1,720,970 research outputs found
Modeling of extrusion-based additive manufacturing for pelletized thermoplastics: Analytical relationships between process parameters and extrusion outcomes
No full textRecent developments in additive manufacturing are moving toward a new trend in material extrusion (ISO/ ASTM 52910:2018), namely, the possibility of printing thermoplastic strands directly from pellets. Pellet additive manufacturing (PAM) is a relatively new manufacturing process that realizes the aforementioned goal. Consequently, the development of models describing the behavior of the entire process remains a matter of research. The present study aims to propose a systematic and parametric analysis based on an analytical model that describes the entire process, from solid pellet conveying to the deposition of molten strands. First, a mathematical model that analytically describes the behavior of a system consisting of a single-screw extruder, a coupling element, and an extrusion nozzle is presented, without considering the effect of the printed strand. The proposed approach allows the calculation of important process variables, such as the mass flow rate, melting profile, and pressure profile for a given screw speed. An experimental setup aimed at predicting the mass flow rate of a real single-screw extruder and computational fluid dy-namics simulations were used to validate the theory presented. The model was then extended to the PAM process, where an additional counterpressure exists because of the strand being deposited on the build plate. The goal is to predict the screw-speed which allows to extrude a prescribed mass flow rate. Subsequently, the effects of the printing nozzle speed and layer height on the process outcomes were investigated. A good agreement with similar trends already predicted in modeling the counterpressure in fused filament fabrication was found (c) 2022 CIRP
Numerical analysis of the effect of topography on deposition from dilulte pyroclastic density xurrents
No full textPyroclastic density currents (PDCs) vary between two end members, concentrated and dilute. When a PDC interacts with an uneven topography, the flow field variables (velocity, pressure, bulk density, particle concentration) may drastically change near the flow-substrate boundary. These changes may significantly affect the sediment flux and the resulting deposits can record the effects in their facies architecture. Here we show, by means of numerical simulations, how a dilute pyroclastic density current interacts with four different types of simple topography, namely: flat, one hill, one valley and two hills. Our numerical scheme treats the very fine particles as being in full thermo-mechanical equilibrium with the volcanic gas, i.e. a dusty gas. A dusty gas-air mixture is defined as a mixture of dusty gas and atmospheric air. The trajectories of the coarser particles or discrete phase (three grain-size classes of 1 mm, 5 mm and 10 mm and density of 1500 kg/m(3)) are tracked as Lagrangian particles that interact with the dusty gas-air mixture through two-way momentum and energy coupling. Numerical results are used to analyze the local effects of topography on the deposition of the Lagrangian particles, by monitoring with time and space the local changes at the boundary between the current and the substrate. The results show that the sediment flux in the flow boundary zone increases near the stoss sides of hills and in the valleys, relative to the flat reference case, whereas it decreases along the lee flanks and on top of the hills. We use the sediment flux in the flow boundary zone and the grain-size distribution of the Lagrangian particles as proxies of the deposit features, and by these parameters we qualitatively compare simulations with deposits of known eruptions
Polynomial chaos assessment of design tolerances for vortex-induced vibrations of two cylinders in tandem
No full textAbstractThe presence of aerodynamics loadings makes the design of some classes of elastic structures, as, for instance, marine structures and risers, very challenging. Moreover, capturing the complex physical interaction between the structure and the fluid is challenging for both theoretical and numerical models. One of the most important phenomena that appear in these situations is vortex-induced vibrations. The picture is even more complicated when multiple elastic elements are close enough to interact by modifying the fluid flow pattern. In the present work, we show how the common design practice for these structures, which is entirely based on deterministic simulations, needs to be complemented by the uncertainty quantification analysis. The model problem is a structure constituted by two elastically mounted cylinders exposed to a two-dimensional uniform flow at Reynolds number 200. The presence of a manufacturing tolerance in the relative position of the two cylinders, which we consider to be a source of uncertainty, is addressed. The overall numerical procedure is based on a Navier–Stokes immersed boundary solver that uses a flexible moving least squares approach to compute the aerodynamics loadings on the structure, whereas the uncertainty quantification propagation is obtained by means of a nonintrusive polynomial chaos technique. A range of reduced velocities is considered, and the quantification, in a probabilistic sense, of the difference in the performances of this structure with respect to the case of an isolated cylinder is provided. The numerical investigation is also complemented by a global sensitivity analysis based on the analysis of variance.</jats:p
Fluid Mechanics of Deformable Aortic Prostheses
No full textThe simultaneous replacement of a diseased aortic valve, aortic root and ascending aorta with a composite graft equipped with a prosthetic valve is a nowadays standard surgical approach in which the Valsalva sinuses of the aortic root are sacrificed and the coronary arteries are reconnected directly to the graft (Bentall procedure). In practice, two different polyethylene terephthalate (Dacron) prostheses are largely used by surgeons: a standard straight graft and a graft with a bulged portion that better reproduces the aortic root anatomy (Valsalva graft). The aim of the present investigation is to study the effect of the graft geometry, with its pseudo-sinuses, on the the flowfield, with particular attention to the coronary entry-flow, and on the stress concentration at the level of coronary-root anastomoses during the cardiac cycle. A bi-leaflet mechanical valve with curved leaflets is considered, attached to the two different prostheses. Two cylindrical channels, reproducing the very early coronary vasculature are connected to the grafts. An accurate three-dimensional numerical method, based on the immersed boundary technique, is proposed to study the flow inside deformable geometries. Direct numerical simulations of the flow inside the prostheses under physiological pulsatile inflow conditions are presented. The dynamics of the leaflets (considered rigid) is obtained by a fully-coupled fluid-structure-interaction approach, while a weak-coupled approach is employed for the deforming roots, in order to reduce the computational cost, using optimized solvers for both the fluid and structural problems. The Dacron material is modeled as orthotropic, with an inversion of the material properties in longitudinal and circumferential direction for the skirt region of the Valsalva prosthesis. Coronary perfusion is reproduced modulating in time the porosity, and thus the resistance, of the coronary channels. The results indicate that while the pseudo-sinuses do not significantly influence the coronary entry-flow, their presence allows for smaller levels of stresses at the level of coronary-root anastomoses, potentially reducing post-operative complications.</jats:p
Linear stability analysis of fluid-structure interaction problems with an immersed boundary method
No full textIn this work, we present a novel approach to perform the linear stability analysis of fluid-structure interaction problems. The underlying idea is the combination of a validated immersed boundary solver for the nonlinear coupled dynamics with Krylovbased techniques to obtain a robust and accurate global stability solver for elastic structures interacting with incompressible viscous flows. The computation of the leading eigenvalues of the linearized system is carried out in a matrix-free framework by adopting a classical Krylov subspace method. The proposed algorithm avoids the complex analytical linearization of the equations while retaining all the relevant aspects of the fully-coupled fluid-structure system. The methodology has been tested for several cases involving two-dimensional incompressible flows around elastically mounted circular cylinders. The obtained results show a good quantitative agreement with those available in the literature. Finally, the method was applied to investigate the linear stability of the laminar flow past two elastically mounted cylinders in tandem configuration at Re = 100, revealing the existence of two complex dominant modes. For low values of the reduced velocity U*, only one mode is found to be unstable and related to the stationary wake mode. The loss of stability of the second mode at U* = 4 marks the beginning of the lock-in region. We also show that for U* = 5 the modes interact, giving rise to the beating phenomenon observable in the nonlinear time evolution of the system. For larger values of the reduced velocity, the linear dynamics is governed by one dominant mode characterized by wider oscillations of the rear cylinder, matching the results of the nonlinear simulations.(c) 2022 Elsevier Ltd. All rights reserved
On the influence of the aortic root geometry on blood flow after a bileaflet mechanical heart valve implant: a numerical study.
Full text linkThe simultaneous replacement of a diseased aortic valve, aortic root and ascending aorta with a prosthesis is known as Bentall procedure (Bentall and De Bono in Thorax 23:338, 1968). This is a nowadays standard surgical approach in which the Valsalva sinuses of the aortic root are sacrificed and the coronary arteries are reconnected directly to the graft. The important function of the natural sinuses in the presence of the natural valve is well established; however, very little information is available about whether or not their presence can affect the functioning of a prosthetic hi-leaflet valve and the coronary flow. In the present work, we study the effect of the aortic root geometry on the blood flow through such devices, focusing the attention on the coronary entry-flow. Three root geometries have been considered, two mimicking the prostheses used in practice by surgeons (a straight tube, and the more recent tube with a circular pseudo-sinus), and a third maintaining the natural shape with three sinuses, Obtained by Reul et al. (J Biomech 23:181-191, 1990) by averaging numerous angiographies of the aortic root in healthy patients. Direct numerical simulations of the flow inside the three prostheses, assumed as undeformable, under physiological pulsatile inflow conditions are presented. The dynamics of the valvular leaflets is obtained by a fully-coupled fluid structure-interaction approach and the coronary perfusion is reproduced by modulating in time an equivalent porosity, an thus the resistance, of the coronary channels. The results indicate that the sinuses do not significantly influence the coronary entry flow, in agreement with the in vivo observations of De Paulis et al. (Eur J Cardio-thorac Surg 26:66-72, 2004). Nevertheless, the peak pressure at the joints of the coronary arteries is smaller in the natural-like aortic root geometry. The latter also produces a further beneficial effect of a reduction in the leaflets' angular velocity at the closure onto the valvular ring. These phenomena, if confirmed in more realistic clinical conditions, suggest that the use of a prothesis with physiologic sinuses would potentially reduce the local pressure peak, with the associated risk of post-operative bleeding and pseudo-aneurysm formation. It would also reduce the haemolysis effects caused by the red blood cells squashing between impacting solid artificial surfaces
siRNA-Chitosan Complexes in Poly(lactic-co-glycolic acid) Nanoparticles for the Silencing of Aquaporin-1 in Cancer Cells
Full text linkA large number of studies document the strong expression of aquaporin-1 (AQP1) in tumor microvessels and correlate this aberrant expression with higher metastatic potential and aggressiveness of the malignancy. Although small animal experiments have shown that the modulation of AQP1 expression can halt angiogenesis and induce tumor regression, effective and safe strategies for the tissue specific inhibition of AQP1 are still missing. Here, small interference RNA-chitosan complexes encapsulated in poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) are proposed for the intracellular delivery of siRNA molecules against AQP1. These NPs are coated with poly(vinyl alcohol) (PVA), to improve stability under physiological conditions, and demonstrate a diameter of 160 nm. The partial neutralization of the negatively charged siRNA molecules with the cationic chitosan enhances the loading by 5-fold, as compared to that of the free siRNA molecules, and allows one to modulate the release kinetics in the pH-dependent manner. At pH = 7.4, mimicking the conditions found in the systemic circulation, only the 40% of siRNA is released at 24 h post incubation; whereas at pH = 5.0, recreating the cell endosomal environment, all siRNA molecules are released in about 3 h. These NPs show no cytotoxicity on HeLa cells up to 72 h of incubation. In the same cells, transfected to overexpress AQP1, a silencing efficiency of 70% is achieved at 24 h post treatment with siRNA-loaded NPs. Confocal microscopy analysis of NP uptake demonstrates that siRNA molecules accumulate perinuclearly and in the nucleus. Given the stability, preferential release behavior, and well-known biocompatibility properties of PLGA nanostructures, these siRNA-loaded NPs hold potential for the efficient and safe in vivo silencing of AQPs via systemic administration.A large number of studies document the strong expression of aquaporin-1 (AQP1) in tumor microvessels and correlate this aberrant expression with higher metastatic potential and aggressiveness of the malignancy. Although small animal experiments have shown that the modulation of AQP1 expression can halt angiogenesis and induce tumor regression, effective and safe strategies for the tissue specific inhibition of AQP1 are still missing. Here, small interference RNA-chitosan complexes encapsulated in poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs) are proposed for the intracellular delivery of siRNA molecules against AQP1. These NPs are coated with poly(vinyl alcohol) (PVA), to improve stability under physiological conditions, and demonstrate a diameter of 160 nm. The partial neutralization of the negatively charged siRNA molecules with the cationic chitosan enhances the loading by 5-fold, as compared to that of the free siRNA molecules, and allows one to modulate the release kinetics in the pH-dependent manner. At pH = 7.4, mimicking the conditions found in the systemic circulation, only the 40% of siRNA is released at 24 h post incubation; whereas at pH = 5.0, recreating the cell endosomal environment, all siRNA molecules are released in about 3 h. These NPs show no cytotoxicity on HeLa cells up to 72 h of incubation. In the same cells, transfected to overexpress AQP1, a silencing efficiency of 70% is achieved at 24 h post treatment with siRNA-loaded NPs. Confocal microscopy analysis of NP uptake demonstrates that siRNA molecules accumulate perinuclearly and in the nucleus. Given the stability, preferential release behavior, and well-known biocompatibility properties of PLGA nanostructures, these siRNA-loaded NPs hold potential for the efficient and safe in vivo silencing of AQPs via systemic administration. © 2013 American Chemical Society
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
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