INCIPIENT MOTION OF NON-COHESIVE SEDIMENT UNDER ICE COVER-AN EXPERIMENTAL STUDY

Based on a series of experiments under both ice-covered and free surface conditions, the present article discusses the role of flow velocity and critical shear Reynolds number for incipient motion of bed material. The influence of the resistance coefficients of both the underside of the ice cover and the channel bed on the location of the maximum velocity has been discussed. In addition, the impacts of ice and composite resistance coefficients on flow velocity for incipient motion of bed material have been assessed. The diagram describing the critical shear Reynolds number and the dimensionless shear stress for the incipient motion of sediment under ice covered conditions with different under cover resistance coefficient has been established. The effects of grain size on densimetric Froude number for incipient motion of bed material have been investigated. A relationship between the densimetric Froude number for incipient motion of bed material and the median grain size of bed material as well as the roughness coefficient of channel bed and roughness coefficient of ice cover has been established.

CHARACTERISTIC ANALYSIS OF THE PLUNGING OF TURBIDITY CURRENTS

Turbidity currents are flows driven by suspended sediment of flood-induced turbid river water with excess density.Such currents are often the governing factor in reservoir sedimentation by transporting fine materials over long distances and delivering the majority of deposition,which thus reduces the storage capacity.Therefore,the design and operation of a reservoir requires an accurate prediction of its occurrence condition and plunging position,which is the objective of the present study.This article presents a verified algebraic slip mixture model including momentum,continuity and algebraic velocity expressions to simulate 2-D turbidity currents.Test experiments in a multiphase flume were carried out.Reynolds number,sediment concentration and densimetric Froude number were used as parameters in the occurrence condition analysis.The plunging of turbidity currents may produce reflux and backflow due to the diving flow at the surface of the clear water.The similar experimental results were also obtained by PIV measurements

Numerical simulation of scouring funnel in front of bottom orifice

The scouring funnel in front of a bottom orifice under the condition of fixed water levels is simulated by using an Eulerian two-phase model, with onsideration of the flow-particle and particle-particle interactions. The predictions of the scouting funnel shape agree well with laboratory measurements. The flow-field characteristics of the two phases and the influences of the hydraulic and geometric parameters on the shape of the scouring funnel are analyzed on the basis of the computation results. It is revealed that the non-dimensional maximum scour hole parameters, the depthdm / do, the length l,. / do, and the half-width w / do, are linearwith the densimetric Froude number Fro , the main parameter describing the scour hole, the centerline scour depth Dc and the half-scour width Wr vary according to a power law, and the transverse scour profiles exhibit strong similarities, the velocity distribution of the water is confined within the sink-like area near the orifice, and the mutual impact of the flows at the azimuthal sections and the resistances of the walls and the sand layer produce a vortex in the scour hole, that makes the sand particles to be suspended in the water, the exchanging water in the pore water is the main contributor in forcing the sand to move, and transporting the sand in the same direction as the pore water along azimuthal sections.

An experimental study on plunging depth of density currents

Mass density of the current flows is the one of the important problem in the hydraulics of the dam reservoir. Plunge point occurs when the mass density current penetrates in the stagnant fluid. Recognition the place of this point is very important because of clearing the boundary of the density current flow and ambient fluid. In this study the influences of bed slope and hydraulic parameters on plunging depth were experimentally investigated. The results show that the slope has a minor effect on the plunging depth. The height of plunging depth is increased by increasing the density of the current flow. Also increasing the densimetric Froude number caused of decreasing the plunging depth. Finally an equation was proposed to estimate the plunging depth using as function of flow characteristics.