Adrian C. H. Lai

Two-phase modeling of sediment clouds

A sediment cloud release in stagnant ambient fluid occurs in many engineering applications. Examples include land reclamation and disposal of dredged materials. The detailed modeling of the distinct characteristics of both the solid and fluid phases of the sediment cloud is hitherto unavailable in the literature despite their importance in practice. In this paper, the two-phase mixing characteristics of the sediment cloud are investigated both experimentally and theoretically. Experiments were carried out to measure the transient depth penetration and the lateral spread of the sediment cloud and its entrained fluid using the laser induced fluorescence technique, with a range of particle sizes frequently encountered in the field (modeled at laboratory scale). A two-phase model of the sediment cloud that provides detailed predictions of the mixing characteristics of the individual phases is also proposed. The entrained fluid characteristics are solved by an integral model accounting for the buoyancy loss (due to particle separation) in each time step. The flow field induced by the sediment cloud is approximated by a Hill’s spherical vortex centered at the centroid and with the size of the entrained fluid. The particle equation of motion under the effect of the induced flow governs each computational particle. A random walk model using the hydrodynamic diffusion coefficient is used to account for the random fluctuation of particles in the dispersive regime. Overall, the model predictions of the two-phase mixing characteristics are in good agreement with the experimental data for a wide range of release conditions.

Spreading Hypothesis of a Particle Plume

AbstractIntegral models of single-phase plumes are often closed using the entrainment hypothesis, which assumes the entrainment velocity is proportional to a characteristic plume velocity, but the corresponding theoretical development for modeling particle plumes has received less attention. In this paper an integral model is developed by proposing a new spreading hypothesis for particle plumes, in which the fluid phase spreading rate is taken as that of a single-phase plume, whereas the particle phase spreading rate is a function of the particle phase and fluid phase average vertical velocity. The change in momentum flux of the particle and fluid phases are calculated by considering the cross-sectional integrated buoyant and drag forces acting on the particles, and their reaction force acting on the fluid. Mixing characteristics of the particle plume can then be determined. The model was validated by laboratory particle plume experiments conducted for various particle sizes and initial plume-to-particle-...

Mixing of a Rosette Jet Group in a Crossflow

Partially treated wastewater is often discharged into coastal waters through an outfall diffuser fitted with clustered ports on risers. On each riser the effluent is discharged through two to eight ports arranged circumferentially, in the form of a rosette-shaped buoyant jet group. The near field mixing of such a jet group in a tidal flow is determined by the merging and interaction of coflowing, oblique-flowing, cross-flowing, and counterflowing jets. Despite numerous studies, a general predictive method for such complex jet groups has not been reported; ocean outfall design is often based on comprehensive physical model experiments. The mixing of merging nonbuoyant and buoyant jets issuing from a rosette outfall riser into an ambient current is studied experimentally by using the laser-induced fluorescence technique. Detailed cross-sectional measurements of the scalar concentration field downstream of the bent-over jets are made. The trajectories of multiple and individual jets discharging at various an...

Scaling Particle Cloud Dynamics: From Lab to Field

AbstractOpen-water disposal of sediment is an important component in many coastal engineering projects. Numerous studies have focused on small-scale dynamics and claimed their results can be scaled up by cloud number scaling. However, this scaling method is largely empirical and unexamined. The present paper confirms that the cloud number scaling provides a rational way to extrapolate small-scale lab experiments or numerical simulations to the field operations.

Behavior of Sediment Clouds in Waves

AbstractThe effects of regular surface waves on the descent of instantaneously released sediments were investigated experimentally. The detailed wave characteristics, including wave height (H), wave period (T), and wave phase of release, were controlled systematically through a wave synchronization system. The results showed that the sediment cloud was passively advected by the wave orbital motion, and there was no significant phase lag between the sediments and surrounding water particles. The motion of the center of mass and the growth rate of the sediment cloud averaged over four representative wave phases of release were found to be similar to those in a stagnant ambient condition. However, a shift (in the range of ±H/2) in the horizontal equilibrium position of the oscillatory motion was observed and could be related to the wave phase of release. A passive advection model was proposed to predict the motion of a sediment cloud in the wave environment by superimposing the wave orbital velocity onto the...

Multiple tandem jet interaction in a crossflow

We formulate a general semi-analytical model for multiple tandem jet interaction in a crossflow. For an array of buoyant jets arranging in tandem, the rear jets experience a reduced effective crossflow velocity due to the blockage and sheltering effect of the leading jet. The jet entrainment is modelled by a distribution of point sinks, and the blockage and sheltering effect is modelled by a distribution of doublets — both along the jet axis. The reduction of rear jet effective velocity as observed in previous experiments is successfully predicted by the model.

Mixing of Coflowing Jets in Rivers

Domestic and industrial wastewater effluents are often discharged into rivers after some treatment. An effective way of rapidly reducing effluent concentrations to acceptable levels within a mixing zone is to discharge the effluent in the form of multiple jets into a coflowing river flow at some distance from the riverbank. An accurate and efficient predictive model of multiple coflowing jets is required for impact and risk assessment of environmental discharges into rivers. In the present study, a new semi-analytical model for multiple coflowing jets is developed using the method of superposition based on Reichardt’s hypothesis. Jet excess momentum fluxes are hypothesized to be Gaussian and additive, which is supported by both theory and experimental observations. The predicted jet mixing characteristics of multiple coflowing jets from the present model compares favorably with basic experimental data. The present model is able to predict the entire concentration field, and thus mixing zone, downstream of an effluent discharge, and has potential application in river discharges.