The transition of a line plume to round plume

Environmental Fluid Mechanics - Buoyant turbulent plumes are often categorized by their geometry and described as either round plumes, issuing from a point source, or line/planar plumes, issuing...     

Buoyancy distribution in a filling box segmented by a planar jet

Environmental Fluid Mechanics - Air curtains are produced by thin vertical planar air jets and are used to prevent exchange flows between two fluids with a horizontal density gradient. They have...     

Energetics of mixing for the filling box and the emptying-filling box

The mixing efficiency of a plume in a filling box and an emptying-filling box is calculated for both transient and steady states. The mixing efficiency of a plume in a filling box in an asymptotic state is 1/2, independent of the details of this state or how the plume is modelled. The mixing efficiency of a plume in an emptying-filling box in steady state is \(1 - \xi \), where \(\xi = h/H\), the depth of the ambient layer h normalised by the height of the box H. A deeper mixed layer therefore corresponds to a higher mixing efficiency. These results shed light on the interpretation of mixing efficiencies of open and closed systems.

     

Cooling tower plume abatement and plume modeling: a review

Visible plumes above wet cooling towers are of great concern due to the associated aesthetic and environmental impacts. The parallel path wet/dry cooling tower is one of the most commonly used approaches for plume abatement, however, the associated capital cost is usually high due to the addition of the dry coils. Recently, passive technologies, which make use of free solar energy or the latent heat of the hot, moist air rising through the cooling tower fill, have been proposed to minimize or abate the visible plume and/or conserve water. In this review, we contrast established versus novel technologies and give a perspective on the relative merits and demerits of each. Of course, no assessment of the severity of a visible plume can be made without first understanding its atmospheric trajectory. To this end, numerous attempts, being either theoretical or numerical or experimental, have been proposed to predict plume behavior in atmospheres that are either uniform versus density-stratified or still versus windy (whether highly-turbulent or not). Problems of particular interests are plume rise/deflection, condensation and drift deposition, the latter consideration being a concern of public health due to the possible transport and spread of Legionella bacteria.

     

Experimental investigation of meandering jets in shallow reservoirs

Meandering flows in rectangular shallow reservoirs were experimentally investigated. The characteristic frequency, the longitudinal wave length and the mean lateral extension of the meandering jet were extracted from the first paired modes, obtained by a proper orthogonal decomposition of the surface velocity field measured by large scale PIV. The depth-normalised characteristic lengths and the Strouhal number were then compared to the main dimensionless numbers characterizing the experiments: Froude number, friction number and reservoir shape factor. The normalised wave length and mean lateral extension of the meandering jet are neither correlated with the Froude number nor with the reservoir shape factor; but a clear relationship is found with the friction number. Similarly, the Strouhal number is found proportional to a negative power of the friction number. In contrast, the Froude number and the reservoir shape factor enable to predict the occurrence of a meandering flow pattern: meandering jets occur for Froude number greater than 0.21 and for a shape factor smaller than 6.2.     

Experimental investigation of cross-shore sandbar volumes

Sandbars are critical to the cross-shore movement of sediment. Prediction of cross-shore sandbar volumes requires knowledge about the functional relationship of sediment transport rate conditions with waves, currents, base slope, sediment property and water depth. In this study, experiments on cross- shore sediment transport were carried out in a laboratory wave channel for initial base slopes of 1/8, 1/10 and 1/15. Using regular waves with different deep-water wave steepness generated by a pedal-type wave generator, bar volumes caused by cross-shore sediment transport are investigated for beach materials with the medium diameter of d₅₀?=?0.25, 0.32, 0.45, 0.62 and 0.80 mm. A non-dimensional equation for sandbar volume was obtained by using linear and non-linear regression methods through the experimental data and was compared with previously developed equations in the literature. The results have shown that the experimental data fitted well to the proposed equation with respect to the previously developed equations.     

Meiofauna response to a dynamic river plume front

 The benthic response to a plume front was studied in two areas of the northern Adriatic (Mediterranean Sea) differently influenced by the Po River freshwater input. Sediment samples were collected in June 1996 and February 1997 from 12 stations. The adopted sampling strategy was able to identify the front line in real time by satellite images and to locate sampling stations along an inner–outer plume gradient in order to cover the benthic area beneath the river plume, where enhanced biological production was expected, and open-sea sediments not directly influenced by freshwater inputs. Meiofaunal parameters were compared to the physical conditions and to phytodetritus inputs, organic matter accumulation and bacterial secondary production. The sediments of the Adriatic Sea were characterised by high concentrations of phytopigments (0.6 to 13.9 μg g⁻¹ for chlorophyll a and 1.2 to 17.7 μg g⁻¹ for phaeopigments) and biopolymeric organic carbon (0.15 to 3.02 mg g⁻¹). The plume system extended for a large sector of the northern Adriatic. In the northern area, a large and highly dynamic plume area was coupled with a sediment organic matter concentration significantly higher than in open-sea sediments. In the southern sector, where the plume area and the front line did not change markedly during the year, plume–benthic coupling was evident only in the sediments beneath the front, and corresponded to phaeopigment accumulation. Bacterial parameters and secondary production were high and significantly higher in the frontal area than at open-sea stations. Meiofauna density (1342 to 8541 ind. 10 cm⁻²) did not change either by season or between areas and was significantly correlated with phaeopigments and bacterial secondary production. Meiofauna displayed different responses to plume inputs in the two sampling areas. In the northern sector, meiofauna density was coupled with organic matter distribution and displayed highest values beneath the plume. In the southern sector, the densities of copepods, turbellarians and kinorhynchs displayed highest values under the front in summer, and the same applied to total meiofauna density in winter. Juvenile decapods and copepod nauplii significantly increased their densities in sediments beneath the front. Data presented in the present study suggest that plume inputs and frontal systems, enhancing phytodetritus accumulation and benthic bacterial response, might influence density, composition and distribution of meiofaunal assemblages. As river plumes are highly variable systems affecting the trophic characteristics of the sediments underneath, their dynamics should be considered when analysing mesoscale spatial changes of meiofaunal assemblages. Received: 30 November 1999 / Accepted: 24 May 2000     

Experimental investigation of bubbly flow and turbulence in hydraulic jumps

Many environmental problems are linked to multiphase flows encompassing ecological issues, chemical processes and mixing or diffusion, with applications in different engineering fields. The transition from a supercritical flow to a subcritical motion constitutes a hydraulic jump. This flow regime is characterised by strong interactions between turbulence, free surface and air–water mixing. Although a hydraulic jump contributes to some dissipation of the flow kinetic energy, it is also associated with increases of turbulent shear stresses and the development of turbulent eddies with implications in terms of scour, erosion and sediment transport. Despite a number of experimental, theoretical and numerical studies, there is a lack of knowledge concerning the physical mechanisms involved in the diffusion and air–water mixing processes within hydraulic jumps, as well as on the interaction between the free-surface and turbulence. New experimental investigations were undertaken in hydraulic jumps with Froude numbers up to Fr = 8.3. Two-phase flow measurements were performed with phase-detection conductivity probes. Basic results related to the distributions of void fraction, bubble frequency and mean bubble chord length are presented. New developments are discussed for the interfacial bubble velocities and their fluctuations, characterizing the turbulence level and integral time scales of turbulence representing a “lifetime” of the longitudinal bubbly flow structures. The analyses show good agreement with previous studies in terms of the vertical profiles of void fraction, bubble frequency and mean bubble chord length. The dimensionless distributions of interfacial velocities compared favourably with wall-jet equations. Measurements showed high turbulence levels. Turbulence time scales were found to be dependent on the distance downstream of the toe as well as on the distance to the bottom showing the importance of the lower (channel bed) and upper (free surface) boundary conditions on the turbulence structure.     

Composition of a deep scattering layer overlying a mid-ocean ridge hydrothermal plume

Three sets of zooplankton trawls with multiple nets were deployed in June 1990 within a deep (2000 m) scattering layer overlying the central hydrothermal vent field on the Endeavour segment of Juan de Fuca Ridge in the northeast Pacific. Trawl data were collected concurrently with temperature, salinity, light attenuation and acoustic (150 kHz) backscatter profiles. We describe the composition, size distribution and biomass of zooplankton collected in the net samples, and compare biomass distributions with physical characteristics of the hydrothermal plume. The nine discrete trawl samples (1 mm mesh) contained zooplankton biomass of between 0.3 and 21 mg dry wt m^-3 with the highest biomass samples coincident with large and positive (+20 dB) acoustic backscatter anomalies observed above the top of the hydrothermal plume. Lowest biomass samples were coincident with small, negative (-5 dB) backscatter anomalies within the core of the plume. Results suggest that the region within a hundred meters of the top of the plume was a zone of enhanced zooplankton concentration associated with nutrition enrichment related to the plume. In contrast, the plume core was a zone of faunal depletion, presumably linked to adverse plume chemistry. The species composition and size distribution profiles from net samples revealed that the epi-plume assemblage contained several trophic levels of bathypelagic fauna, but did not contain benthic larvae or vent-related benthopelagic fauna.     

Experimental investigation of the near-surface flow dynamics in downburst-like impinging jets

Environmental Fluid Mechanics - Downbursts are strong downdrafts that originate from thunderstorm clouds and create vigorous radial outflows upon hitting the ground. This study is part of the...     

Experimental investigation of false positive errors in auditory species occurrence surveys

False positive errors are a significant component of many ecological data sets, which in combination with false negative errors, can lead to severe biases in conclusions about ecological systems. We present results of a field experiment where observers recorded observations for known combinations of electronically broadcast calling anurans under conditions mimicking field surveys to determine species occurrence. Our objectives were to characterize false positive error probabilities for auditory methods based on a large number of observers, to determine if targeted instruction could be used to reduce false positive error rates, and to establish useful predictors of among-observer and among-species differences in error rates. We recruited 31 observers, ranging in abilities from novice to expert, who recorded detections for 12 species during 180 calling trials (66,960 total observations). All observers made multiple false positive errors, and on average 8.1% of recorded detections in the experiment were false positive errors. Additional instruction had only minor effects on error rates. After instruction, false positive error probabilities decreased by 16% for treatment individuals compared to controls with broad confidence interval overlap of 0 (95% CI:--46 to 30%). This coincided with an increase in false negative errors due to the treatment (26%;--3 to 61%). Differences among observers in false positive and in false negative error rates were best predicted by scores from an online test and a self-assessment of observer ability completed prior to the field experiment. In contrast, years of experience conducting call surveys was a weak predictor of error rates. False positive errors were also more common for species that were played more frequently but were not related to the dominant spectral frequency of the call. Our results corroborate other work that demonstrates false positives are a significant component of species occurrence data collected by auditory methods. Instructing observers to only report detections they are completely certain are correct is not sufficient to eliminate errors. As a result, analytical methods that account for false positive errors will be needed, and independent testing of observer ability is a useful predictor for among-observer variation in observation error rates.     

Experimental and numerical investigation of density current over macro-roughness

The dynamics of density current over a bottom covered by macro-roughness elements were investigated by laboratory experiments and a computational model using large eddy simulations. The macro-roughness considered had significant size in comparison with the scale of density current. Five different roughness conditions were considered, namely flat bottom (for reference), half spheres, fine gravels, medium gravels, and large gravels. These bottom conditions had variations in roughness element size, shape, angularity, and spatial configuration. The density current was a lock-exchange type with a density difference of 1% between the two fluids initially separated by a gate in the middle. In the computational model, the roughness was captured using two different methods depending on the size of the roughness elements. For the large roughness elements, i.e., the half spheres and the medium and large gravels, an immersed boundary method was used to resolve the surface of each gravel, which was obtained through 3D laser scanning. The realistic and physically correct placement of these scanned objects in the simulation domain was achieved using a computer tool which can detect the collision of rigid bodies and simulate their dynamics. For the fine gravels, a rough wall function was used. The computational model was validated with the data measured in the experiments, including front position and velocity, and point velocity measurement within the current. The results show that density currents over macro-roughness have distinct behavior from those over a smooth boundary. The characteristics (size, angularity, and pavement pattern) of the macro-roughness play a key role in the current development. Macro-roughness significantly retards the front propagation and enhances entrainment.     

Analytical solutions for flow induced by a vertically distributed turbulent plume

Environmental Fluid Mechanics - Analytical solutions are developed for the flow induced by a vertically distributed turbulent plume in an otherwise quiescent uniform environment. The plume...     

Experimental investigation of channel flow through idealized isolated tree-like vegetation

Environmental Fluid Mechanics - Riparian and floodplain tree-like emergent vegetation alter significantly the flow field and lead to complicated three-dimensional flow patterns, characterized by...     

Verification of the plume rise/dispersion model USPR: plume rise for single stack emissions

Despite proliferation of the use of air pollution models for regulatory application, major discrepancies still occur between models and also between models and observations, especially when oversimplistic models are used. The problem of predicting plume rise (and subsequently ground level concentrations) from a single source is evaluated here in terms of an integral plume rise and dispersion model (USPR) which encompasses both bouyant rise and turbulent spreading; thus avoiding the problems of the concatenation of separate plume rise and dispersion models. The wide range of validity of the USPR model is demonstrated is terms of plume rise by comparison with the highly buoyant GCOS and Kincaid plumes as well as with dense effluents. It is also shown to be in agreement with Briggs' two-thirds law when the restrictions applicable to the latter model are imposed.     

Determination of the entrainment coefficient of a pure plume using the salt-bath technique

Environmental Fluid Mechanics - The entrainment coefficient of the pure plume, the ratio of the radial velocity of the entraining fluid at the edge of the plume to the axial velocity, is...     

Feeding in a larval fish assemblage: the nutritional significance of an estuarine plume front

Gut fullness of larval fishes was used to determine the nutritional significance of an estuarine plume front off Botany Bay, SE Australia, on three days in March/April 1990. Fishes were captured in three different water masses (estuarine plume, front and shelf water), each separated by 200 m, using a 260 m mesh purse-seine net. Overall, the gut-fullness index (GFI) of 260 fish larvae combined from eight families (Gerreidae, Mugilidae, Mullidae, Sparidae, Blenniidae, Kyphosidae, Monodactylidae, Pomacentridae), was significantly greater in the plume and front water on two of the sampling occasions, but no difference was detected on the third occasion. Trends in GFI among families were inconsistent with respect to the front. The mugilid (Liza argentea) fed equally and abundantly in all water masses (gut fullness>90%), while the kyphosid (Kyphosus spp.) had a significantly greater GFI in the plume compared to the shelf water. In general, the response of GFI to the front varied between dates and amongst taxa. Diet analysis showed that mugilids selected for copepod nauplii within the plume and shelf water [alpha selectivity index (ASI) of (0.22 to 0.98)] and for harpacticoid copepods within the front (ASI=0.92), representing characteristic diets identified by canonical discriminant analysis. Kyphosids selected positively for copepod nauplii in all three water masses on both occasions (ASI>0.96). In general, diets of larval fish were taxon-specific, and responded variably to the three habitats. Comparison of diets across water masses was complicated by the large number of families, which rarely occurred in all water masses. The nutritional significance of an estuarine plume front varies between species of larval fish, and there are no obvious trends that can be applied to the larval fish community in general.     

Experimental investigation into the effects of suspended sediment on fertilisation, larval survival and settlement in a scleractinian coral

Laboratory and field experiments were used to determine whether high (≃100 mg l⁻¹), low (≃50 mg l⁻¹) and control (≃0 mg l⁻¹) levels of suspended sediment affected fertilisation, larval survival, and larval settlement in the scleractinian coral Acropora digitifera (Dana, 1846). Both high- and low-sediment treatments significantly decreased fertilisation, but post-fertilisation embryonic development was not inhibited by suspended sediments. Larval survival and larval settlement were significantly reduced in high- and low-sediment treatments. No difference was found between high- and low-sediment treatments in any of the three post-spawning processes investigated, suggesting that they are susceptible to sediment concentrations which are not exceptionally high even under natural conditions (>50 mg l⁻¹). The introduction of an additional stress in the form of high levels of suspended sediments coupled with naturally high variability in recruitment may have a considerable effect on the successful supply and settlement of coral larvae to a reef. Given that many coral communities are open reproductive systems, the consequences of disturbance events are not likely to be restricted to the impact area. Recruitment to a population may be reduced significantly in the presence of high levels of suspended sediments because of effects on larval survival and settlement. Recruitment of larvae to adjacent populations may also be affected due to a decreased fertilisation success and potential increases in mortality of larvae passing through the affected site. Received: 13 August 1998 / Accepted: 22 July 1999