Author(s): Subhasish Dey; Sk Zeeshan Ali
Keywords: Fluvial Hydraulics; Turbulent Flow; Sediment Transport; Sediment Suspension
Abstract: In sediment suspension, the turbulence production in the near-bed flow zone lifts up the fine particles beyond the bedload layer, sustaining them in a suspension mode of transport. The settling tendency of a particle is principally balanced by the turbulent diffusion in order to achieve a dynamic equilibrium. The suspended load transport plays an effective role in governing the fluvial morphodynamics, because it remains the major fraction of the total sediment transport in a fluvial system. The suspended sediment concentration is usually determined with respect to a reference concentration, which prevails at a reference level. In general, the extent of the reference level is considered to be the extremity of the bedload layer, which is the source of the sediment particles to come in suspension. Rouse (1937) was the pioneer to put forward an analytical solution for the suspended sediment concentration in vertical with the aid of Fickian diffusion together with the logarithmic law of velocity. The major drawback of the Rouse equation is that the sediment concentration disappears at the free surface. After Rouse (1937), significant advances were made to derive enhanced versions of the distribution of suspended sediment concentration in vertical (Dey, 2014). In this study, the hydrodynamics of sediment suspension, including the mechanics and the turbulent structure of two-phase flow system is analyzed, considering the dynamic coupling among the sediment concentration, flow velocity and the two-phase flow energetics. The continuity, momentum and energy equations of fluid and solid phases are separately considered to obtain the generalized equations of the two-phase flow system. The system of equations is simultaneously solved by means of suitable closure relationships. The results reveal that the sediment concentration and the turbulent kinetic energy (TKE) reduce with an increase in Rouse number. In addition, the TKE flux, TKE diffusion rate and TKE production rate increase as the Rouse number increases, whereas the TKE dissipation rate reduces. The turbulence length scales show that the Prandtl’s mixing length reduces with an increase in Rouse number. By contrast, the Taylor microscale and the Kolmogorov length scale increase as the Rouse number increases. References Dey, S. (2014). Fluvial hydrodynamics: Hydrodynamic and sediment transport phenomena, Springer, Berlin. Rouse, H. (1937). Modern conceptions of the mechanics of fluid turbulence. Transactions of the American Society of Civil Engineers 102, 463–505.