Development Of Accurate Numerical Models Research Articles

Comparison between DNS and RANS approaches for liquid metal flows around a square rod bundle

The thermal-hydraulic characteristics of liquid metal flows around rod bundles are of great interest for the research and design of fourth generation nuclear reactors. Currently, a large research effort is aimed at the development of accurate numerical models for low Prandtl number fluid flows, since the data available in the literature are quite scarce. Direct Numerical Simulation (DNS) is undoubtedly the most accurate approach, but its large requirements of computational resources and time make it less practical than other simplified methods such as the Reynolds-Average Navier Stokes (RANS) approach. The present paper provides a comparison between numerical results of a flow of liquid Lead-Bismuth Eutectic (LBE) at Pr=0.031 around four vertical cylindrical rods arranged in a square lattice, obtained by DNS and RANS. Several turbulence models are considered, including the standard k−ε, k−ω SST, and two Reynolds stress models, namely the ones by Launder, Reece and Rodi (LRR), and Speziale, Sarkar and Gatski (SSG). The accuracy of these models is assessed by comparing the mean Nusselt number, the pressure drop, and local field distributions with those obtained by DNS.

A comparative numerical-experimental investigation on the tensile behaviour of bonded, rivetted and hybrid composite joints configurations

This paper deals with the development of accurate numerical models to predict the tensile mechanical behaviour of composite joints. The problems of damage initiation and propagation in composite joints for an economic solution with minimized oversizing is addressed. Different types of joints (Single Lap Joint, Single Strap Joint, Joggle Joint) and couplings (bonded, rivetted, hybrid) have been combined to find out the most suitable and reliable joining technique depending on the design constraints. On the basis of preliminary DCB and ENF tests, the characteristics of the adhesive material (stress limits and fracture energies) have been determined. These data have been employed for the numerical simulations on bonded and hybrid (bonded/rivetted) joints. The introduced numerical models, taking into account geometrical non-linearity and progression of damage in composite plies, have been preliminary validated by ad-hoc experimental tests to demonstrate their effectiveness in simulating the mechanical behaviour of general composite joints.

An effective numerical modelling strategy for FRCM strengthened curved masonry structures

Fabric Reinforced Cementitious Matrix (FRCM) composites are currently considered a very effective solution for strengthening masonry constructions. However, the mechanical interactions governing the response and the strength of FRCM reinforced masonry structures are very complex, especially in the case of curved structures. Moreover, these interactions involve several interfaces between different materials. Thus, the development of accurate numerical models for curved FRCM reinforced masonry structures comes up against several difficulties, and models too complex for practical applications can be obtained. In addition, several mechanical parameters needed for the calculations are generally inaccessible by conventional experimental tests.Here, a suitable numerical modelling strategy for FRCM strengthened curved masonry structures is proposed to combine the accuracy in simulating the actual behaviour in terms of stiffness, strength and collapse mechanisms with a reasonable simplicity, making the proposed approach usable also by practitioners, by adopting commercial codes and at a moderate computational effort. The relatively small number of mechanical parameters characterizing the model can be determined by ordinary experimental tests on materials or by literature formulations.The proposed modelling strategy is validated with respect to experimental data found in literature concerning a FRCM reinforced masonry barrel vault, and then is employed for studying the seismic capacity of the vault through a pushover analysis. A broad sensitivity analysis sheds light on the effect of variations of the mechanical parameters on the predicted overall behaviour, showing the robustness of the results obtainable through the proposed approach concerning inaccuracies in the determination of the parameters often very difficult to determine by ordinary experimental tests on masonry structures.

The Influence of Gas\u2013Liquid Interfacial Transport Theory on Numerical Modelling of Plasma Activation of Water

Plasma activated water has shown great promise in a number of emerging application domains; yet the interaction between non-equilibrium plasma and liquid represents a complex multiphase process that is difficult to probe experimentally, necessitating the development of accurate numerical models. In this work, a global computational model was developed to follow the concentrations of aqueous reactive species in water treated using a surface barrier discharge in ambient air. While the two-film theory has long superseded other methods of modelling mass transfer in such areas of research as environmental and aerosol science, plasma modelling studies continue to use equilibrium and one-film theories. The transport of reactive species across the gas–liquid interface was therefore treated using the one-film and two-film theories, with the results compared to ascertain which is most appropriate for PAW modelling studies. Comparing the model-predicted concentrations to those measured, it was shown that concentrations of aqueous H+ and NO3− ions were better represented by the two-film theory, more closely fitting experimental measurements in trend and in magnitude by a factor of ten, while HNO2 and NO2− showed a slightly worse fit using this theory. This is attributed to the assumption in two-film theory of a gas-phase stagnant film which provides additional resistance to the absorption of hydrophilic species, which is absent in the one-film theory, which could be improved with a more accurate value of the Sherwood number for each species.

Investigation and Validation of Numerical Models for Composite Wind Turbine Blades

As the world shifts to using renewable sources of energy, wind energy has been established as one of the leading forms of renewable energy. As the requirement for wind energy increases, so too does the size of the turbines themselves, where the latest turbines are 10 MW with a turbine diameter in excess of 190 m. The design and manufacture of the blades for these turbines will be critical if they are to last for the design life, where the accuracy of the numerical models used in the design process is paramount. Therefore, in this paper, three independent numerical models have been created using three available finite element method packages—ABAQUS, ANSYS, and CalculiX—and the results were compiled. Following this, the accuracy of the models has been evaluated and validated against the results from an experimental testing campaign. In order to complete the study, a 13 m full-scale wind turbine blade has been used, which has been subjected to static testing in both the edgewise and flapwise directions. The results from this testing campaign, along with the blade mass and natural frequencies, have been compared to the results from the independent numerical models. The differences in the models, along with other sources of error, have been discussed, which includes recommendations on the development of accurate numerical models.

Damage assessment and partial failure mechanisms activation of historical masonry churches under seismic actions: Three case studies in Mantua

Abstract The structural safety and seismic performance assessment of historical masonry churches belonging to cultural heritage is currently a topic of increasing interest in Italy. Recent Italian seismic events have proved that severe damage or collapse can be caused to such typology of structures even by earthquakes of small-to-moderate magnitude. This paper describes the damage occurred in three important historical masonry churches of the outstanding cultural heritage in Mantua, Northern Italy, after the 2012 Emilia earthquake and presents the results of extensive numerical investigations carried out on detailed FE models with an elasto-plastic damage constitutive law for masonry. Historical and documentary research, on-site surveys and visual inspections were fundamental to identify the critical elements and obtain a thorough detailed knowledge of the churches for the development of accurate numerical models. The results of advanced FE analyses were used to investigate the causes of the crack patterns observed after the earthquake, indicating that the damage distribution detected in the churches can be simulated satisfactorily by the numerical approach adopted. Moreover, this study provides additional information on the seismic response of the three churches analyzed under higher PGA than those registered in Mantua during the 2012 Emilia earthquake. Comparisons among the seismic response of the churches are made in terms of predicted damage distribution, maximum normalized displacements and energy density dissipated by tensile damage.

Development of Accurate Numerical Models for Bending of Aluminum Tailored Blanks

Nowadays the main target in the automotive field is the realization of lightweight and safe components. In this way it is possible to reduce costs and improve fuel consumption and, at the same time, enhance passenger safety. The use of tailored blanks has increased considerably in the automotive industry. Tailored blanks are a combination of different thicknesses or different materials, obtained by welding together two or more blanks, used in particular in car body panels. A new requirement in the automotive sector is the application of aluminum tailored blanks. The main target of this paper is the development of accurate numerical models for bending tailored blanks made from thin aluminum sheets, joined by laser welding, without filler metal. The FE bending simulations have been carried out using an explicit solver. The accuracy of the numerical models has been estimated and improved through a comparison with the results from an experimental study. The experimental tests have been performed using bending testing equipment, designed and developed by the authors. Three different bending radii have been tested. Tailored blanks, used as specimens, have been made by laser welding of thin Al6061 sheets. The considered outputs, used for the numerical-experimental comparison, are the punch force and the bending angle. The experimental results have been compared with the numerical ones in order to verify the accuracy of the FE model related to thickness and radius variations.

On the mechanical stability of porous coated press fit titanium implants: A finite element study of a pushout test

Pushout tests can be used to estimate the shear strength of the bone implant interface. Numerous such experimental studies have been published in the literature. Despite this researchers are still some way off with respect to the development of accurate numerical models to simulate implant stability.In the present work a specific experimental pushout study from the literature was simulated using two different bones implant interface models. The implant was a porous coated Ti–6Al–4V retrieved 4 weeks postoperatively from a dog model. The purpose was to find out which of the interface models could replicate the experimental results using physically meaningful input parameters.The results showed that a model based on partial bone ingrowth (ingrowth stability) is superior to an interface model based on friction and prestressing due to press fit (initial stability).Even though the present study is limited to a single experimental setup, the authors suggest that the presented methodology can be used to investigate implant stability from other experimental pushout models. This would eventually enhance the much needed understanding of the mechanical response of the bone implant interface and help to quantify how implant stability evolves with time.

Measurement of microslips of subsurface fatigue crack faces due to rolling loads

The development of accurate numerical models of the growth of subsurface fatigue cracks due to rolling load needs reliable experimental data on the actual interactions between the crack faces. The distribution of microslips that occur between the crack faces and the interaction between the crack faces as a function of the magnitude and the position of the load with respect to the crack tip are of particular interest. In this study, the microslips of the faces of a subsurface fatigue crack developed in a railhead made of 900A steel were measured by the grating holographic interferometry (GHI) technique. Reconstruction of each double-exposed hologram made it possible to obtain four interferograms at multiple off-axis angles. On the basis of the interferograms, two-dimensional maps of the displacement vector components were determined, making it possible to calculate the distribution over the crack length of the crack face microslips and the crack opening and closing. A specifically designed experimental stand allowed the application of a stepwise increase in the normal load and consequently stepwise rolling of a constant load along the edge of the specimen.

Mixed finite element methods for computing groundwater velocities

AbstractCentral to the understanding of problems in water quality and quantity for effective management of water resources is the development of accurate numerical models to stimulate groundwater flows and contaminant transfer. We discuss several important difficulties arising in modeling of subsurface flow and present promising numerical procedures for alleviating these problems. Furthermore, we describe mixed‐finite element techniques for accurately approximating fluid velocities, and review computational results on a variety of hydrologic problems.