Introduction

Partially-treated wastewater discharge from marine outfall diffusers often contains organic solids that may settle close to the source, giving rise to the formation of sludge banks and affecting benthic ecology. The purpose of this research is to develop a general model to predict the particulate transport and the resultant deposition on the ocean floor in the near and intermediate field of the jet discharge.

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Experimental Setup

The experiments are performed with a horizontal turbulent jet in otherwise stagnant water, in a 1 m x 1 m x 0.6 m high water tank. Fine sand particles as well as synthetic particles are used in the tests. An hourglass type sediment supply system has been specially developed for effectively controlling the source conditions for a continuous and steady discharge of particles from a turbulent water jet.

Particle feed system of horizontal sediment jet

Flow visualization of sediment jet

(Run S153J40, C0 = 4.26 g/L, ws = 1.51 cm/s, (a) non-uniform C0 at source ; (b) uniform C0 .

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Results

Aided by flow visualization technique, experimental observations show that the particle distribution in the jet cross-section exhibits significant heterogeneity, and higher concentration of particles is observed in the lower jet section from where the particles start to settle out. The experimental observation of a momentum jet with a dilute suspension shows that the jet spreading characteristics is not materially affected by the presence of solid particles.

For a horizontal momentum jet, the key length scale is the momentum-settling length scale lm. The sediment deposition rate per unit x-distance can be successfully interpreted in terms of lm and be described by a log-normal distribution. For a buoyant jet, an additional jet momentum length scale ls is needed to describe the dynamics of the plume rise and radial spreading region.

Normalized deposition rates as function of downstream distance. (this study)

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Two-layer Sediment Jet Model

A Lagrangian sediment jet model is developed to predict the mixing of dilute particle-laden momentum jets under general flow conditions. The model is based on top-hat profiles for the jet mixing and the particle sedimentation. The particles are assumed to fall out from the bottom of the jet once the settling velocity exceeds the entrainment velocity. After the particles leave the jet edge, their trajectories are tracked in a Lagrangian framework. To account for the non-uniform particle distribution in the jet, a two-layer theory is proposed to account for the particle mass transfer between the upper and lower jet sections. The near field computation is terminated when the jet reaches a boundary (e.g. free surface). Predictions of sediment deposition rate are in excellent agreement with the experimental data (using an interfacial mixing coefficient of 0.7). The 30% velocity reduction due to flow turbulence is of the same order as measured velocity turbulent intensities across a jet.

(a)

(b)

Comparison of predicted and measured longitudinal sediment deposition rates for horizontal buoyant jets

(a) S153B52, ls =0.0602, lm = 0.3414, lm > 3ls

(b) S199B50, ls = 0.0794, lm = 0.1584, lm < 3ls

(a)

(b)

The comparison of one-layer and two-layer prediction, (a) horizontal deposition rate; (b) particle concentration along the jet.

(a)

(b)

(c)

(d)

Comparison of predicted and measured longitudinal sediment deposition rates for horizontal momentum jets

(a) FJ34; (b) CJ54; (c) S153J58; (d) S199J66.

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Conclusion

Horizontal momentum sediment jets in stagnant water are studied with special reference to bottom deposition rates. A Lagrangian sediment jet model is developed for prediction. A two-layer theory is proposed for the particle mass transfer between the upper and lower jet sections to account for the non-uniform particle distribution in the jet. Predictions of sediment deposition rates are in excellent agreement with the experimental data.

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