Formation and stability of oxygen-rich bubbles that shape photosynthetic mats

Gas release in photic-zone microbialites can lead to preservable morphological biosignatures. Here, we investigate the formation and stability of oxygen-rich bubbles enmeshed by filamentous cyanobacteria. Sub-millimetric and millimetric bubbles can be stable for weeks and even months. During this time, lithifying organic-rich laminae surrounding the bubbles can preserve the shape of bubbles. Cm-scale unstable bubbles support the growth of centimetric tubular towers with distinctly laminated mineralized walls. In environments that enable high photosynthetic rates, only small stable bubbles will be enclosed by a dense microbial mesh, while in deep waters extensive microbial mesh will cover even larger photosynthetic bubbles, increasing their preservation potential. Stable photosynthetic bubbles may be preserved as sub-millimeter and millimeter-diameter features with nearly circular cross-sections in the crests of some Proterozoic conical stromatolites, while centrimetric tubes formed around unstable bubbles provide a model for the formation of tubular carbonate microbialites that are not markedly depleted in (13)C.

Self-sustained nonlinear waves in traffic flow

In analogy to gas-dynamical detonation waves, which consist of a shock with an attached exothermic reaction zone, we consider herein nonlinear traveling wave solutions to the hyperbolic ("inviscid") continuum traffic equations. Generic existence criteria are examined in the context of the Lax entropy conditions. Our analysis naturally precludes traveling wave solutions for which the shocks travel downstream more rapidly than individual vehicles. Consistent with recent experimental observations from a periodic roadway [Y. Sugiyama, N. J. Phys. 10, 033001 (2008)], our numerical calculations show that nonlinear traveling waves are attracting solutions, with the time evolution of the system converging toward a wave-dominated configuration. Theoretical principles are elucidated by considering examples of traffic flow on open and closed roadways.

Experimental evaluation of a mathematical model for predicting transfer efficiency of a high volume-low pressure air spray gun

The transfer efficiency of a spray-painting gun is defined as the amount of coating applied to the workpiece divided by the amount sprayed. Characterizing this transfer process allows for accurate estimation of the overspray generation rate, which is important for determining a spray painter's exposure to airborne contaminants. This study presents an experimental evaluation of a mathematical model for predicting the transfer efficiency of a high volume-low pressure spray gun. The effects of gun-to-surface distance and nozzle pressure on the agreement between the transfer efficiency measurement and prediction were examined. Wind tunnel studies and non-volatile vacuum pump oil in place of commercial paint were used to determine transfer efficiency at nine gun-to-surface distances and four nozzle pressure levels. The mathematical model successfully predicts transfer efficiency within the uncertainty limits. The least squares regression between measured and predicted transfer efficiency has a slope of 0.83 and an intercept of 0.12 (R2 = 0.98). Two correction factors were determined to improve the mathematical model. At higher nozzle pressure settings, 6.5 psig and 5.5 psig, the correction factor is a function of both gun-to-surface distance and nozzle pressure level. At lower nozzle pressures, 4 psig and 2.75 psig, gun-to-surface distance slightly influences the correction factor, while nozzle pressure has no discernible effect.

On the use of computational fluid dynamics in the prediction and control of exposure to airborne contaminants-an illustration using spray painting

Computational fluid dynamics (CFD) is employed to simulate breathing-zone concentration for a simple representation of spray painting a flat plate in a cross-flow ventilated booth. The results demonstrate the capability of CFD to track correctly changes in breathing-zone concentration associated with work practices shown previously to be significant in determining exposure. Empirical data, and models verified through field studies, are used to examine the predictive capability of these simulations and to identify important issues in the conduct of such comparisons. A commercially available CFD package is used to solve a three-dimensional turbulent flow problem for the velocity field, and to subsequently generate particle trajectories for polydisperse aerosols. An in-house algorithm is developed to convert the trajectory data to breathing-zone concentrations, transfer efficiencies and aerosol size distributions. The mesh size, time step, duration of the simulation, and number of particles per size interval are all important variables in achieving convergent results.

Modeling breathing-zone concentrations of airborne contaminants generated during compressed air spray painting

This paper presents a mathematical model to predict breathing-zone concentrations of airborne contaminants generated during compressed air spray painting in cross-flow ventilated booths. The model focuses on characterizing the generation and transport of overspray mist. It extends previous work on conventional spray guns to include exposures generated by HVLP guns. Dimensional analysis and scale model wind-tunnel studies are employed using non-volatile oils, instead of paint, to produce empirical equations for estimating exposure to total mass. Results indicate that a dimensionless breathing zone concentration is a nonlinear function of the ratio of momentum flux of air from the spray gun to the momentum flux of air passing through the projected area of the worker's body. The orientation of the spraying operation within the booth is also very significant. The exposure model requires an estimate of the contaminant generation rate, which is approximated by a simple impactor model. The results represent an initial step in the construction of more realistic models capable of predicting exposure as a mathematical function of the governing parameters.

Application of a tracer gas challenge with a human subject to investigate factors affecting the performance of laboratory fume hoods

The results of a "user" tracer gas test were applied to investigate the effects of various parameters on hood containment ability and to evaluate accepted methods to classify hood performance. This user tracer gas test was performed with a human subject standing in front of the hood. Based on the data collected, face velocity, its variability, and cross drafts are important in determining hood leakage. Results indicate that the temporal variability of face velocity may deserve as much consideration as its spatial variability, a parameter more traditionally recognized as being important. The data collected indicate that hoods with horizontally sliding sash doors perform better with the doors positioned to provide a center opening rather than when all of the doors are pushed to one side. The observed smoke patterns suggest that this trend is caused by the location and instability of vortices formed along the perimeter edge when all doors are pushed to one side. The results of manikin tracer gas tests and the user tracer gas test are inconsistent, suggesting that more research is needed to determine how best to evaluate whether a hood protects its users.

Experimental and numerical studies on the impact of work practices used to control exposures occurring in booth-type hoods

The observation that the between-worker variance component of exposure is significant for those performing the same tasks suggests that work practices are an important determinant of exposure. Decisions to implement engineering controls may be less than optimal if these work practices are not carefully identified. This study examines the position of the worker with respect to an object and the airflow direction in a large booth-type hood, and its implications for control of exposure. Experiments are conducted in a wind-tunnel using a mannequin and tracer gas techniques to measure exposures in the various positions at different air velocities. Smoke-wire, flow-visualization techniques are employed to correlate the exposures with the airflow patterns. Numerical predictions of these flow patterns and exposures compare favorably with experimental data, despite limitations. Further work is underway to examine more realistic situations such as spray-painting applications.

Three-dimensional finite-element simulation of a turbulent push-pull ventilation system

A finite-element formulation with penalty approach to enforce continuity is employed here to simulate the three-dimensional velocity field resulting from a simple push-pull ventilation configuration. An analytic expression for the length scale and a transport equation for turbulent kinetic energy are coupled with the momentum equations. A coaxial square hood and jet are arranged with cross-draught perpendicular to the common centreline. Numerical predictions of the velocity and turbulence kinetic energy fields are evaluated in the plane of symmetry with hot film anemometry, and smoke-wire flow visualizations. The agreement of the simulated jet trajectories with flow visualizations is reasonable, as are velocities. Predictions of turbulence kinetic energy are not as good, particularly near the hood face. Despite the limitations the numerical approach is useful in assessing the impact of cross-draughts on the push-pull arrangement.

Computational simulation of worker exposure using a particle trajectory method

The velocity field downstream of a worker is approximated with a discrete vortex algorithm. This information is used to calculate trajectories of massless tracer 'particles' released from a point-source of contaminant. Concentrations in the plane of this source are estimated by averaging over a number of such trajectories. Approximations include: (1) representing the worker by a two-dimensional elliptical cylinder; and (2) representing tracer gas contaminant by massless particles generated without momentum. These particles are transported by both vortex shedding and turbulent diffusion. Computer-predicted mean concentrations in the near-wake region downstream of the worker compare well with results from wind-tunnel tracer gas experiments employing a mannequin. Subsequently, the concept of a computational breathing zone is introduced, and predictions of worker exposure are made. These simulations of time-integrated breathing zone concentration also compare well with measured values.

The effect of contaminant source momentum on a worker's breathing zone concentration in a uniform freestream

Several factors affecting breathing zone concentration were examined in a paint spray booth by using a tracer gas method. The variables in the study include contaminant momentum, the presence of a flat plate downstream of the worker, the distance between the contaminant source and the body, and the worker's motion. A dramatic reduction in breathing zone concentration was observed when the spray gun emitted contaminants with high momentum. Reductions of 30-50% were observed because of the other variables. The source momentum effect was studied, subsequently, in a wind tunnel by measuring the breathing zone concentration of a mannequin with various flows through jets of different diameter, at varying freestream velocities. A functional relationship was determined between nondimensional breathing zone concentration and contaminant source momentum. This relationship is supported by numerical simulations. The effect of contaminant momentum on the near-wake flow field is discussed in conjunction with results from the numerical simulations.

Modeling a worker's exposure from a hand-held source in a uniform freestream

The phenomenon of boundary layer separation can be an important factor in determining a worker's exposure to toxic airborne pollutants. A conceptual model was developed to understand this phenomenon and to predict the average concentration in the reverse flow region downstream of a worker in a uniform freestream. Subsequently, the assumptions of this model were tested experimentally in wind tunnel studies. On the basis of these results, a revised model is presented and validated by using a tracer gas method. The revised model provides a reasonable estimate of the average concentration in the reverse flow region of the mannequin. Empirical models are presented that relate both the average concentration in the reverse flow region and the breathing zone concentration to the body dimensions and the freestream air velocity. Applications and limitations of the results are discussed.

Airflow pattern around a worker in a uniform freestream

The effect of boundary layer separation on worker exposure is an important factor in the design of local exhaust ventilation. Three-dimensional airflow around a mannequin is examined by using flow visualization techniques and hot-film anemometry. Above the chest, a downwash effect is noted; from the chest to the elbows, a combination of downwash and vortex shedding is observed; and from the waist to the hip, vortex shedding appears to be dominant. A coherent vertical flow structure is observed close to the body. Vortex shedding frequency is determined by using hot-film anemometry. The dimensions of the reverse flow region and the area of the vortices are estimated from flow visualization videos.

Discrete vortex methods for the simulation of boundary layer separation effects on worker exposure

The discrete vortex method is a numerical technique for the solution of the two-dimensional Navier-Stokes equations in vorticity-transport form. The technique is employed here, with appropriate modifications, to simulate boundary layer separation around a worker and to assess the implications for exposure. Approximations include: (1) representation of the worker as a two-dimensional elliptical cylinder; and (2) contaminant transport by vortex shedding exclusively. The model results in estimates of breathing zone concentration that are in reasonable agreement with laboratory wind tunnel experiments.

Computer simulation in the design of local exhaust hoods for shielded metal arc welding

Computer simulations were used to examine competing exhaust hood configurations for shielded metal arc welding. The welder's breathing zone concentration appears to be an inverse linear function of the computer-predicted hood capture efficiency. Hood aspect ratio, hood flow, and the welder's position relative to the hood all have a significant effect on the breathing zone concentration. The height of the hood above the welding surface showed no significant effect in reducing breathing zone concentration. Further examination of breathing zone concentration as a function of capture efficiency is needed before reliable design methods can be developed using this parameter.

Prediction and measurement of velocity into flanged slot hoods

A model describing the three-dimensional velocity field into a flanged slot hood has been developed using potential flow theory. Modeling the slot as an elliptical aperture allows use of the potential function to develop expressions for the velocity components (vx,vy,vz) at any point (x,y,z). Experiments were performed to measure velocities in front of six slot hoods. Experimental results were compared with velocities predicted by two models: an equal area ellipse with the same length to width ratio as the slot and an ellipse inscribed within the slot.

Empirical validation of theoretical velocity fields into flanged circular hoods

Previously presented theoretical models of the three-dimensional velocity field into a flanged circular hood, both with and without crossdraft, are examined by hot film anemometry. A final model with empirical modifications is selected and validated. Computer generated streamline maps, which enable visualization of the effects of crossdrafts on hood performance, are presented. The theoretical basis for capture efficiency using the model is discussed.