Philip J. W. Roberts

Application of optical techniques to the study of plumes in stratified fluids

Digital techniques for the correction of signal distortions that arise in planar laser-induced fluorescence (PLIF) measurements (by PC-based video digitizing systems) of jets and plumes and the production of flow statistics in both time and space are reviewed. The entire concentration field is repeatedly imaged in 130 s intervals over hundreds of thousands a points in a plane. By the recognition of signal distortion sources and the employment of corrections, a clearer picture of tracer concentrations may be realized. Fluorescence studies are made with a planar sheet of laser light 430 mm tall and 1.5 mm thick. The fluorescence excitation produced from trace concentrations of Rhodamine 6G is used to visualize and measure the propagation of a jet or plume in a density stratified laboratory tank. The emitted light is collected by a CCD camera in a 512 × 480 pixel format over a 940 × 715 mm field of view. The captured images are corrected for transverse laser sheet intensity distribution; laser beam attenuation; refraction; lens vignette; time varying and spatial noise; digitization aspect ratio; camera response. The measurement and methods of correction are discussed in detail. The resulting image data can then be used to collect tracer concentration statistics for jets and plumes. Instantaneous (i.e. over 130th of a second intervals), average, maximum, minimum, standard deviation, and coefficient of variation are given as introductory examples of image statistics realizable for a buoyant jet.

A Multidisciplinary Study of Spatial and Temporal Scales Containing Information in Turbulent Chemical Plume Tracking

This report describes the results of a multidisciplinary study of turbulent chemical plume tracking of blue crabs and autonomous agents. The study consists of a coordinated investigation of animal behavior, fluid mechanics, strategy simulations, and chemical sensing. The objective is to provide a comprehensive understanding of chemical plume tracking in a single biological system and to prescribe strategies that are effective for autonomous agents. The consensus of the study is that spatial variation in the plume, measured by sampling at multiple locations simultaneously, yields information that is useful for plume tracking. Behavioral investigations reveal that blue crabs demonstrate the ability to detect the chemical plume and use lateral movements to avoid losing contact with the odor. Blue crabs move rapidly towards the source, strongly suggesting that temporal comparisons of odor properties are not employed during navigation. Analysis of the concentration fields reveals that a spatial correlation between spanwise-separated sensors indicates the relative direction of the plume centerline over short time periods provided the sensor spacing is scaled appropriately relative to the plume. Similarly, simulations of tracking strategies reveal an optimal separation for the sensors at a distance roughly equal to the plume width; both smaller and larger sensor spans degrade tracking performance. The simulations further reveal an optimal sensor size above which the fine details of the concentration distribution are obscured and below which there is insufficient contact with the odor to enable effective navigation. Finally, analysis of the chemical signal shows that the frequency dependent correlation function between two (or more) sensors indicates the relative position of the source.

Plume tracking and dilution of effluent from the Boston sewage outfall.

Field surveys were conducted on the Boston sewage outfall plume to test and certify the outfalls initial dilution in the near field and to investigate its dispersion in the far field. Rhodamine WT dye was added to the effluent at the treatment plant at a constant concentration over a 6-h period and tracked offshore over three days. During the near-field surveys, the current was flowing closely parallel to the diffuser, resulting in the wastefield spreading laterally as a dynamic density current at a rate that was closely predicted by theoretical equations. The plume was submerged by the oceanic density stratification, with a minimum initial dilution of about 102 within a few tens of meters from the diffuser. The initial dilution and the other near-field characteristics were in good agreement with predictions of mathematical models and with the physical model study on which the diffuser design was based. After a travel time of 24h, the dye patch was still intact and oceanographic mixing and dispersion had increased dilution by a factor of about two to more than 200:1. After 48h, the plume had broken into large patches, and most dilutions considerably exceeded 400 with an average dilution of order 1000. For the approximately 52h that the dye patch was followed in the far field, mixing was due to lateral diffusion; vertical mixing was negligible. This slow vertical mixing is due to the stable density stratification in the water column. The outfall is performing as designed. The field surveys provided a strong confirmation of the ability of small-scale laboratory model studies to replicate and predict the near-field characteristics of ocean wastewater outfalls. They also increase the confidence that mathematical models can be used to reliably estimate initial dilution under other effluent flows, oceanographic conditions, and stratification regimes.

Surface dilution of round submerged buoyant jets

The behavior of a submerged vertical round buoyant jet as it interacts with a free surface in an unstratified, stagnant receiving fluid is considered. Conventional analyses for estimating dilution within such a jet consider only the mixing due to the rising jet and neglect further dilution in the region where the jet is deflected and begins to spread radially along the surface. The present study explicitly accounts for the entrainment within a region that typically extends a few flow depths away from the source. This region is analyzed as a radial internal hydraulic jump and computational results indicate dilution increases even greater than within the submerged jet.Two experimental studies were performed to measure dilution within the radially spreading flow region and supplement data previously collected in other studies. The data indicate that the surface dilution directly above the jet can be predicted on the basis of allowing for a thin blocking layer at the surface. The data also indicate a three to...

Experimental Studies on Vertical Dense Jets in a Flowing Current

Experiments were performed using three-dimensional laser-induced fluorescence on turbulent vertical dense jets in flowing currents typical of brine disposal from seawater desalination plants. The flows are complex and different phenomena can dominate at different locations and at different current speeds, indicating that predicting these flows numerically will be quite challenging. At low current speeds, the rising and falling flows are almost vertical with some interference between them and the bottom flow spreads upstream as a wedge. At higher current speeds the wedge is expelled; the ascending flow is still almost vertical, but the descending flow is more gradual so the jet impacts the lower boundary farther downstream. Dilutions at the terminal rise height and impact point increase with increasing current speeds. Cross-sectional profiles of tracer concentration are neither axially or self-similar. In the descending flow, at low or intermediate current speeds, the plume is much taller than it is wide, the peak concentration occurs much closer to the top, and fluid can detrain from the jet. At higher current speeds, the profiles initially approach radial symmetry, but develop a kidney shape due to formation of two counter-rotating vortices farther downstream. These vortices cause the jet to almost completely bifurcate after impacting the bottom.

Marine wastewater outfalls and treatment systems.

Wastewater disposal by marine outfalls is proven and effective and is a reliable and cost effective solution with minimal environmental impacts. The design and siting of submarine outfalls is a complex task that relies on many disciplines including oceanography, civil and environmental engineering, marine biology, construction, economics, and public relations. Marine Wastewater Outfalls and Treatment Systems brings these disciplines together and outlines all tasks involved in the planning and design of a wastewater system involving a marine outfall. This book concerns the design of marine wastewater disposal systems: that is an ocean outfall plus treatment plant. All aspects of outfall design and planning are covered, including water quality design criteria, mathematical modelling of water quality and dilution, gathering required oceanographic data, appropriate wastewater treatment for marine discharges, construction materials for marine pipelines, forces on pipelines and outfall design, outfall hydraulics, outfall construction, tunnelled outfalls, operation and maintenance, monitoring, case studies are discussed and methods for gaining public acceptance for the project are presented. Finally, costs for many outfalls around the world are summarized and methods for estimating costs are given. This is the first book to consider all aspects of marine outfall planning and construction. The authors are all extensively involved with outfall schemes and aware of recent developments. The science and technology of all aspects of outfall discharges into coastal waters and estuaries of treated municipal or industrial wastewater has advanced considerably over the past few years. Marine Wastewater Outfalls and Treatment Systems provides an up to date and comprehensive summary of this rapidly developing area. ISBN: 9781843391890 (Print) ISBN: 9781780401669 (eBook)

New experimental techniques for validation of marine discharge models

Abstract A new three-dimensional Laser-Induced Fluorescence (3DLIF) system was applied to study the mixing of a produced water outfall discharge typical of that used in offshore production operations. The outfall consists of a pipeline terminating in a multiport diffuser discharging into water about 12 m deep. Scaled laboratory experiments were done using conditions based on measured oceanographic conditions near the site. The current was perpendicular to the diffuser, and the receiving water was linearly stratified. The model was undistorted and the jet densimetric Froude number was the same as in the prototype. This scaling results in correct modeling of the source volume, momentum, and buoyancy fluxes, and equality of other line source dimensionless parameters. The experiments were conducted in a constant refractive-index environment which enabled detailed three-dimensional concentration measurements in the near field mixing zone to be obtained despite the density variations in the flow field. The mixing processes were found to be complex. The jets discharging upstream were quickly swept downstream, where they began merging with the downstream jets. Lateral mixing caused the concentration profiles to quickly become laterally homogeneous. Initially, dilution increased rapidly with distance from the diffuser, but farther away the rate of increase of dilution slowed as the turbulence became affected by the ambient density stratification. The dilution eventually becomes approximately constant; the location where this occurs defines the end of the near field, where mixing is primarily due to turbulence and other processes induced by the discharge itself. The end of the near field is marked by collapse of the self-induced turbulence under the influence of the ambient stratification. The results were compared to predictions of commonly used mathematical models of marine mixing zones: RSB, UM3, and CORMIX. The best predictions of the near field dilution and the length of the near field were by RSB. UM3 gave good predictions of the near field dilution, but CORMIX overestimated the dilution and rise height substantially. It is believed that this overestimation by CORMIX is due to its approximation of the diffuser as a row of vertically discharging jets of equivalent total momentum flux; this is clearly a poor approximation to the diffuser conditions modeled here. 3DLIF is an exciting new technique that can give unprecedented insight into the complex hydrodynamics and processes occurring in buoyancy-modified mixing zones and to improvement of near field mathematical models. Experiments on other discharge configurations are continuing.

Multiport Diffusers for Dense Discharges

AbstractComprehensive laboratory experiments on multiport diffusers for dense effluents such as brine into stationary receiving waters are reported. Tracer concentration fields were mapped in three dimensions by laser-induced fluorescence (3DLIF). The effect of port spacing is described by s/dF where s is the port spacing, d the nozzle diameter, and F the jet densimetric Froude number. For s/dF>∼2, the jets did not merge, and the results followed expected asymptotic solutions for single jets. For s/dF<2 the jets merged, but did not follow expected asymptotic line source solutions. As the port spacing was reduced, the rise height and other geometrical variables decreased. The dilutions also decreased, but much more rapidly than predicted. These were caused by Coanda effects and merging. The Coanda effect caused an under pressure on the interior jet surfaces which caused them to curve more sharply inwards. This shortened their trajectories, reducing the external surface area available for entrainment. Jet m...

Dense Jet Discharges in Shallow Water

AbstractExtensive Experiments on inclined dense jets typical of brine discharges into shallow water are reported. The experiments were conducted with nozzles oriented at 30°, 45°, and 60° to the horizontal and the spatial variations of tracer concentrations were measured by three-dimensional laser-induced fluorescence (3DLIF). Three flow regimes were identified: deep water, surface contact, and shallow water. The regimes depend on the value of dF/H, where d is the nozzle diameter, F the jet densimetric Froude number, and H the water depth; criteria were presented for the transitions between them. Flow images revealed complex three-dimensional interactions with the free surface, especially for steep nozzle angles in shallow water. Dilutions at critical points and their locations were measured. For deep water, all results followed those previously reported for fully submerged jets. As the depth decreases (or dF/H increases) to the surface contact regime, dilutions begin to decrease. Tracer concentration pro...

Field and Model Studies of the Boston Outfall

Near- and far-field oceanographic measurements of the plume from the Boston outfall were made over two days in April 2001. The results were consistent with the original physical model studies on which the diffuser design was based. New laboratory experiments were run to simulate the actual conditions at the time of the field tests using three-dimensional laser-induced florescence (3DLIF) to measure dilution. The 3DLIF results were very close to the field observations and provided considerable additional insight into the near-field mixing processes: the plumes from each port of the multiport risers merge, first with other ports on the same riser, and then with similar plumes from adjacent risers; mixing transverse to the current direction then gradually erases any vestiges of lateral concentration variations. The field results were consistent with predictions of the mathematical model NRFIELD. Measurements were also made in the far field on the second sampling day. For the observed travel times of up to 24...