Stratified Zone Research Articles

Detrainment from a turbulent plume produced by a vertical line source of buoyancy in a confined, ventilated space

AbstractNew experiments are presented which explore the dynamics of a turbulent buoyant plume produced by a vertically distributed linear source of buoyancy of strength $f$ per unit height. In a uniform environment, the plume volume flux increases with height from the base of the source, $z$, as $q(z) = {2^{-1/3}} {\pi }^{2/3} \alpha ^{4/3} f^{1/3} z^2$ where the entrainment coefficient, $\alpha = 0.09\pm 0.01$. In an enclosed space, with a net upward vertical ventilation flow $Q_V$, the buoyant plume generates a steady ambient stratification. The lowest part of the space, $z<h_i$, where $q(h_i)=Q_V$, is filled with fluid supplied by the ventilation flow and there is a net upflow in the ambient. Above this, $z>h_i$, the ambient fluid is linearly stratified with a reduced gravity gradient $f/Q_V$, and has no net vertical motion. Instead, for $z>h_i$, the time-averaged volume flux in the plume equals the ventilation flow. The intermittent entrainment of ambient fluid into the plume is now matched by intermittent detrainment from the plume, and the mean buoyancy in the plume relative to the ambient remains constant. The supply of fresh ventilation fluid to the ambient in the linearly stratified zone only occurs through the local detrainment and consequent horizontal intrusion of fluid from the plume. This has key implications for design of ventilation systems, in which there may be vertically distributed sources of buoyancy.

Traditional Irrigation Practices, High Crop Diversification and Multiple Agricultural Cycles in Wastewater Irrigation Farming in Peru Urban Areas, Zambia

Studies conducted on peri urban wastewater irrigation agriculture in Zambia have not adequately tackled issues pertaining to farmer managed irrigation systems in heavy metal contaminated wastewater irrigation farming.The study focused on characteristics of farmer managed irrigation systems in wastewater irrigation farming contaminated with heavy metals at New Farm in Mufulira and Chilumba Gardens in Kafue. The study objectives were to ascertain the nature of irrigation practices and types of crops grown at the two study sites in Zambia. It was hypothesised that: (i) the nature of irrigation practices and types of crops were not significantly different at the two study sites in different seasons. (ii) the agro-ecological factors did not significantly influence the nature of irrigation practices and types of crops. The method comprised observation of sources of water supply, methods of irrigation and types of crops grown coupled with agro-ecological factors such as soil, slope and drainage at field plots located at regular intervals along transects established in the stratified sampling zones. The results indicated that farmer managed irrigation systems at the two study sites were characterised by multiple sources of water supply, multiple methods of irrigation, relatively high number of crops per study site equivalent to twenty types of crops grown per study site and relatively high number of crops per field plot ranging from one crop to ten crops per field plot coupled with multiple cycles of agricultural practices. The nature of irrigation practices and types of crops were influenced by a combination of agro-ecological factors. In conclusion, the farmer managed irrigation systems in wastewater irrigation farming were characterised by traditional irrigation practices, high crop diversification and multiple cycles of agricultural practices which were typical of traditional farmer managed irrigation systems. It can be argued that the crop cultivators have adapted the rural traditional irrigation systems’ practices to the peri urban wastewater irrigation systems in Zambia which confirmed the findings from other studies in developing countries. The study findings will be used to select and implement appropriate agricultural practices which can mitigate the negative effects of heavy metal contamination.

On summer stratification and tidal mixing in the Taiwan Strait

On continental shelves, a front that separates the sea into well-mixed and stratified zones is usually formed in warm seasons due to spatial variations of tidal mixing. In this paper, using eight years of in situ hydrographic observations, satellite images of sea surface temperature (SST) and chlorophyll-a (Chl-a) concentration, and results of a tidal model, we investigate summer stratification in the Taiwan Strait and its dependence on tidal mixing, upwelling, and river diluted water plumes. In most regions of the strait the dominant role of tidal mixing in determining the thermohaline structure is confirmed by the correlation between the two; there are some regions, however, where thermohaline structure varies in different ways owing to significant influences of upwelling and river diluted water plumes. The well-mixed regions are mainly distributed on the Taiwan Bank and in the offshore regions off the Dongshan Island, Nanao Island, and Pingtan Island, while the northern and central Taiwan Strait and the region south of the Taiwan Bank are stratified. The critical Simpson-Hunter parameter for the region is estimated to be 1.78.

Angular momentum transport by internal waves in stellar interiors

Internal gravity waves are excited in stellar interiors thanks to turbulent convective regions, to opacity bumps and to tides when there is a close stellar or planetary companion. Then, they propagate in stably stratified stellar radiation zones where they deposit/extract angular momentum by damping processes and corotation resonances. This modifies the rotational evolution as well as the internal mixing in stars. In this review, we will present the state of the art for modelling such transport mechanisms with a peculiar focus on the most recent advances taking into account the influence of rotation, shear, and magnetic fields on internal gravity waves. Next, we will discuss consequences for stars dynamical evolution and what can be learnt from stellar seismology.

Influence of physical forcing on mesozooplankton communities at the Ushant tidal front

The pelagic ecosystem around Ushant tidal front was studied over a spring–neap tidal cycle in September 2009. The system was divided in a mixed coastal zone, the frontal zone and a stratified oceanic zone, which remained stable from spring to neap tide. Standing stock, size structure and composition of the phytoplankton and mesozooplankton communities varied more intensely between zones than with the tidal cycle. Localized changes in chlorophyll a occurred due to forcing by wind and tides. At the front, a diatom bloom intensified from spring to neap tide, but generally phytoplankton communities maintained stable proportions and were numerically dominated by pico- and nanophytoplankton. Highest mesozooplankton diversity was encountered in the frontal zone where organisms from both sides are blended; yet highest biomass was found in the stratified zone 30nm west of the front. Temporal changes in the abundance of several zooplankton groups occurred and mirror trophic dynamics, physical processes and phytoplankton production. Differences between normalized biomass spectra from LOPC and image analysis were strongest during spring tide. The spectral slope steepened from a theoretical equilibrium value in the stratified zone to <−1 in the mixed zone reflecting the greater importance of small sized mesozooplankton in coastal marine ecosystems.

A Dynamic-Zone-Based Coordinated Ramp-Metering Algorithm With Queue Constraints for Minnesota's Freeways

Following about 40 years of successful deployment of coordinated traffic-responsive ramp control, a new generation is being developed for Minnesota's freeways based on density measurements, rather than flow rates. This was motivated from recent research indicating that the critical value of density at which capacity is observed is less sensitive and more stable than capacity, thereby allowing the opportunity for more effective control. The main goals of the new approach are to delay the onset of the breakdown and accelerate system recovery when ramp metering is disabled due to the violation of maximum allowable ramp waiting times. This is obtained by a dynamic zone partitioning of the freeway network to identify critical bottleneck locations and coordinated balancing of ramp delays, which aims to avoid mainline breakdown. The effectiveness of the new control strategy is assessed by comparison with the currently deployed version of the stratified zone metering algorithm through microscopic simulation of a real 12-mi 17-ramp freeway section. Simulations show a decrease in delays of mainline and ramp traffic and an improvement of 8% in overall system delays while avoiding maximum ramp delay violations.

The status of dental caries and related factors in a sample of Iranian adolescents

To describe the status of dental caries in a sample of Iranian adolescents aged 14 to 18 years in Qazvin, and to identify caries-related factors affecting this group. Qazvin was divided into three zones according to socio-economic status. The sampling procedure used was a stratified cluster sampling technique; incorporating 3 stratified zones, for each of which a cluster of school children were recruited from randomly selected high schools. The adolescents agreed to participate in the study and to complete a questionnaire. Dental caries status was assessed in terms of decayed teeth (DT), filled teeth (FT) and decayed, missing and filled teeth (DMFT). A multivariate regression analysis was used to determine statistically significant associations between DMFT and other variables. The study sample comprised 380 adolescents, 188 (49.5%) being male. The mean age of the adolescents was 15.42 (SD= 1.1) years, and the mean DMFT was 2.61 (SD=1.89). Boys had significantly higher DMFT scores than girls (P <0.05). The multivariate regression analyses revealed a significant relation between high DMFT scores and such variables as increasing age, male gender, lower levels of parental education, higher family income, lower frequency of tooth brushing and dental flossing, having a history of no visits to the dentist, and bad perception of own oral health. The present study reveals that Iranian adolescents have a poor oral hygiene, as very few subjects brush and floss their teeth on a regular basis. Although the incidence of caries was found to be moderate, it was influenced by demographic factors such as age and gender in addition to socio-behavioral variables such as family income, the level of education attained by parents, the frequency of dental brushing and flossing, and both the frequency and type of visit to a dentist.

The impact of magnetic fields on stellar structure and evolution

Magnetic fields have been detected at the surface of many stars, where they are responsible for various activity phenomena, and presumably they are present also in their deep interior. They channel stellar winds to large distance, thus increasing the loss of angular momentum. In stably stratified radiation zones, they tend to impose uniform rotation, and may thus interfere with the rotational mixing operating in these regions and influence the evolution of the host star. Moreover, certain magnetic configurations are known to be unstable, and they could produce some mixing of their own. We shall discuss these problems in the light of the latest developments.

Mechanism of formation of concentrically laminated spherules: implication to Randall’s plaque and stone formation

We report on the formation of calcium phosphate multi-laminated spherules via a polymer-induced liquid-like precursor (PILP) process. In this non-classical crystallization route, the precipitation of liquid-like amorphous calcium phosphate (ACP) particles is promoted using anionic polypeptide additives, and these droplets coalesce to form globules that later crystallize into spherulites. During crystallization of the amorphous globules, the polymer additive, as well as the waters of hydration, is excluded ahead of the crystallization front, but some polymer becomes entrapped within diffusion-limited zones. This results in the formation of concentric laminations with layers of variable density from organic-rich inclusions. The striking resemblance of these spherules with the crystals of the Randall's plaque and other laminated stones suggests that such biological structures may form via an amorphous precursor process as well. Given the organic-rich environment present in the urinary tract, one might expect a large amount of organic materials to become entrapped within the stratified zones of a forming stone during this type of solidification and transformation process.

Spatial and Temporal Evolution of Biogeochemical Processes Following In Situ Capping of Contaminated Sediments

In situ capping has recently emerged as a remedial method for contaminated sediments and involves placing a layer of clean material at the sediment-water interface. The biogeochemical response of native sediment following capping, as well as the redoxenvironmentsthatdevelopwithinthe cap, are currently unknown. Column experiments were performed using voltammetric microelectrodes to characterize spatial and temporal distributions of biogeochemical processes in capped sediments under stagnant and upflow conditions. Oxygen penetration into sand caps extended only a few centimeters, thus maintaining underlying sediment anaerobic. Chemical species indicative of heterotrophic organic matter degradation (Mn2+, Fe2+, organic--FeIII(aq), FexSy(aq), sigmaH2S) were observed in stratified zones below the oxic layer. The majority of the overlying cap was subject to iron-reducing conditions under stagnant flow, while upflow conditions led to a compression of the redox zones toward the cap-water interface. Controls confirmed that sediment capping induced an upward, vertical shift of biogeochemical processes into the overlying cap, with redox stratification conserved. The redox conditions within the cap, specifically the predominance of iron reduction, should allow for reductive contaminant attenuation processes to extend into the overlying cap. These findings improve our understanding of the dynamics of biogeochemical processes following capping of contaminated sediments.

First Observations of Benthos and Seston from a Submersible in the Lower St. Lawrence Estuary

Six dives with the submersible PISCES IV have permitted a unique description of the benthic and pelagic environments of a large, deep Canadian estuary. The estuarine floor and continental slopes are divided into five depth-dependent benthic zones. In order of decreasing depth are the Bathyal Trough Zone, the lnfaunal Zone, the Ophiura Zone, the Ice Rafting Zone and the Wave Base Zone. The zonal boundaries are based on changes in the faunal community, sediment texture, current energy, level of bioturbation and suspended particulate loading. Biological resuspension appears important in the Bathyal Trough and lnfaunal Zones. Current resuspension dominates the Ophiura and Ice Rafting Zones with storm waves additionally reworking the Wave-Base sediments. Seston characteristics are strongly influenced by the source and dynamics of the host water mass. The Surface Layer, the entrained outflow from the St. Lawrence River, is the source of most suspended matter found beneath. Large particles, mostly organo-mineral aggregates, become even larger with depth and indicate a rapid downward transfer of suspended sediment. The upper Intermediate Layer is complicated by stratified zones of turbulence that temporarily reduce the floe size. With the absence of such fine water structure, the lower Intermediate Layer is characterized by long chains of marine "snow" joined by delicate filaments. The Bottom Layer, a zone of increased turbulence, had aggregates breaking up into a haze of fine particles.

Development and Comparative Evaluation of Ramp Metering Algorithms Using Microscopic Traffic Simulation

Freeway ramp metering is an efficient freeway control that can ameliorate freeway congestion by limiting the number of vehicles entering the freeway. This study presents development and comparative evaluation of five ramp controls including no control, time-of-day plan, and three well-known ramp metering algorithms; ALINEA, FLOW and Stratified Zone through the use of microscopic traffic simulation. In this paper, ramp controls were developed and evaluated using Microscopic Traffic Simulation, AIMSUN NG. The simulation model of the Pacific Motorway in Queensland, Australia was used as a test-bed model. The results from the study indicated that ramp metering was basically found to be able to improve network performances up to 40% compared to no control. In terms of network and freeway mainline performance, ALINEA was superior to other algorithms under both normal and high traffic demand, whereas, Stratified Zone was the best algorithm for on-ramp performance. In terms of on-ramp performance, ALINEA was found to generate the lowest on-ramp performance. FLOW was found to be slightly superior to Stratified Zone. However, the trend was opposite for on-ramp performance. Gini coefficient was applied to measure the road user equity under the implementation of different algorithms. The results showed that FLOW was the most equitable algorithm, whereas, ALINEA was the worst. The results also indicated that network performance and on-ramp performance were trade-off in the presence of ramp metering operation.

Improving Queue Size Estimation for Minnesota's Stratified Zone Metering Strategy

Stratified zone metering (SZM), a successor of zone metering, was recently implemented in the United States in the Twin Cities. SZM not only restricts metering rates subject to the freeway capacity constraints but also limits the on-ramp waiting time to a predetermined threshold. In SZM, accurate queue size estimation is crucial because inaccurate queue size can violate the maximum wait time constraint or reduce the service quality of the main-line system. Currently, a uniform and pre-calibrated regression equation is adopted for ramp queue size estimation for all ramps. It has been verified that such an estimation method may lead to outstanding estimation errors for ramp queues. In this paper, different ramp queue estimation algorithms are proposed for ramps with different categories, depending on the counting errors of queue and passage detectors. For ramps with minor counting error, queue size is estimated on the basis of flow conservation. For ramps with significant counting error on passage detectors but minor counting error on queue detectors, the flow conservation model can still be applied, but the traffic counts of passage detectors are replaced by the “green counts,” which are calculated by the ramp metering rate. For ramps with significant count errors from both queue and passage detectors, a Kalman filtering model is adopted. To verify the proposed methods, surveillance video data were used to compare actual and estimated queue sizes. Results indicate that the proposed methods greatly improve the accuracy of queue size estimation as compared with the original SZM queue size estimation.

Impact of tidal front on the distribution of bacterioplankton in the southern Yellow Sea, China

Three surveys were carried out in anchovy spawning periods in southern Yellow Sea in May and June 2001, and June 2002. Chlorophyll a (Chl- a) concentration, bacterioplankton abundance, biomass and their variations along the zone of tidal fronts were investigated. The results showed that (1) high Synechococcus abundance distributed more often in frontal area and middle-surface layer of a stratified zone; and (2) the maximal abundance of bacteria occurred in stratified and mixed zone.

Comprehensive Evaluation of New Integrated Freeway Ramp Control Strategy

A new integrated ramp control strategy, recently deployed by the Minnesota Department of Transportation in the Twin Cities metropolitan area, is evaluated. This strategy, stratified zone metering (SZM), takes into account real-time ramp demand and queue size information and aims to strike a balance between two competing objectives: improving freeway efficiency and preventing excessive ramp delays. In this study, the SZM strategy was compared to the earlier ZONE metering strategy as well as to the no-control alternative. Comprehensive metrics were generated through rigorous microsimulation to assess critical aspects of the strategy's performance. The evaluation results are consistent with qualitative field observations and confirm that SZM strategy improves freeway efficiency when compared with the no-control alternative, reduces freeway travel time and delay, improves freeway speed, smooths freeway flow, and reduces the number of stops. More important, excessive queue spillbacks and ramp delays were significantly reduced under SZM control when compared with the earlier ZONE metering strategy. However, the nonrestrictive metering rates under SZM may cause the freeway proper to be overloaded; this leads to compromised freeway performance as compared with the more restrictive ramp control. This is especially true for heavy traffic demands, when higher ramp volumes are allowed on the freeway.

Comprehensive Evaluation of New Integrated Freeway Ramp Control Strategy

A new integrated ramp control strategy, recently deployed by the Minnesota Department of Transportation in the Twin Cities metropolitan area, is evaluated. This strategy, stratified zone metering (SZM), takes into account real-time ramp demand and queue size information and aims to strike a balance between two competing objectives: improving freeway efficiency and preventing excessive ramp delays. In this study, the SZM strategy was compared to the earlier ZONE metering strategy as well as to the no-control alternative. Comprehensive metrics were generated through rigorous microsimulation to assess critical aspects of the strategy's performance. The evaluation results are consistent with qualitative field observations and confirm that SZM strategy improves freeway efficiency when compared with the no-control alternative, reduces freeway travel time and delay, improves freeway speed, smooths freeway flow, and reduces the number of stops. More important, excessive queue spillbacks and ramp delays were signi...

Improving Minnesota's Stratified Ramp Control Strategy

Minnesota's most recent stratified zone ramp metering (SZM) strategy, the successor of the ZONE metering algorithm, has been deployed in the Minneapolis–St. Paul area since late 2002. The SZM strategy is intended to maximize freeway throughput while keeping ramp waiting times below a predetermined threshold. Preliminary evaluation results confirmed the overall effectiveness of the SZM strategy but also revealed that the freeway performance was compromised by the restrictive maximum ramp waiting constraint. Consequently, improvements were sought. These focus on a better determination of the minimum release rate for each ramp and its integration with the overall SZM strategy. Both the current and the enhanced SZM strategies are tested in two freeway sites under various demand scenarios through a state-of-the-art microscopic simulator. The simulation results indicate that the enhanced SZM strategy is effective for delaying and decreasing the freeway congestion and results in smoother freeway traffic flow compared with the original SZM strategy without violating the maximum ramp delay requirement.

Improving Minnesota's Stratified Ramp Control Strategy

Minnesota's most recent stratified zone ramp metering (SZM) strategy, the successor of the ZONE metering algorithm, has been deployed in the Minneapolis-St. Paul area since late 2002. The SZM strategy is intended to maximize freeway throughput while keeping ramp waiting times below a predetermined threshold. Preliminary evaluation results confirmed the overall effectiveness of the SZM strategy but also revealed that the freeway performance was compromised by the restrictive maximum ramp waiting constraint. Consequently, improvements were sought. These focus on a better determination of the minimum release rate for each ramp and its integration with the overall SZM strategy. Both the current and the enhanced SZM strategies are tested in two freeway sites under various demand scenarios through a state-of-the-art microscopic simulator. The simulation results indicate that the enhanced SZM strategy is effective for delaying and decreasing the freeway congestion and results in smoother freeway traffic flow com...

Thermocline mixing in summer stratified continental shelf seas

A key process in the shelf sea pumping of CO2 to the open ocean is vertical mixing in the summer stratified zones of continental shelf seas. Here, we present measurements of profiles of the rate of dissipation of turbulent kinetic energy (ɛ), from which vertical mixing is inferred, for several locations in the summer stratified zones of the NW European shelf seas. At both sites we find significant thermocline mixing, with ∼20% of the observed turbulent dissipation taking place within the thermocline. We then present evidence to demonstrate that the thermocline mixing is a result of shear instability within the thermocline. This is contrary to the conventional view that vertical mixing in shelf seas is largely driven by boundary shear stresses.

Methodology for Performance Optimization of Ramp Control Strategies through Microsimulation

Freeway ramp control has been successfully implemented since the mid-1960s as an efficient and viable freeway management strategy. However, the effectiveness of any ramp control strategy is largely dependent on site-specific customization and calibration, preferably before its deployment. A general methodology for such performance optimization of ramp control strategies is proposed in a microscopic simulation environment as an alternative to trial-and-error field experimentation. The applicability of the methodology is demonstrated by implementation on Minnesota's new stratified zone metering (SZM). Further, the effect of external factors, such as traffic demand variation and incidents, on SZM control and optimization results was also studied. Results show that the optimization methodology is highly effective depending on the optimization objective, test site characteristics, and demand levels.