Boundary impingement and attachment of horizontal offset dense jets

A horizontal outfall near the seabed is occasionally adopted in engineering practice for the submerged discharge of dense jets. For example, for desalination discharges, it is done due to cost considerations and the relative ease of construction, while for the discharge of mine tailings the configuration is taken to minimize the mixing of the tailings with the overlying ambient waters. Upon exiting the nozzle, the horizontal dense jet will curve towards and impinge on the seabed due to the negative buoyancy. The boundary interactions would intensify if the offset between the outfall and seabed, or the bed proximity, reduces. If the outfall is placed sufficiently close to the boundary, a dynamic attachment, also known as Coanda attachment, can take place. The Coanda attachment is undesirable for desalination discharges since the mixing and dilution of the brine discharge will be significantly impaired. At the same time, it is desirable for mine tailing discharges for the same reason. Quantitative assessment of the boundary impingement and attachment of the offset dense jet is thus of interest for the specific design objectives. In the present study, an experimental investigation of this subject was conducted using Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) as the experimental techniques. Coanda-influenced and free horizontal offset dense jets were investigated by varying the discharge densimetric Froude number, Fr, as well as the bed proximity parameter z(0)/L-M. Various impingement characteristics are established through the analysis of the experimental results. In particular, it is found that the velocity and concentration axes begin to diverge at the impingement point due to the differences in boundary conditions at the bottom surface. (C) 2010 International Association of Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved.