1,721,041 research outputs found
Dry deposition of particle on urban areas
Full text linkDry deposition process is recognized as an important pathway among the removal processes of radioactive pollutants in atmosphere. There is not a unique and accepted theoretical description of involved dry deposition phenomena due to the complexity of the fluid-dynamic processes that influence the deposition flux, but also because there is a lack of experimental data covering all scenarios of interest. In this paper, that is the result of a National Research Program a research activity conducted by DEIM Department of the University of Palermo and ENEA and funded by the Italian Minister of Economic Development, a new schema for parameterization of particle dry deposition velocity on urban area is proposed. The work required comparisons with some experimental data reported in literature for different particle deposition scenarios. The results show that the proposed approach can catch some aspects of phenomena involved in dry deposition processes for the examined environmental conditions with good agreement
Simulation of Jets with a Finite Element Navier-Stokes Solver and a Multilevel VOF Approach
No full textAbstract. An accurate finite element Navier-Stokes solver is presented to simulate axis-symmetric and three-dimensional incompressible jets. It is based on a multilevel approach where the Volume-of-Fluid (VOF) scalar function is defined on a high resolution grid and then is projected on a coarser grid where the velocity field is computed. With this approach it is possible to consider grid spacings that ensure a high value of the ratio between the local radius of the interface curvature and the cell size even for three-dimensional computational domains. Since the number of cut cells is usually much smaller than the total number of cells in the computational domain, we have implemented a compact memorization of the VOF data based on a sparse matrix storage approach
A FEM solver coupled to a multilevel VOF method for simulation of axisymmetric jets and to a front-tracking method for simulation of spreading droplets
No full textIn this work we review a few studies on the performance of the finite element approach in direct numerical simulations of liquid jet injection and droplet impact on dry surfaces. We consider the single-fluid formulation of the incompressible Navier-Stokes equations, discretize these equations on a fixed grid with the finite element method, and solve the resulting algebraic system with a multigrid technique. The finite element method allows a variational formulation of the capillary force that is implemented both with a volumetric approach and by using the Laplace-Beltrami operator. In order to study the injection of a liquid jet in a gas environment at low Reynolds numbers, we have implemented a multilevel volume of fluid (VOF) method because it can deal with strong topology changes in an automatic way. We have also considered a front-tracking algorithm to study the spreading of a droplet on a dry solid surface, because there is the need of a very precise positioning of the contact point, while there is no breaking or topology change. The results of several numerical experiments are presented and discussed for both cases
Coupling a VOF method and an interface normal transport equation for two-phase flow
No full textIn a direct numerical simulation (DNS) of two-phase flows many techniques, such as the Volume-of-Fluid (VOF), surface markers and the level set methods, have been used to track the evolution of a line or a surface. These methods have been successfully applied to industrial and natural flows such as sea waves, jet atomization, splashes, droplet/bubble
oscillations which involve breakup and coalescence. In direct numerical simulations the local interface normal vector and curvature are needed to compute the capillary force. These geometrical quantities are usually calculated from the spatial derivatives of a scalar function at any instant in time. A new methodology for calculating the interface normal vector is here proposed. The normals are advected along the streamlines together with the volume fraction data in order to reconstruct more accurately the interface.The accuracy and performance of the new method are demonstrated via standard test cases with a prescribed velocity field and
the results are compared with those of traditional VOF methods
A geometrical predictor-corrector fluxing scheme and its application to the volume fraction function
No full textWe present a multidimensional Eulerian advection method for interfacial and incompressible flows in two-dimensional Cartesian geometry. In the scheme we advect the grid nodes backwards along the streamlines to compute the pre-images of the grid lines. These pre-images are approximated by continuous, piecewise-linear lines. The enforcement of the discrete version of the incompressibility constraint is a very important issue to determine correctly the flux polygons and to reduce considerably the integration, discretization and interpolation numerical errors. The proposed method compares favourably with other previous multidimensional advection methods
as long as the initial interface line is well reconstructed. Conversely, we show that when the interface is very fragmented the total numerical error is completely dominated by the reconstruction error and in these conditions it is very difficult to assess which advection scheme is the most reliable one
Simulation of axisymmetric jets with a finite element Navier-Stokes solver and a multilevel VOF approach
No full textA multilevel VOF approach has been coupled to an accurate finite element Navier-Stokes solver in axisymmetric geometry for the simulation of incompressible liquid jets with high density ratios. The representation of the color function over a fine grid has been introduced to reduce the discontinuity of the interface at the cell boundary. In the refined grid the automatic breakup and coalescence occur at a spatial scale much smaller than the coarse grid spacing. To reduce memory requirements, we have implemented on the fine grid a compact storage scheme which memorizes the color function data only in the mixed cells. The capillary force is computed by using the Laplace-Beltrami operator and a volumetric approach for the two principal curvatures. Several simulations of axisymmetric jets have been performed to show the accuracy and robustness of the proposed schem
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
A multilevel domain decomposition approach for studying coupled flow applications
No full textIn this paper, a multilevel domain decomposition approach based on multi-grid methods for obtaining fast solutions for coupled engineering flow applications arising on complex domains is presented. The proposed technique not only allows solutions to be computed efficiently at the element level but also helps us to achieve proper accuracy, load balancing and computational efficiency. Numerical results presented demonstrate the robustness of the proposed technique
A Two-step Approach for the Preliminary Evaluation of the Thermal-hydraulics and Safety of the ELSY Open Square Core Design
No full textSeveral innovative solutions for a liquid metal fast reactor design are being investigated within the framework of the ELSY (the acronym for the European Lead-cooled System) Sixth Framework Programme of URATOM. Among these solutions the open-assembly core option adopted by ENEA for the preliminary neutronic analysis requires a new approach to the thermal-hydraulic core design. In addition, it could have important implication on the reactor safety. Therefore a two-step approach at the T/H analysis of the core is being followed:
Utilization of a one-dimensional RELAP5 model for independent channels analysis for a prompt verification of the thermal-hydraulic and safety of the core neutronic design. Development of a dedicated tool for the evaluation of the intermingled effects between assembly flows
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