The knowledge of exposure to the airborne particle emitted from three-dimensional (3D) printing activities is becoming a crucial issue due to the relevant spreading of such devices in recent years. To this end, a low-cost desktop 3D printer based on fused deposition modeling (FDM) principle was used. Particle number, alveolar-deposited surface area, and mass concentrations were measured continuously during printing processes to evaluate particle emission rates (ERs) and factors. Particle number distribution measurements were also performed to characterize the size of the emitted particles. Ten different materials and different extrusion temperatures were considered in the survey. Results showed that all the investigated materials emit particles in the ultrafine range (with a mode in the 10-30-nm range), whereas no emission of super-micron particles was detected for all the materials under investigation. The emission was affected strongly by the extrusion temperature. In fact, the ERs increase as the extrusion temperature increases. Emission rates up to 1×10 particles min were calculated. Such high ERs were estimated to cause large alveolar surface area dose in workers when 3D activities run. In fact, a 40-min-long 3D printing was found to cause doses up to 200 mm .

Although the interpersonal distance represents an important parameter affecting the risk of infection due to respiratory viruses, the mechanism of exposure to exhaled droplets remains insufficiently characterized. In this study, an integrated risk assessment is presented for SARS-CoV-2 close proximity exposure between a speaking infectious subject and a susceptible subject. It is based on a three-dimensional transient numerical model for the description of exhaled droplet spread once emitted by a speaking person, coupled with a recently proposed SARS-CoV-2 emission approach. Particle image velocimetry measurements were conducted to validate the numerical model. The contribution of the large droplets to the risk is barely noticeable only for distances well below 0.6 m, whereas it drops to zero for greater distances where it depends only on airborne droplets. In particular, for short exposures (10 s) a minimum safety distance of 0.75 m should be maintained to lower the risk below 0.1%; for exposures of 1 and 15 min this distance increases to about 1.1 and 1.5 m, respectively. Based on the interpersonal distances across countries reported as a function of interacting individuals, cultural differences, and environmental and sociopsychological factors, the approach presented here revealed that, in addition to intimate and personal distances, particular attention must be paid to exposures longer than 1 min within social distances (of about 1 m).

SUMMARYThis paper presents a numerical comparison of two different numerical procedures based on the characteristic-based split (CBS) algorithm for the solution of incompressible flows. The first procedure is based on the original semi-implicit version of the CBS, introduced in the early days of the scheme development. The second was proposed more recently in order to have a fully explicit version of the scheme, and is based on the introduction of an artificial compressibility type of formulation. The interest in fully explicit schemes has increased because of the easy and efficient parallelization. The main differences between the two versions of the schemes are highlighted in this work, and a numerical comparison is presented on the basis of several benchmark solutions. Both steady and unsteady flows are considered. With the explicit version of the scheme a dual time stepping procedure is needed to recover unsteady solutions.

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The increased traffic emissions and reduced ventilation of urban street canyons lead to the formation of high particle concentrations as a function of the related flow field and geometry. In this context, the use of advanced modelling tools, able to evaluate particle concentration under different traffic and meteorological conditions, may be helpful.In this work, a numerical scheme based on the non-commercial fully explicit AC-CBS algorithm, and the one-equation Spalart-Allmaras turbulence model, was developed to perform numerical simulations of fluid flow and ultrafine particle dispersion in different street canyon configurations and under different wind speed and traffic conditions. The proposed non-commercial numerical tool was validated through a comparison with data drawn from the scientific literature.The results obtained from ultrafine particle concentration simulations show that as the building height increases the dispersion of particles in the canyon becomes weaker, due to the restricted interaction between the flow field in the street canyon and the undisturbed flow. Higher values of approaching wind speed facilitate the dispersion of the particles. The traffic effect has been evaluated by imposing different values of particles emission, depending on the vehicles type, with the lowest concentration values obtained for the Euro 6 vehicles, and the highest for High Duty Vehicles. A parametric analysis was also performed concerning the exposure to particles of pedestrians in different positions at the road level as a function of street canyon geometry, traffic mode, and wind speed. The worst exposure (1.25 × 10 6 part./cm 3 ) was found at the leeward side for an aspect ratio H/W = 1, wind speed of 5 m/s when High Duty Vehicles traffic was considered.

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SUMMARYIn this paper, porous medium-free fluid interface problems in the presence of high source terms are studied for the first time by using a stable, accurate and efficient Artificial Compressibility (AC) CharacteristicBased Split (CBS) algorithm. The interest in the AC-CBS scheme has increased, as its matrix-inversion free procedure offers the possibility of an easy and efficient parallelization. The present algorithm, recently introduced for the solution of porous media flows, presented some difficulties in solving complex interface problems. In this paper, the AC-CBS scheme has been specifically developed to overcome such difficulties and produce an accurate, stable, and efficient solution for the generalized porous medium flow equations for forced, free, and mixed convection through interfaces. In order to obtain the present results, a stability analysis for the AC-CBS scheme for the solution of problems with high source terms is carried out for the first time. The efficiency and the accuracy of the AC-CBS algorithm are verified through comparison with analytical, numerical, and experimental data available in the literature. The authors consider three different types of interface benchmark problems: (i) forced convection in a horizontal channel; (ii) mixed convection in a vertical channel; and (iii) natural convection in a vertically divided cavity. The first models for flow through fluid saturated porous media were based on the phenomenological relation proposed by Darcy [17]. Later, two major extensions to the Darcy model, widely used in many engineering fields [18], were due to Forchheimer [19] and Brinkman [20]. In particular, the non-linear Forchheimer term takes into account the effects of moderate and high Reynolds numbers, whereas the Brinkman extension makes it possible to impose a no-slip boundary condition at the impermeable wall and also to match the momentum equations at the porous medium/free fluid interface without having a jump in velocity. From the physical point of view, non-linear terms in the momentum conservation equation appears as a consequence of large filtration velocities, which make the drag due to solid obstacles comparable with the surface drag due to friction [21].With the advent of computational power, flow modeling in porous media received a significant boost and the model known as generalized model, based on the volume averaging analysis, was introduced by Whitaker [22], who showed that by matching the Brinkman-Darcy and Stokes equations the continuity of velocity is retained, but a jump in the shear stress is produced [21]. Obviously, irrespective of the model used to describe the porous medium flow, in problems where an interface between a porous medium and a free fluid exists, a set of proper matching conditions is needed at the interface itself. An analysis of interface conditions, as a function of the flow velocity and the porous medium properties, is then crucial. A comprehensive comparative analysis of the hydrodynamic and thermal interfacial conditions between a po...