Mixing of turbulent advected line puffs

Abstract

(Uncorrected OCR) Abstract of the thesis entitled �ixing of Turbulent Advected Line Puffs�submitted by CHU Chi Keung Paul for the degree of Doctor of Philosophy at the University of Hong Kong in June 1996. i Abstract Wastewater after land-based treatment is often discharged in the form of buoyant jet into a river or tidal flow via a sewage outfall. The initial dilution achieved by the outfall is of great importance to the effectiveness of such a disposal scheme. Limited by inadequate experimental techniques, detailed information on the scalar and flow field of the two important asymptotic regimes of a buoyant jet in crossflow, the momentum-dominated far field (MDFF) and buoyancy-dominated far field (BDFF), are scarce in previous studies. In particular, the dilution prediction and our physical understanding of the vortex-pair flow in the MDFF need much improvement. The methodology of advected line puffs (ALP) and thermals (ALT), which overcomes some major difficulties in performing experiment in the MDFF and BDFF, is adopted in the present investigation to study the mixing of a turbulent buoyant jet in crossflow. Digital image processing techniques are applied to quantify the visual properties of ALP/ALT from the flow visualization (side view). The results strongly support the feasibility of the present methodology. The numerical coefficients of the self-similar solutions in MDFF/BDFF are obtained. Contrary to previous studies, the spreading rates of ALP and ALT are found to be quite similar. The cross-sectional instantaneous and time-averaged scalar field of ALP are measured using non-intrusive laser-induced fluorescence techniques; the cross-sectional flow field is determined using laser doppler anemometry. Detailed information about the turbulence structure in the developed stage including instantaneous maximum concentration, turbulent intensity and maximum vertical velocity are obtained. The flow is highly intermittent with coherent rotating turbulent patches that randomly move around the entire section. The intermittency is generally less than 1, 7 < 1; ambient fluid can be present anywhere in the cross-section. The time-averaged measurements show clearly the kidney-shaped (often bifurcated) scalar field and the vortex-pair flow. A similar study of jet in coflow is also performed. Using the top-hat average velocity as the characteristic velocity in a spreading hypothesis, an integral model is formulated and the predictions show good agreement with the experiments. The correlation of conceptual model parameters with physical entities is established. It is found that the edge defined by the top-hat profile is a proper representation of jet boundary in both the near and far field. The significant loss of ALP impulse, as predicted by a 2D line puff turbulence model, which implies the existence of added mass in the MDFF, is experimentally confirmed. Solely based on the experimentally determined spreading rate, [5 ~ 0., and added mass coefficient, k ~ , an integral model can be developed. The predictions compare favorably with the present and ii previous experimental results. Flow visualization of the major vortical features of a jet in crossflow are studied in relation to the mixing. Although the wake vortices have little effect on the scalar field, contrary to previous studies, it is observed that their origin is not necessarily related to the wall boundary layer but is closely related to the vortex shedding in the near field.