The purpose of this research is to improve the current understanding of the mixing processes and dilution of negatively buoyant jets impacting an inclined boundary in a stationary environment. This work has direct application to the discharge of brine from a desalination plant, but has broader implications for other industrial discharges. Improvements in our current understanding were be gained from a comprehensive experimental program and comparisons with associated numerical models. The predictive capabilities of these numerical models have key benefits for industry in design and environmental impact assessment processes, and outcomes of compliance and monitoring processes. In this context predictive capabilities of flow scale, dilution and location are particularly important for these complex flows. Thus the primary motivation for this study is embedded in the application to discharges from large-scale desalination plants that typically result in jets of dense effluent into the ocean, near the sea floor, and it is necessary to demonstrate that these discharges meet environmental constraints within a prescribed mixing zone. The relatively dense effluent initially rises towards the ocean surface because of its initial momentum, but subsequently falls back to the ocean floor because of its negative buoyancy.

There is a long history (70 years +) of research into the near field mixing and dilution of positively buoyant jets, as they are the result of the more common fresh-water municipal discharge into a marine environment. While the behaviour of these flows are governed by the same principles, there is significantly less research into the behaviour of negatively buoyant discharges. It has been noted by previous researchers that there is a lack of robust experimental results in this area, in particular assessing the dilution characteristics in the impact region, when the flow returns to the ocean floor (bottom boundary). The most recent studies have provided some clarity on this issue, but these studies have been limited to flows impacting a horizontal boundary. Consequently this research program included a comprehensive experimental program that generated high quality dilution data for negatively buoyant discharges impinging on boundaries with different slopes. Experiments were carried for discharge angles ranging from 30◦ to 60◦ because these are representative of potential industrial applications. The boundary angle was varied from 0◦ to 20◦, which is consistent with previous numerical studies for these discharges on sloped boundaries. Variations in the initial discharge densimetric Froude number provided opportunities to assure the quality of the data because the flow is expected to scale on this dimensionless parameter. A Laser-Induced Fluorescence (LIF) system was employed to measure planar concentration fields along the centreline of the flow. This measuring system has provided detailed information about the scale, dilution and location of the flow in the impact region.

Impact point dilution and location data have been extracted from a unique and comprehensive experimental data set that covers lower boundary (seabed) angles from 0◦ to 20◦ , discharge angles of 30◦ , 45◦ and 60◦ , non-dimensional source heights ranging from -0.09 to 5.04, and initial densimetric Froude numbers ranging from 10.98 to 52.77. This data has been compared with predictions from a relatively simple semi-analytical model. These comparisons show that predictions of the impact point dilution, and the horizontal and vertical coordinates of the impact point, are in good agreement with the measured data. This agreement supports the use of the model to predict conditions at the impact point for lower boundary (seabed) inclinations up to 20◦ below a horizontal reference plane. This agreement also suggests that the mixing prior to reaching the impact point is not adversely affected by the presence of the lower boundary.

The boundary data presented demonstrates that the mixing achieved in the immediate proximity of the impact point is most sensitive to boundary inclination when the effluent is released at 60◦, and that notable additional mixing (a 26% increase) can be achieved within 2 boundary length scales of the impact point. The relationship between the boundary length and the flow path length to the impact point enables scaling of the above results to a range of marine disposal scenarios for large scale desalination facilities.

This new information is potentially valuable when considering compliance of these different scenarios in the context of pre-defined regulatory mixing zones, or alternatively in defining appropriate regulatory mixing zones for a given discharge scenario.