Flow mechanisms in horizontal sediment-laden jets

Abstract

Particle-laden jets are an important type of multiphase flow which can be found in various natural and technical processes. This study focuses on the flow mechanisms in a horizontally discharging sediment-laden jet that is of particular interest in environmental science and engineering.

Experimental techniques and mathematical models are developed to investigate horizontal sediment-laden jets, both for the buoyant and non-buoyant jet discharge cases. In the laboratory, the separation of images of the fluid and the particulate phases is achieved by harnessing light signals of visualization at different wavelengths. Whole field measurements of velocities of the two phases are made by the adoption of particle image velocimetry (PIV) algorithms. Numerical models are developed in two approaches with regard to the treatment of the particulate phase. In the Lagrangian approach, individual sediment particles are tracked while the flow field of the fluid phase is computed with large-eddy simulation (LES). This simulation successfully captures the transient nature of the particle-laden flow. In the Eulerian approach, a two-phase model is used to obtain steady flow simulations in a much shorter computation time.

The experimental and numerical results for the horizontal momentum jets show that, at low initial particle concentrations, the sediment particles generally follow the jet flow but with some levels of deficit velocities. In the upper layer of the jet the particles do not follow the fluid flow as well as in its lower layer. More particles are observed in the lower layer than in the upper one. For the momentum-dominated zone of a horizontal buoyant jet, the flow exhibits similar behaviors as the horizontal particle-laden momentum jet, except that there are some slight modifications from the effects of buoyancy. In the bending zone of the buoyant jet, the effects of buoyancy become significant. Notably, the locations of maximum velocity magnitude and those of maximum turbulence intensity are well separated in this zone. A strong correlation of particle abundance and high turbulence intensity is observed in the lower outer jet layer in this bending zone.

Significant modifications to the global behaviors of horizontal sediment jets are observed as the particle concentration increases to relatively high levels. The jet trajectories are brought downwards by the particle loads and the jet widths are also increased. For the flow regime being investigated, turbulence intensity in the fluid flow is found to be increased by the presence of sediment particles.

The results suggest that turbulence helps suspend sediment particles in horizontally discharging jets. A Stokes number is proposed to represent the ability of particles to follow the fluid flow. It is defined as St=W_s/U_j , where ws is the particle settling velocity in still fluid and Uj is the jet exit velocity, which indirectly governs the turbulence characteristics of the jet flow. The advecting large eddies in a turbulent jet are found to play the role of organizing particles in patches. Interaction and coalescence between particle-concentrated eddies may result in the sudden drop of a group of particles, which contributes to sediments falling from a horizontal jet in the form of particle-rich “fingers”.