Direct numerical simulation of turbulent channel flow over porous walls

We perform direct numerical simulations (DNS) of a turbulent channel flow over porous walls. In the fluid region the flow is governed by the incompressible Navier--Stokes (NS) equations, while in the porous layers the Volume-Averaged Navier--Stokes (VANS) equations are used, which are obtained by volume-averaging the microscopic flow field over a small volume that is larger than the typical dimensions of the pores. In this way the porous medium has a continuum description, and can be specified without the need of a detailed knowledge of the pore microstructure by indipendently assigning permeability and porosity. At the interface between the porous material and the fluid region, momentum-transfer conditions are applied, in which an available coefficient related to the unknown structure of the interface can be used as an error estimate. To set up the numerical problem, the velocity-vorticity formulation of the coupled NS and VANS equations is derived and implemented in a pseudo-spectral DNS solver. Most of the simulations are carried out at $Re_\tau=180$ and consider low-permeability materials; a parameter study is used to describe the role played by permeability, porosity, thickness of the porous material, and the coefficient of the momentum-transfer interface conditions. Among them permeability, even when very small, is shown to play a major role in determining the response of the channel flow to the permeable wall. Turbulence statistics and instantaneous flow fields, in comparative form to the flow over a smooth impermeable wall, are used to understand the main changes introduced by the porous material. A simulations at higher Reynolds number is used to illustrate the main scaling quantities.Comment: Revised version, with additional data and more in-depth analysi

Evolution of the scalar dissipation rate downstream of a concentrated line source in turbulent channel flow

The dissipation rate, εθ, of a passive scalar (temperature in air) emitted from a concentrated source into a fully developed high-aspect-ratio turbulent channel flow is studied. The goal of the present work is to investigate the return to isotropy of the scalar field when the scalar is injected in a highly anisotropic manner into an inhomogeneous turbulent flow at small scales. Both experiments and direct numerical simulations (DNS) were used to study the downstream evolution of εθ for scalar fields generated by line sources located at the channel centreline (ys/h = 1.0) and near the wall (ys/h = 0.17). The temperature fluctuations and temperature derivatives were measured by means of a pair of parallel cold-wire thermometers in a flow at Reτ = 520. The DNS were performed at Reτ = 190 using a spectral method to solve the continuity and Navier-Stokes equations, and a flux integral method (Germaine, Mydlarski & Cortelezzi, J. Comput. Phys., vol. 174, 2001, pp. 614-648) for the advection-diffusion equation. The statistics of the scalar field computed from both experimental and numerical data were found to be in good agreement, with certain discrepancies that were attributable to the difference in the Reynolds numbers of the two flows. A return to isotropy of the small scales was never perfectly observed in any region of the channel for the downstream distances studied herein. However, a continuous decay of the small-scale anisotropy was observed for the scalar field generated by the centreline line source in both the experiments and DNS. The scalar mixing was found to be more rapid in the near-wall region, where the experimental results exhibited low levels of small-scale anisotropy. However, the DNS, which were performed at lower Reτ, showed that persistent anisotropy can also exist near the wall, independently of the downstream location. The role of the mean velocity gradient in the production of εθ (and therefore anisotropy) in the near-wall region was highlighted

Design of an air-flow microchamber for microparticles detec

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.A novel device, able to funnel a suspension of micrometric particles in air into a microchamber equipped with a capacitive sensor, has been designed for the detection and characterization of particulate matter (PM) in air. Numerical simulations have been performed to predict the trajectory of the microparticles through the PDMS microchamber where the sensor is located. The feasibility of detecting single PM10 particles has been demonstrated by our experiments, where sequences of single industrial talc particles (average diameter of 8 μm) have been detected and counted by a capacitive sensor. Our results indicate that radical miniaturization of air quality monitors is possible and, therefore, pervasive monitoring of air pollution will be soon feasible

Assessing the sensitivity of leeches as indicators of water quality

The objective of this work was assessing the sensitivity of leeches to several water quality attributes in lowlands streams.Weused occupancymodelling that account explicitly for detectability, to estimate the influence of four variables (dissolved oxygen, 5-days biochemicals oxygen demand, conductivity, and dissolved inorganic nitrogen) affecting nine species. We described the sensitivity as a change in the occupancy along the range of water quality attributes.We found at least one species of Helobdella in 81% of sites and Helobdella, as genus, was detected along the entire gradient of each attribute. However, differences in the sensitivitywere observed between species. For example, if we analyse the sensitivity of the genus Helobdella to dissolved oxygen, we can say that it is very tolerant. However, if we analyse the response to dissolved oxygen of each one of the species of Helobdella,wewill realize that H. michaelseni, and H. simplex showed a high occupancy at high levels of dissolved oxygen; while H. hyalina and H. triserialis lineata showed high occupancy at low levels. Describe the sensitivity of the species in terms of occupancy, offers a new methodology to understand how the species behave along a stressor gradient.Instituto de Limnología "Dr. Raúl A. Ringuelet"Facultad de Ciencias Naturales y Muse

Biological and molecular characterization of PNH-like lymphocytes emerging after Campath-1H therapy

Campath-1H, an anti-CD52 monoclonal antibody, is therapeutically active in lymphoproliferative and autoimmune diseases. After Campath-1H therapy, lymphocytes with a paroxysmal nocturnal haemoglobinuria (PNH) phenotype have been reported to emerge. We characterized a PNH-like lymphocyte population emerging after Campath-1H therapy, in a patient with fludarabine refractory B-cell chronic lymphocytic leukaemia (B-CLL). We demonstrated a reduction in PIG-A mRNA levels compared with controls, and of all cytokines tested [interleukin (IL)-4, IL-13, IL-2, interferon(IFN)-gamma, IL-6, IL-10, and tumour necrosis factor (TNF)-alpha], except transforming growth factor (TGF)-beta. Given the inhibitory activity of TGF-beta, its elevated levels may contribute to the selective pressure of Campath-1H, leading to the emergence of PNH-like lymphocyte

Features and heterogeneities in growing network models

Many complex networks from the World-Wide-Web to biological networks are growing taking into account the heterogeneous features of the nodes. The feature of a node might be a discrete quantity such as a classification of a URL document as personal page, thematic website, news, blog, search engine, social network, ect. or the classification of a gene in a functional module. Moreover the feature of a node can be a continuous variable such as the position of a node in the embedding space. In order to account for these properties, in this paper we provide a generalization of growing network models with preferential attachment that includes the effect of heterogeneous features of the nodes. The main effect of heterogeneity is the emergence of an "effective fitness" for each class of nodes, determining the rate at which nodes acquire new links. The degree distribution exhibits a multiscaling behaviour analogous to the the fitness model. This property is robust with respect to variations in the model, as long as links are assigned through effective preferential attachment. Beyond the degree distribution, in this paper we give a full characterization of the other relevant properties of the model. We evaluate the clustering coefficient and show that it disappears for large network size, a property shared with the Barab\'asi-Albert model. Negative degree correlations are also present in the studied class of models, along with non-trivial mixing patterns among features. We therefore conclude that both small clustering coefficients and disassortative mixing are outcomes of the preferential attachment mechanism in general growing networks.Comment: 16 pages, 6 figures, revte

Évaluation de l'utilisation d'îles forestières par des espèces de perroquets dans une savane néotropicale

Understanding the effect of habitat fragmentation is a fundamental yet complicated aim of many ecological studies. Beni savanna is a naturally fragmented forest habitat, where forest islands exhibit variation in resources and threats. To understand how the availability of resources and threats affect the use of forest islands by parrots, we applied occupancy modeling to quantify use and detection probabilities for 12 parrot species on 60 forest islands. The presence of urucuri (Attalea phalerata) and macaw (Acrocomia aculeata) palms, the number of tree cavities on the islands, and the presence of selective logging,and fire were included as covariates associated with availability of resources and threats. The model-selection analysis indicated that both resources and threats variables explained the use of forest islands by parrots. For most species, the best models confirmed predictions. The number of cavities was positively associated with use of forest islands by 11 species. The area of the island and the presence of macaw palm showed a positive association with the probability of use by seven and five species, respectively, while selective logging and fire showed a negative association with five and six species, respectively. The Blue-throated Macaw (Ara glaucogularis), the critically endangered parrot species endemic to our study area, was the only species that showed a negative association with both threats. Monitoring continues to be essential to evaluate conservation and management actions of parrot populations. Understanding of how species are using this natural fragmented habitat will help determine which fragments should be preserved and which conservation actions are needed.La compréhension de l'effet de la fragmentation de l'habitat est l'objectif fondamental mais compliqué de nombreuses recherches en écologie. La savane de Beni est un milieu forestier naturellement fragmenté dans lequel les îles forestières diffèrent sur le plan des ressources et des menaces. Afin de comprendre à quel point la disponibilité des ressources et les menaces ont une influence sur l'utilisation d'îles forestières par les perroquets, nous avons appliqué des modèles de présence pour quantifier l'utilisation et la probabilité de détection de 12 espèces de perroquets sur 60 îles forestières. La présence de palmiers canne (Acrocomia aculeata) et de palmiers urucuri (Attalea phalerata), le nombre de cavités d'arbres sur les îles et la présence de coupes sélectives et de feu ont été inclus comme covariables associées à la disponibilité des ressources et aux menaces. L'analyse de la sélection de modèles a indiqué que les variables relatives aux ressources et celles relatives aux menaces expliquaient l'utilisation d'îles forestières par les perroquets. Pour la plupart des espèces de perroquets, les meilleurs modèles ont confirmé les prédictions. Le nombre de cavités était positivement associé avec l'utilisation d'îles forestières chez 11 espèces. La superficie de l'île et la présence de palmiers canne étaient positivement associées avec la probabilité d'utilisation chez 7 et 5 espèces, respectivement, tandis que la coupe sélective et le feu étaient négativement associés chez 5 et 6 espèces, respectivement. L'Ara canindé (Ara glaucogularis), espèce de perroquet en voie de disparition critique endémique à notre aire d'étude, a été la seule espèce qui a montré une association négative avec les deux menaces. Le suivi demeure essentiel pour l'évaluation des mesures de conservation et de gestion des populations de perroquets. La compréhension de l'utilisation de ce milieu naturellement fragmenté par les espèces contribuera à déterminer quelles parcelles devraient être protégées et quelles mesures de conservation sont nécessaires.Facultad de Ciencias Naturales y Muse