne analyse théorique démontre la similitude des profils de vitesse avec ou sans transport de sediment. Les profils de vitesse logarithmiques avec coefficient de trainee sont done applicables pour les écoulements chargés de sediments. Cette loi permet de représenter les profils de vitesse avec maximum sous la surface libre. Les sediments en suspension: (1) augmentent les pertes d'énergie;(2) affaiblissent la diffusion turbulente verticale et augmentent Ie gradient de vitesse; et (3) affectent les profils de vitesse en raison de la concentration moyenne et du gradient de densité. La comparaison avec des données de laboratoire en canal étroit confirme que: (1) la loi logarithmique avec coefficient de trainee s'applique bien aux données expérimentales; (2) la concentration moyenne et Ie gradient de densité diminuent la constante de von Karman; et (3) pour un rapport largeur-profondeur donné, la concentration de sediments augmente légèrement Ie coefficient de trainee tandis que Ie gradient de densité exerce peu d'influence sur ce coefficient.

Application of Modified Log-Wake Law in Open-Channels

“…By contrast, although the velocity dip phenomenon has been reported for a long time (Leighly 1932), our understanding of it is poor and only a few studies on it can be found in the literature (Sarma et al 1983(Sarma et al , 2000Chiu and Said 1995;Chiu and Tung 2002;Moramarco et al 2004;Guo and Julien 2001). The velocity dip phenomenon can hardly be modeled with log-wake velocity profiles because it imposes a velocity increase with distance from the boundary.…”

Role of suspended‐sediment particle size in modifying velocity profiles in open channel flows

“…Yet the traditional approach based on the Reynolds-averaged Navier-Stokes equations (RANS) and a closure model for the turbulent correlations continues to be most applicable to water resources engineering practice. Within the framework of RANS, Vanoni [1946] proposed the concept of reduced von Karman coefficient, followed by Einstein and Chien [1955], Vanoni and Nomicos [1960], and Guo and Julien [2001], etc. Coleman [1981Coleman [ , 1986 argues that the mean velocity follows the logarithmic profile as in a clear water flow in the near-bed region and deviates from that according to the law of the wake near the free surface, with the wake intensity varying as a function of sediment concentration.…”

Shear Stress in Smooth Rectangular Open-Channel Flows

“…According to the Schwarz-Christoffel transformation (Spiegel 1993, p. 204), the delimitation CH is found as (Guo 1998;Guo and Julien 2002) …”

“…For example, one must know boundary shear stress to study a velocity profile (Guo and Julien 2001;BabaeyanKoopaei et al 2002). One must separate the bed shear stress from the total shear stress to estimate bed-load transport in openchannel flows.…”