In order to understand environmental impact and to design sustainable effluent disposal systems, it is necessary to predict the impact of a polluting or natural environmental discharge. The challenging task is to predict the unknown trajectory and mixing of an arbitrarily-inclined buoyant plume in a density-stratified crossflow. 

Common polluting discharges include domestic sewage discharges from outfalls, thermal discharges from power stations (cooling water), brine from desalination plants, and effluents from industry (breweries, dye and bleaching, paper mills, restaurants, etc). The usual public perception is any discharge into the environment is evil. In reality, the discharge (plume) is mixed by the turbulent vortices in the environment; the effluent is dispersed and diluted, leading to a continuous and rapid reduction in pollutant concentration. The degree of mixing (and hence the environmental impact) is determined by the effluent characteristics (e.g. density, pollution load, discharge velocity and orientation, geometric dimensions) and the environmental conditions (local ambient density, current velocity and direction, density stratification). With a suitable combination of prior treatment, proper design and siting of discharge location, environmentally sustainable and economically viable waste disposal options can be developed. In many coastal cities (e.g. Sydney, Shanghai, and Hong Kong), partially-treated sewage and industrial effluents are discharged through a submerged outfall system into the adjacent sea. A capability to predict the impact of environmental discharges is essential to define the degree of waste removal required, and to define a meaningful mixing zone outside of which water quality objectives are legally enforced.

VISJET is a general predictive, PC-based, flow visualization tool to portray clearly the evolution and interaction of the multiple buoyant jets discharged at different angles to the ambient tidal current. It combines an extensively validated Lagrangian jet mixing model, JETLAG, with computer graphics techniques to trace the path and mixing characteristics of a group of arbitrarily inclined jets in three-dimensional space, in a uniform or density-stratified crossflow. JETLAG has been adopted for use for the environmental impact assessment of the Hong Kong Strategic Sewage Disposal Scheme (SSDS) and the Stage II Shanghai Combined Sewerage Project, and the post-operation monitoring of the Sydney Deep Water Ocean Outfall Scheme. 

Unlike traditional Eulerian models, VISJET caters for virtually any combination of discharge, ambient flow, and stratification conditions. The model provides informative graphics and enables data interrogation - including the determination of the degree of merging of a group of interacting plumes in user-defined cross-sections. Alternatively, it can be used for computing a single arbitrarily-inclined buoyant plume in a current, with a 3D trajectory - for a variety of applications. Development is also underway to generalize the graphics and visualization approach to other important applications including atmospheric discharges.

Reference

Lagrangian modeling and visualization of rosette outfall plumes
Joseph H.W. Lee, Valiant Cheung, Wenping Wang, Sebastian K.B. Cheung
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