Computational analysis of transport in three-dimensional heterogeneous materials: An OpenFOAM®-based simulation framework

Porous and heterogeneous materials are found in many applications from composites, membranes, chemical reactors, and other engineered materials to biological matter and natural subsurface structures. In this work we propose an integrated approach to generate, study and upscale transport equations in random and periodic porous structures. The geometry generation is based on random algorithms or ballistic deposition. In particular, a new algorithm is proposed to generate random packings of ellipsoids with random orientation and tunable porosity and connectivity. The porous structure is then meshed using locally refined Cartesian-based or unstructured strategies. Transport equations are thus solved in a finite-volume formulation with quasi-periodic boundary conditions to simplify the upscaling problem by solving simple closure problems consistent with the classical theory of homogenisation for linear advection–diffusion–reaction operators. Existing simulation codes are extended with novel developments and integrated to produce a fully open-source simulation pipeline. A showcase of a few interesting three-dimensional applications of these computational approaches is then presented. Firstly, convergence properties and the transport and dispersion properties of a periodic arrangement of spheres are studied. Then, heat transfer problems are considered in a pipe with layers of deposited particles of different heights, and in heterogeneous anisotropic materials

A Systematic Analysis of the Memory term in Coarse-Grained models: the case of the Markovian Approximation

The systematic development of Coarse-Grained (CG) models via the Mori-Zwanzig projector operator formalism requires the explicit description of several terms, including a deterministic drift term, a dissipative memory term and a random fluctuation term. In many applications, the memory and fluctuation terms are related by the fluctuation-dissipation relation and are, in general, more challenging to derive than the drift term. In this work we analyse an approximation of the memory term and propose a rational basis for a data-driven approach to an approximation of the memory and fluctuating terms which can be considered included in the class of the Markovian ones.Comment: 39 pages, 2 Figure

HPV related diseases in males: a heavy vaccine-preventable burden

Human Papillomavirus (HPV) has a significant impact in male?s health, as cause of clinical manifestations ranging from genital warts to several cancers of the anogenital and aero-digestive tract. HPV types which most frequently affect men are 6,11,16 and 18, included in the HPV quadrivalent vaccine, recently approved for use in males by Food and Drug Administration (FDA) and European Medicines Agency (EMA). Although several data about the safety and efficacy of quadrivalent vaccine are available, the implementation of proper immunization plans dedicate to male?s population can- not ignore the knowledge of the characteristics of the disease in men, which in some aspects should be clarify, in particular clearance of type-specific HPV infections and transmission dynamics. Purpose of this review is to summarise the main information about the burden and the natural history of the HPV related disease in males

SIMULATION OF FLOW AND PARTICLE TRANSPORT AND DEPOSITION IN POROUS MEDIA WITH COMPUTATIONAL FLUID DYNAMICS

The simulation of transport and deposition of colloidal particles in porous media finds important applications in many engineering and environmental problems, such as particle filtration, catalytic processes carried out in filter beds, chromatographic separation and aquifer remediation. This study focuses in particular on remediation of contaminated groundwater via direct injection of nano-sized zerovalent iron particles, which have been shown to be able to efficiently degrade a large variety of contaminants. Application of this technology on full scale applications poses a number of challenges, the most important of which regards the mobility of the particles and their delivery to the contaminated site in the soil. Particles migration is usually quantitatively expressed by a single parameter: the deposition efficiency in the porous bed, whose theoretical reference lies in the classical colloid filtration theory, which moreover further subdivides the process of deposition in the three mechanisms by which particles can reach the solid grain: Brownian diffusion, steric interception, and gravitational sedimentation. This theory, however, has been developed only for very simple geometrical representations of the porous media and a narrow range of fluid conditions. The difficulties in investigating this kind of systems from the experimental point of view have prevented the development of accurate models able to account for the high degree of complexity which characterizes a porous medium, both in the grain arrangement and in their shape. The aim of this study is therefore to simulate the transport of the nanoparticles and their interaction with the porous media (at the microscopic scale), in order to improve the current understanding of these phenomena and obtain predictive models for the deposition efficiency of the colloids on the surface of the grains constituting the porous medium; moreover, eventually, to evaluate the effectiveness of the zerovalent iron technology. Several two and three dimensional microscale (the order of millimiters) representations of grain packings with different degrees of complexity were analyzed. First, two dimensional random arrangements of spheres were considered. Then, the analysis was extended to domains reconstructed from SEM images of a real porous medium. The work was then expanded in three dimensions, first considering simplified domains constituted by irregular packings of spheres, and finally geometries constituted by grains of realistic shapes. These last geometries were created using an algorithm simulating the grain sedimentation process in porous media (Settledyn). Flow field and particle transport was then investigated using finite volume CFD codes (Fluent and OpenFoam), solving the Navier-Stokes equations for the flow and using an Eulerian approach for the colloid transport, eventually obtaining, for each case, an estimate of the colloidal transport efficiency. After having validated the methodology used in this work by comparing our results with proved analytical results available for simplified cases, new predictive equations for each of the individual contributions of the three deposition mechanisms were derived, highlighting the differences from the theoretical model due to the wider range of operating conditions investigated and/or the different geometrical characteristics of the porous media

Solving the Klein-Gordon equation using Fourier spectral methods: A benchmark test for computer performance

The cubic Klein-Gordon equation is a simple but non-trivial partial differential equation whose numerical solution has the main building blocks required for the solution of many other partial differential equations. In this study, the library 2DECOMP&FFT is used in a Fourier spectral scheme to solve the Klein-Gordon equation and strong scaling of the code is examined on thirteen different machines for a problem size of 512^3. The results are useful in assessing likely performance of other parallel fast Fourier transform based programs for solving partial differential equations. The problem is chosen to be large enough to solve on a workstation, yet also of interest to solve quickly on a supercomputer, in particular for parametric studies. Unlike other high performance computing benchmarks, for this problem size, the time to solution will not be improved by simply building a bigger supercomputer.Comment: 10 page

Numerical simulation of droplet impact on wettability-patterned surfaces

© 2020 American Physical Society. Numerical simulations have unexplored potential in the study of droplet impact on nonuniform wettability surfaces. In this paper, we compare numerical and experimental results to investigate the application potential of a volume-of-fluid method utilized in OpenFOAM. The approach implements the Kistler model for the dynamic contact angle of impacting droplets. We begin with an investigation into the influence of the most important solver parameters to optimize the computational setup and reach the best compromise between computational cost and solution errors, as assessed in comparison to experimental results. Next, we verify the accuracy of the predictions for droplet impact on uniformly hydrophilic or superhydrophobic surfaces. Benchmarking the maximal spreading factor, contact, and spreading times, as well as contact-line behavior, we show strong agreement between the present numerical results and the models of Pasandideh-Fard, Phys. Fluids 8, 650 (1996)PHFLE61070-663110.1063/1.868850 and Clanét, J. Fluid Mech. 517, 199 (2004)JFLSA70022-112010.1017/S0022112004000904. Lastly, we demonstrate the capability of the model to accurately predict outcome behaviors of droplets striking distributed-wettability surfaces, which introduce 3D outcome characteristics, even in orthogonal impact. The model successfully predicts droplet splitting and vectoring, as reported in the experiments of Schutzius, Sci. Rep. 4, 7029 (2014)2045-232210.1038/srep07029. Finally, we demonstrate a configuration wherein a droplet centrally strikes a circular disk of different wettability than its surrounding domain. The main contribution of the present paper is a numerical model capable of accurately simulating droplet impact on spatially nonuniform wettability patterns of any foreseeable design

Mechanisms of dispersion in a porous medium

This paper studies the mechanisms of dispersion in the laminar flow through the pore space of a three-dimensional porous medium. We focus on preasymptotic transport prior to the asymptotic hydrodynamic dispersion regime, in which solute motion may be described by the average flow velocity and a hydrodynamic dispersion coefficient. High-performance numerical flow and transport simulations of solute breakthrough at the outlet of a sand-like porous medium evidence marked deviations from the hydrodynamic dispersion paradigm and identify two distinct regimes. The first regime is characterised by a broad distribution of advective residence times in single pores. The second regime is characterised by diffusive mass transfer into low-velocity regions in the wake of solid grains. These mechanisms are quantified systematically in the framework of a time-domain random walk for the motion of marked elements (particles) of the transported material quantity. Particle transitions occur over the length scale imprinted in the pore structure at random times given by heterogeneous advection and diffusion. Under globally advection-dominated conditions, i.e., Péclet numbers larger than 1, particles sample the intrapore velocities by diffusion and the interpore velocities through advection. Thus, for a single transition, particle velocities are approximated by the mean pore velocity. In order to quantify this advection mechanism, we develop a model for the statistics of the Eulerian velocity magnitude based on Poiseuille’s law for flow through a single pore and for the distribution of mean pore velocities, both of which are linked to the distribution of pore diameters. Diffusion across streamlines through immobile zones in the wake of solid grains gives rise to exponentially distributed residence times that decay on the diffusion time over the pore length. The trapping rate is determined by the inverse diffusion time. This trapping mechanism is represented by a compound Poisson process conditioned on the advective residence time in the proposed time-domain random walk approach. The model is parameterised with the characteristics of the porous medium under consideration and captures both preasymptotic regimes. Macroscale transport is described by an integro-differential equation for solute concentration, whose memory kernels are given in terms of the distribution of mean pore velocities and trapping times. This approach quantifies the physical non-equilibrium caused by a broad distribution of mass transfer time scales, both advective and diffusive, on the representative elementary volume (REV). Thus, while the REV indicates the scale at which medium properties like porosity can be uniquely defined, this does not imply that transport can be characterised by hydrodynamic dispersion

Adjuvants and alternative routes of administration towards the development of the ideal influenza vaccine.

Vaccination is universally considered as the principal measure for the control of influenza, which represents a significant burden worldwide, both from a health-care and a socio-economic viewpoint. Conventional non-adjuvanted trivalent influenza vaccines (TIVs) have been recognized as having some deficiencies, such as suboptimal immunogenicity particularly in the elderly, in patients with severe chronic diseases and immunocompromized, indeed, those groups of the population at higher risk of developing severe complications following influenza infection, when compared to healthy adults. Moreover, the protection offered by conventional vaccines may be reduced by periodic antigenic drifts, resulting in a mismatch between the circulating and vaccinal viral strains. Another gap regarding currently available vaccines is related to the egg-based manufacturing system for their production: not only the length of time involved with the latter but also the limited capacity of this platform technology represent a major limitation for the active prevention of influenza, which is particularly important in the case of a new pandemic strain. New technologies used in vaccine composition, administration and manufacture have led to major advances during the last few years, and clinical researchers have continued to work hard, investigating several different strategies to improve the performance of influenza vaccines: namely, the addition of different adjuvants (i.e., MF59- and AS03-vaccines, virosomal formulations), the use of alternative routes of administration or manufacture (i.e., intradermal, nasal and oral vaccines and cell culture- and reverse genetic-based vaccines) or of high doses of antigen, and the development of DNA-vaccines, or the use of conserved viral epitopes (i.e., the extracellular portion of the M2 protein, the nucleoprotein and some domains of the hemagglutinin), in the attempt to produce a "universal target" antigen vaccine. The knowledge acquired represents a fundamental challenge for the control of influenza. An overview of the most recent and interesting results, some of which gained from our own research experience, particularly concerning two successful approaches, of those outlined above, namely the use of: (i) the oil-in-water MF59-adjuvant, and (ii) the intradermal (ID) route for vaccine administration, through a novel microinjection system, will be reported and discussed, together with the possible implications and perspectives to optimize immunization policies against influenza in the near future

Fatherhood and wage inequality in Britain, Finland, and Germany

Objective This study investigates whether and how fatherhood shapes the wage distribution in Britain, Finland, and Germany. Background Existing research debates whether fatherhood is associated with greater wages. However, it remains unclear whether the association between fatherhood and wages varies along the wage distribution as well as institutional contexts. To explore this, we compare three countries that differ in their wage bargaining institutions and family policies. Method We use unconditional quantile regression on longitudinal data from the 1995 to 2016 waves of the Finnish Linked Employer Employee data, German Socio-Economic Panel, and UK Longitudinal Household Study. To control for selection into fatherhood, we combine quantile regressions with fixed effects techniques. Results Results show little evidence of substantial fatherhood wage effects along men's wage distribution. In all countries, fathers' higher wages at the median and top of the wage distribution are mostly accounted for by selection, but fatherhood shifts the bottom part of the distribution to the left particularly in the UK. Conclusions The extent to which having a child affects men's wages across the wage distribution is similar across three diverse policy contexts. Yet, differences across the wage distribution are larger in the UK. We argue this may be linked to its higher level of inequality typical of liberal labour markets