Modelling of Droplet Capture in an Open-Cell Metal Foam at the Pore and Macroscopic Scales

Open-cell metal foams are often used in applications where particulate and/or droplet capture is important. Here a Computational Fluid Dynamics (CFD) modelling approach is described which models the metal foam at both the pore-scale and the macroscopic scale. At the pore-scale, the detailed internal geometry of the foam is included and the flow field and droplet tracking and capture is modelled explicitly. At this scale, a coefficient is found for each metal foam that relates the distance a droplet can freely travel through the foam to both the droplet diameter and the Darcian velocity in the porous medium. Then, at the macroscopic scale, the coefficient from the pore-scale droplet capture simulations is used in a novel stochastic particle extinction model. Here, the droplets travel through a porous zone and are removed from the model, the probability of which is determined by the coefficient from the pore-scale modelling. A test case is described in which the macroscopic model is verified against the pore-scale model with acceptable levels of accuracy

Experimental study of the pressure loss in aero-engine air-oil separators

The results of extensive experimental testing of an aero-engine air-oil separator are presented and discussed. The study focuses on the pressure loss of the system. Oil enters the device in the form of dispersed droplets. Subsequently, separation occurs by centrifuging larger droplets towards the outer walls and by film formation at the inner surface of a rotating porous material, namely an open-cell metal foam. The work described here is part of a study led jointly by the Karlsruhe Institute of Technology (KIT) and the University of Nottingham (UNott) within a recent EU project. The goal of the research is to increase the separation efficiency to mitigate oil consumption and emissions, while keeping the pressure loss as low as possible. The aim is to determine the influencing factors on pressure loss and separation efficiency. With this knowledge, a correlation can eventually be derived. Experiments were conducted for three different separator configurations, one without a metal foam and two with metal foams of different pore sizes. For each configuration, a variety of engine-like conditions of air mass flow rate, rotational speed and droplet size was investigated. The experimental results were used to validate and improve the numerical modelling. Results for the pressure drop and its dependencies on air mass flow rate and the rotational speed were analysed. It is shown that the swirling flow and the dissipation of angular momentum are the most important contributors to the pressure drop, besides the losses due to friction and dissipation caused by the flow passing the metal foam. It was found that the ratio of the rotor speed and the tangential velocity of the fluid is an important parameter to describe the influence of rotation on the pressure loss. Contrary to expectations, the pressure loss is not necessarily increased with a metal foam installed

Value of syndromic surveillance within the Armed Forces for early warning during a dengue fever outbreak in French Guiana in 2006

<p>Abstract</p> <p>Background</p> <p>A dengue fever outbreak occured in French Guiana in 2006. The objectives were to study the value of a syndromic surveillance system set up within the armed forces, compared to the traditional clinical surveillance system during this outbreak, to highlight issues involved in comparing military and civilian surveillance systems and to discuss the interest of syndromic surveillance for public health response.</p> <p>Methods</p> <p>Military syndromic surveillance allows the surveillance of suspected dengue fever cases among the 3,000 armed forces personnel. Within the same population, clinical surveillance uses several definition criteria for dengue fever cases, depending on the epidemiological situation. Civilian laboratory surveillance allows the surveillance of biologically confirmed cases, within the 200,000 inhabitants.</p> <p>Results</p> <p>It was shown that syndromic surveillance detected the dengue fever outbreak several weeks before clinical surveillance, allowing quick and effective enhancement of vector control within the armed forces. Syndromic surveillance was also found to have detected the outbreak before civilian laboratory surveillance.</p> <p>Conclusion</p> <p>Military syndromic surveillance allowed an early warning for this outbreak to be issued, enabling a quicker public health response by the armed forces. Civilian surveillance system has since introduced syndromic surveillance as part of its surveillance strategy. This should enable quicker public health responses in the future.</p

Human performance and strategies while solving an aircraft routing and sequencing problem: an experimental approach

As airport resources are stretched to meet increasing demand for services, effective use of ground infrastructure is increasingly critical for ensuring operational efficiency. Work in operations research has produced algorithms providing airport tower controllers with guidance on optimal timings and sequences for flight arrivals, departures, and ground movement. While such decision support systems have the potential to improve operational efficiency, they may also affect users’ mental workload, situation awareness, and task performance. This work sought to identify performance outcomes and strategies employed by human decision makers during an experimental airport ground movement control task with the goal of identifying opportunities for enhancing user-centered tower control decision support systems. To address this challenge, thirty novice participants solved a set of vehicle routing problems presented in the format of a game representing the airport ground movement task practiced by runway controllers. The games varied across two independent variables, network map layout (representing task complexity) and gameplay objective (representing task flexibility), and verbal protocol, visual protocol, task performance, workload, and task duration were collected as dependent variables. A logistic regression analysis revealed that gameplay objective and task duration significantly affected the likelihood of a participant identifying the optimal solution to a game, with the likelihood of an optimal solution increasing with longer task duration and in the less flexible objective condition. In addition, workload appeared unaffected by either independent variable, but verbal protocols and visual observations indicated that high-performing participants demonstrated a greater degree of planning and situation awareness. Through identifying human behavior during optimization problem solving, the work of tower control can be better understood, which, in turn, provides insights for developing decision support systems for ground movement management

More Accurate Insight into the Incidence of Human Rabies in Developing Countries through Validated Laboratory Techniques

International audienceNo abstract availabl

Impacts of storms and evolution of the coastline. Marine Geology

International audienc

Modelling the size-dependent collection efficiency of hedgerows for ambient aerosols

A model for estimating the size-segregated particle collection efficiencies of three hedgerow species of different aerodynamic porosities has been developed and tested in the particle size range 0.5–View the MathML source. Collection efficiency (CE) has been described in terms of the coupled effects of the deviation of the approach flow (CEflow) and the filtration through the foliage elements (CEfiltration). Variations in barrier porosities with wind speed, which govern both the flow and the particle collection mechanism, have been modelled using an empirical parameter called the “rigidity factor” (r) for representative leaf-sizes of the three hedge types. Computational fluid dynamics (CFD) allows simulation of velocity fields and turbulence around the barriers, leading to identification of prominent flow separation zones in the wake of denser hedges. Application of different turbulence closure schemes in the CFD model influences the predicted CE only for particles above View the MathML source. Field validations obtained for a porous (hawthorn) and a denser (yew) hedge indicate that CEfiltration ranges between 10 and 35% for “coarse” particles (10–View the MathML source) and 1–5% for “fine” particles (0.5–View the MathML source), the porous hedge showing higher values in the former case and vice versa in the latter. Model predictions of CE over the particle size range studied for the porous hedge lie within 1–30% and for the denser hedge within 0.4–3%

Experimental and tomography-based CFD investigations of the flow in open cell metal foams with application to aero engine separators

Oil-air separation is a key function in aero engines with closed-loop oil systems. Typically, aero engine air/oil separators employ the use of a porous medium such as open cell metal foams, as a secondary separation mechanism. Assessing its impact on overall separation is important since non-captured oil is released overboard. Computational fluid dynamics offers a possibility to evaluate the metal foam separation effectiveness. A pore scale numerical modelling methodology is applied to determine the transport properties of fluid flow through open cell metal foams. Microcomputer tomography scans were used to generate a 3D digital representation of commercial open cell metal foams of different grades. Foam structural properties such as porosity, specific surface, pore size distribution and the minimum size of a representative elementary volume are directly extracted from the CT scans. Subsequently, conventional finite volume simulations are carried out on the realistic tomography based foam samples. Simulations were performed for a wide range of Reynolds numbers. The feasibility of using standard Reynolds-averaged Navier-Stokes (RANS) turbulence models is investigated here. As part of the method validation, samples with varying lengths were simulated. Pressure drop values were compared on a length-normalized basis against in-house experimental data. The oil phase was modelled using a Lagrangian particle tracking approach. Boundary conditions for the oil phase were extracted from a previous CFD simulation of a full breather device in the ground idle regime (worst separation effectiveness). Steady state particle tracking simulations were run for droplet diameters ranging from 0.5-15 μm, and for flow inlet velocities ranging from 10 - 60 m/s. Stochastic tracking was taken into account in order to model the effects of turbulence on the particle trajectories. Simulations were run on different types of foam and the results are compared qualitatively. The procedure has shown that pore scale modelling is a valid tool to capture the flow field and model oil separation inside open cell metal foams. However, at the moment there is no experimental data available for validation of the oil phase modelling