Introduction

In modern outfall designs, the wastewater is typically discharged from an outfall riser in the form of rosette jet groups. A rosette jet group discharging into an ambient current is a complex flow involving mixing, merging and interaction of coflowing, crossflowing, and counter-flowing jets. Existing buoyant jet models can only be applied after approximations and simplifications.

The present study aims at developing a robust model in predicting mixing characteristics of a rosette jet group in both the near and intermediate field.

The mixing characteristics of the rosette jet group beyond the near field are predicted by a dynamic coupling of the near field VISJET model and a 3D shallow water circulation model using a recently developed Distributed Entrainment Sinks Approach (DESA) (Choi and Lee (2007)).

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Experimental Setup

Experiments are carried out in a 12 m by 5 m wide shallow water basin; the water depth is set at 0.3 m. Six, eight, and twelve-nozzle risers are considered. The diffuser discharge flow is measured by a calibrated rotameter. Buoyancy of the source fluid is obtained by addition of ethanol to water. Densities of the source fluid and the ambient are measured using a precision density meter. Crossflow velocity is measured by an Acoustic Doppler Velocimetry (ADV) placed approximately 1 m upstream of the diffuser. Near-field cross-sectional concentration field at various downstream stations and jet trajectories are obtained by using the Laser-Induced Fluorescence (LIF) techniques supplemented by colour dye experiments together with image processing techniques. Far-field mixing characteristics such as gravitational spreading, surface buoyant layer thicknesses are similarly obtained.

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Results and Summary

The comparison of jet trajectories between experiments and VISJET predictions are shown in Fig. 3. It can be seen they agree with each other well. The cross-sectional concentration field of a rosette jet group in various downstream sections are shown in Figure 4. The coflowing jet (lower centre), counter flowing jet (upper centre), and the merging crossflow jets (at the two side) can be clearly seen and well-simulated by the VISJET.

Fig.3: VISJET simulation and experimental results of jet trajectories.

Fig 4: VISJET simulation and experimental results of cross-sectional field.

The composite dilution calculated by the VISJET is compared with the observed experimental dilution of a rosette jet group. The result is shown in Figure 5. It can be seen the predicted dilution agrees reasonably well with the observed dilution.

Fig 5: composite dilution (left: VISJET calculation, right: experiment)

The intermediate field mixing characteristics such as surface gravitational spreading is shown in Figure 6. Predicted buoyant spreading by the DESA is also shown for comparison. It can be seen they are in reasonable agreement.

Fig. 6: Intermediate field mixing characteristics

By using the VISJET and DESA, mixing characteristics of a rosette jet group in the near and intermediate field can be predicted reliably.

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