Václav Matoušek

Experimental investigation of internal structure of open-channel flow with intense transport of sediment

Abstract Gravity-driven open-channel flows carrying coarse sediment over an erodible granular deposit are studied. Results of laboratory experiments with artificial sediments in a rectangular tilting flume are described and analyzed. Besides integral quantities such as flow rate of mixture, transport concentration of sediment and hydraulic gradient, the experiments include measurements of the one-dimensional velocity distribution across the flow. A vertical profile of the longitudinal component of local velocity is measured across the vertical axis of symmetry of a flume cross section using three independent measuring methods. Due to strong flow stratification, the velocity profile covers regions of very different local concentrations of sediment from virtually zero concentration to the maximum concentration of bed packing. The layered character of the flow results in a velocity distribution which tends to be different in the transport layer above the bed and in the sediment-free region between the top of the transport layer and the water surface. Velocity profiles and integral flow quantities are analyzed with the aim of evaluating the layered structure of the flow and identifying interfaces in the flow with a developed transport layer above the upper plane bed.

Analysis of Concentration Profiles in Dense Settling-Slurry Flows

In summer 2012, an extensive measurement campaign was carried out in a new test loop composed of pipes of the internal diameter of 100 mm. Four fractions of glass ballotini were used for preparation of settling slurries of different properties. During the tests, different patterns of slurry flows were observed. The tests included measurements of concentration profiles in a cross section of the slurry pipe.The paper analyzes shapes of measured concentration profiles with an aim to recognize flow patterns and dominating particle-dispersion mechanisms in the observed slurry flows. It is shown that particles are supported both through an interaction with the flowing carrier (turbulent dispersion) and through an interaction with other particles (collisional dispersion). A measure of each particular support is quantified through relating a distribution of concentration with a distribution of solids stresses across a flow discharge area in a pipe.Copyright

Liquid-Solid Flows Above Deposit in Pipe: Prediction of Hydraulic Gradient and Deposit Thickness

A predictive model for the pressure drop in a slurry flow above a stationary bed was presented at the last Liquid-Solid Flow Symposium in 2007. Since then the model has been further refined and validated using experimental data from the tests performed at the Institute of Hydrodynamics ASCR and from the literature. The new version of the model enables to choose a set of appropriate equations in order to predict the hydraulic gradient (the frictional pressure drop) and the thickness of the stationary bed for pipe flows of various average velocities of slurry and delivered concentrations of solids. The paper describes the structure and components of the model and discusses model predictive abilities by comparing model predictions with the new experimental data gained for slurries of two different fractions of sand (respectively fine to medium and medium sands) at various velocities and solids concentrations in a 100-mm pipe.© 2009 ASME

Pipe-Wall Friction in Vertical Sand-Slurry Flows

Friction due to the presence of solid particles suspended in a flow is a result of processes in a relatively thin layer near the pipe wall. Pipe-wall friction generated by particles in permanent contact with pipe wall is relatively well understood. However, very little is known about the friction deriving from sporadic contact (collisions) of particles with the wall. This friction is a major contributor to the frictional pressure drop in many slurry pipeline applications. The paper describes results of extensive laboratory tests of vertical flows of different sand fractions (fine, medium and coarse sands) carried out in the Laboratory of Dredging Engineering of the Delft University. In order to identify mechanisms that govern the solid-particle friction at the pipe wall the paper analyses friction conditions in observed vertical flows. The effects of particle-particle interactions and particle-liquid interactions on the pipe-wall friction are evaluated. One of the interesting phenomena observed in the laboratory was that frictional pressure drops in highly-concentrated flows at high velocities are lower for slurries of medium sand and coarse sand than for slurries of fine sand. The observed trend is believed to be associated with the liquid–lift force acting on solid particles traveling near a pipe wall. This off-wall force seems to be the most effective for medium to coarse particles traveling in highly concentrated mixture in the near-wall region. Thus pressure drops due to the presence of solids in non-stratified flows seem to be primarily produced by the combined effect of the Bagnold collisional force (force that colliding particles exert against the pipe wall) and liquid lift force acting on solid particles in the near-wall zone of the slurry flow.Copyright

Laboratory testing of granular kinetic theory for intense bed load transport

Abstract Collisional interactions in a sheared granular body are typical for intense bed load transport and they significantly affect behavior of flow carrying bed load grains. Collisional mechanisms are poorly understood and modelling approaches seldom accurately describe reality. One of the used approaches is the kinetic theory of granular flows. It offers constitutive relations for local shear-induced collision-based granular quantities - normal stress, shear stress and fluctuation energy - and relates them with local grain concentration and velocity. Depth distributions of the local granular quantities produced by these constitutive relations have not been sufficiently verified by experiment for the condition of intense bed load transport in open channels and pressurized pipes. In this paper, results from a tilting-flume facility including measured velocity distribution and deduced concentration distribution (approximated as linear profiles) are used to calculate distributions of the collision-based quantities by the constitutive relations and hence to test the ability of the kinetic-theory constitutive relations to predict conditions observed in these collision-dominated flows. This test indicates that the constitutive relations can be successfully applied to model the local collisional transport of solids at positions where the local concentration is not lower than approximately 0.18 and not higher than approximately 0.47.

THRESHOLDS OF UPPER-STAGE PLANE BED REGIME FOR INTENSE BED LOAD IN OPEN CHANNEL

Results are discussed of laboratory experiments on criteria determining the transition between the regime of dunes and the upper stage plane bed (UPB) regime and the transition between the UPB regime and the regime of wavy flow. The experiments were carried for 3 fractions of plastic material and two fractions of glass beads in a broad range of flow conditions (different discharges of water and solids and longitudinal bed slopes) in a tilting flume. The experiments reveal that, contrary to expectations, a constant value of the Shields parameter is not an appropriate criterion for the transition between the dune regime and the UPB regime. Instead, the criterion seems to be well represented by a constant value of the longitudinal bed slope. The transition between the UPB regime and the wavy regime is found at a constant value of the densimetric Froude number. Relatively high values of Froude number at the threshold are due primarily to the presence of the collisional transport layer which protects the bed from an influence of undulating water surface.