The evolution of discharges with two and three dimensional trajectories
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
In the literature there is limited data available on the behaviour of discharges with three dimensional trajectories, although these are the most commonly found in the flows exiting (ocean) outfalls. The necessary three dimensional trajectory data requires cumbersome experimental systems and specialised laboratory setup. Therefore, results from two dimensional trajectory experiments are commonly extrapolated to enable prediction of the flow trajectories and dilutions of discharges that follow three dimensional paths. Importantly, there are also still some aspects of the behaviour of discharges with two dimensional trajectories that are not entirely clear. Non-buoyant flows discharged at an angle to the ambient flow, oblique discharges, behave either like a strongly advected jet or a momentum puff, depending on the discharge angle. Previous research indicated that the transition angle lies between 20° and 40°. Furthermore there is no clear distinction made between the cross sectional flow structure of buoyant and non-buoyant discharges in a cross flow, advected thermals and momentum puffs, and flow prediction models, like Visjet or Corjet, which assume these flows spread at the same rate.
The primary objectives of this research are to create a more comprehensive dataset for discharges with three dimensional trajectories; to ascertain the transitional discharge angle that separates flows that behave as a strongly-advected jet or a line momentum puff, and to establish whether there is a difference in the cross sectional concentration profiles of buoyant and non-buoyant discharges in a cross flow. The application of a double Gaussian distribution will be carried out for line advected thermals complimenting earlier work with line momentum puffs. The work feeds into these models and therefore can have an indirect impact on outfall design. A light attenuation system is employed to study the various discharges and the dynamic range is extended by developing a multiple dye system. This enables the evolution of the discharges to be measured over much greater distances. The light attenuation system is described in detail to substantiate the experimental results.
The new data shows that the mean tracer distributions for buoyant and non-buoyant discharges in a cross flow are distinct, with the former having a greater peak separation than the latter. This leads to differences in the relationships between peak and centreline concentrations. In addition, while the experimental spreading rates for the two flows are similar, the different forms of the puff and thermal profiles require distinctly different spreading rates for standardised flow profile models, such as the ‘top hat’ models. Differences are also evident in the conversions needed to estimate peak values from the predictions of these standardised profiles and the implications of these differences are discussed in the context of integral models, which are commonly employed to predict the behaviour of such flows. The experimental data from the oblique discharge experiments showed that flows discharged at acute angles up to 32.4° displayed strongly advected jet behaviour, flows discharged at obtuse angles greater than 39.0° displayed momentum puff behaviour, while the intermediate 35.9° discharge appeared as some combination of the aforementioned flows.
A comprehensive experimental investigation into the behaviour of discharges with 3D trajectories has been carried out. The flows were released horizontally at an angle of 90°, 45°, or at 22° to the ambient current and the ambient to initial velocity ratio varied from 0.0042 to 0.057, extending the range of initial conditions previously considered. The experiments show limited variability in trajectory and dilution results around the average values. This provides the basis for conducting future experiments with fewer repetitions. The flows with initial discharges angles of 90° and 45° to the ambient motion, display initially line momentum puff and afterwards advected thermal behaviour. The consistent appearance of the characteristic double peaked distributions alleviates previously published concerns about the ability to transfer the understanding gained from discharges that follow a two dimensional path. However, the different orientations of the two peaks within these flow regimes introduces additional complexity into the transition region. In experiments with an initial discharge angle of 22° the double peak distribution did not develop until the flow evolved into an advected thermal, which was consistent with expectations based on the experiments with oblique discharges.