Showing papers in "THE INTERNATIONAL JOURNAL OF COMPUTATIONAL METHODS AND EXPERIMENTAL MEASUREMENTS in 2016"
TL;DR: In this article, the slug flow regime in an air-water horizontal pipe flow has been simulated using the CFD technique, and the variables identified to characterise the slug regime are the slug length and slug initiation.
Abstract: In this work, the slug flow regime in an air-water horizontal pipe flow has been simulated using the CFD technique. The variables identified to characterise the slug regime are the slug length and slug initiation. Additionally, the pressure drop and the pressure distribution within the simulated pipe segment have been predicted. The volume of fluid method was employed assuming unsteady, immiscible air-water flow, constant fluid properties and coaxial flow. The model was developed in the STAR-CCM+ environment, and the grid was designed in the three dimensional domain using directed mesh. A grid independency study was carried out through the monitoring of the water velocity at the outlet section. 104,000 hexahedral cells for the entire geometry were decided on as the best combination of computing time and accuracy. The simulated pipe segment was 8 m long and had a 0.074 m internal diameter. Three cases of air-water volume fractions have been investigated, where the water flow rate was pre-set at 0.0028 m3/s, and the air flow rate was varied at three dissimilar values of 0.0105, 0.0120 and 0.015 m3/s. These flow rates were converted to superficial velocities and used as boundary conditions at the inlet of the pipe. The simulation was validated by bench marking with a Baker chart, and it had successfully predicted the slug parameters. The computational fluid dynamics simulation results revealed that the slug length and pressure were increasing as the air superficial velocity increased. The slug initiation position was observed to end up being shifted to a closer position to the inlet. It was believed that the strength of the slug was high at the initiation stage and reduced as the slug progressed to the end of the pipe. The pressure gradient of the flow was realised to increase as the gas flow rate was increasing, which in turn was a result of the higher mean velocity. © 2016 WIT Press
16 citations
TL;DR: This paper presents a methodology based on the Galerkin formulation of the finite elements method to obtain representations of relationships that are defined a priori, between a set of variables: y = z(x1, x2,...., xd).
Abstract: Numerical modelling methodologies are important by their application to engineering and scientific problems, because there are processes where analytical mathematical expressions cannot be obtained to model them. When the only available information is a set of experimental values for the variables that determine the state of the system, the modelling problem is equivalent to determining the hyper-surface that best fits the data. This paper presents a methodology based on the Galerkin formulation of the finite elements method to obtain representations of relationships that are defined a priori, between a set of variables: y = z(x1, x2,...., xd). These representations are generated from the values of the variables in the experimental data. The approximation, piecewise, is an element of a Sobolev space and has derivatives defined in a general sense into this space. The using of this approach results in the need of inverting a linear system with a structure that allows a fast solver algorithm. The algorithm can be used in a variety of fields, being a multidisciplinary tool. The validity of the methodology is studied considering two real applications: a problem in hydrodynamics and a problem of engineering related to fluids, heat and transport in an energy generation plant. Also a test of the predictive capacity of the methodology is performed using a cross-validation method.
13 citations
TL;DR: In this article, the authors investigated the relationship between reinforcing steel and concrete under high-loading rates and found that the bond stress dependence on the loading rate was not as high as the scattering of the results.
Abstract: Understanding the bond behaviour between reinforcing steel and concrete under high-loading rates is becoming more and more important with increasing frequency of natural disasters, impact loadings and a threat of terrorism. This paper aims to obtain a better understanding of the material interactions between the steel rebar and the concrete in the bond zone under different loading rates. During the experimental program push-in tests were conducted under quasi-static and dynamic loading conditions. Both a servo-hydraulic machine as well as an instrumented drop tower were used during the investigation. Samples with short-bond zone in the middle of a cylindrical specimen were used and only a small reinforcement bar diameter (10 mm) was investigated. This approach was chosen to ensure constant bond stress distribution and that the failure occurs during the first pass of the stress wave through the bond zone. Throughout the experimental programme the loading rate was varied from 0.01 mm/s to 8.3 m/s. Bond stress–slip relationships in dependence on the bond stress rate are presented in this paper. The results indicate a bond stress dependence on the loading rate although the scattering of the results is quite high. The experimentally determined dynamic increase factor (DIF) for concrete-steel bond stress is around 1.5 which is a value comparable to other authors.
13 citations
TL;DR: In this paper, a computational approach, based on the solution of Reynolds-averaged-Navier-Stokes (RANS) equations, to describe the flow within and around a diffuser augmented wind turbine (DAWT) is reported.
Abstract: In this paper, a computational approach, based on the solution of Reynolds-averaged-Navier–Stokes (RANS) equations, to describe the flow within and around a diffuser augmented wind turbine (DAWT) is reported. In order to reduce the computational cost, the turbine is modeled as an actuator disc (AD) that imposes a resistance to the passage of the flow. The effect of the AD is modeled applying two body forces, upstream and downstream of the AD, such that they impose a desired pressure jump. Comparison with experiments carried out in similar conditions shows a good agreement suggesting that the adopted methodology is able to carefully reproduce real flow features.
13 citations
11 citations
10 citations
TL;DR: In this article, the axial force and the velocity distribution in the diffuser were analyzed for a diffuser augmented wind turbine (DAWT), and it was shown that the thrust on an unshrouded screen is lower than on a shrouded screen, under the same inflow condition.
Abstract: This study presents an experimental investigation on a diffuser augmented wind turbine (DAWT). A screen mesh is used to simulate the energy extraction mechanisms of a wind turbine in experiment. Different screen porosities corresponding to different turbine loading coefficients are tested. Measurements of the axial force and of the velocity distribution in radial direction are reported. The general purpose is to highlight the dependency between the diffuser and the screen, and to compare the radial velocity distributions in the diffuser between unloaded and loaded conditions. It is shown that the thrust on an unshrouded screen is lower than on a shrouded screen, under the same inflow condition. Moreover, the thrust on the diffuser largely depends on the screen loading. For the present configuration, the thrust on the screen with high loading coefficient contributes for more than 70% of the total thrust on the DAWT. Smoke visualizations and radial velocity profiles reveal that the high loading screen induces flow separation on the outer surface of the diffuser, justifying the results of the thrust measurements. It is also inferred that the flow separation leads to loss of thrust and has a great effect on the total pressure drag. It should be emphasized that the experimental results indicate that the flow field around the diffuser is strongly affected by the choice of screen porosity, that is, turbine loading. And that, the thrust coefficient of the diffuser does not show a linear dependence on the thrust coefficient of the screen. The axial momentum theory, therefore, is not a solid predictor for DAWT performance with high loaded screens.
9 citations
8 citations
TL;DR: In this article, the influence of geometry of charges on the propagation of blast waves was investigated in the field tests of various shape charges (cylinder, sphere, irregular shape) and compared.
Abstract: The purpose of the paper is to investigate the influence of geometry of charges on the propagation of blast waves. Various shape charges (cylinder, sphere, irregular shape) were used in the field tests. The main type of explosive, homemade ANFO (Ammonium nitrate + fuel oil), was used as the most common used explosives in improvised explosive devices used in terrorist attacks. Characteristics of homemade and industrially made ANFO explosives are different. There were comparing charges of various types of industrially produced types of explosives and homemade explosive in the field tests. The blast wave propagation were investigated and compared.
6 citations
TL;DR: In this article, different beam-to-column connections, wet and dry connections, are studied and investigated numerically under the column removal scenario, and a detailed model for the different connections is developed using the Applied Element Method (AEM).
Abstract: Precast concrete components are manufactured in a well-controlled environment. It has been proven to show good behaviour under gravity and lateral loads. However, the beam to column connections remain the critical part in the precast concrete structures under the column loss scenario in a progressive collapse scenario. In this paper, different beam to column connections, wet and dry connections, are studied and investigated numerically under the column removal scenario. A detailed model for the different connections is developed using the Applied Element Method (AEM). Different column removal locations are considered in the study to provide a comprehensive assessment. The performance of the connections is studied in terms of ultimate load capacity and rotational ductility. According to the results obtained, a connection enhancement is suggested to increase the resistance of precast concrete structures to progressive collapse.
6 citations
TL;DR: In this paper, a new dimensionless number valid to predict cavitation in some scenarios where the traditional cavitation number fails is proposed, which can also be used for a better design of experiments with the tube-arrest method as a practical application.
Abstract: Cavitation is the formation of vapour cavities in a liquid due to a local low pressure. The traditional cavitation number is used to predict the occurrence of cavitation in liquid flows through devices such as pumps, propellers or dam spillways. However this number can only be applied when cavitation is produced by changes of dynamic and static pressure in a liquid flow. There are other means to produce cavitation where the traditional cavitation number cannot be applied. The purpose of this research is to formulate a new dimensionless number valid to predict cavitation in some scenarios where the traditional cavitation number fails. The “tube-arrest” method produces cavitation by subjecting a column of liquid to a high acceleration without the need of any velocity between the liquid and the tube. In this scenario the traditional number is not useful due to the low values of relative velocity between liquid and walls. However the dimensionless number reported here predicts accurately the occurrence of cavitation in the “tube-arrest” method, as it is shown by Finite Element Method analysis. There is another scenario where the dimensionless number is tested successfully that is the bulk of a liquid downstream of a closing valve. A systematic comparison between the values of the dimensionless number and the occurrence of cavitation predicted by the FEM analysis is given. On the other hand the values of the traditional cavitation number are calculated and it is shown that these values are meaningless in these scenarios. In contrast, the agreement between the prediction of the dimensionless number and the simulations is excellent. It is concluded that the new dimensionless number predicts cavitation in scenarios where the traditional number is meaningless. It can also be used for a better design of experiments with the “tube-arrest” method as a practical application.
TL;DR: In this paper, a blast retrofit technique for concrete structures using carbon fiber-reinforced polymer (CFRP) layers was investigated for use in large infrastructure systems with the overarching goal of preventing against major loss of life and considerable damage that would require extensive repair.
Abstract: A blast retrofit technique for concrete structures using carbon fiber-reinforced polymer (CFRP) layers was investigated for use in large infrastructure systems with the overarching goal of preventing against major loss of life and considerable damage that would require extensive repair. Large-scale experiments were conducted and the retrofit behavior was investigated for application on relatively large reinforced concrete walls subjected to blast-like loadings. The experimental program utilized the University of California San Diego (UCSD) Blast Simulator. The Blast Simulator is able to induce various blast-like shock waves to the test specimen in a controlled laboratory environment. The performance of this blast retrofit was tested and then analyzed using SDOF and finite element modeling methods. A finite element model was created using LS-DYNA and utilized contact algorithms for the CFRP-concrete interface. Results and comparisons between the two analysis methods are given.
TL;DR: In this article, a gas-solid cyclone separator was used in a complex cement production plant to achieve a 5% improvement in the separation efficiency and thermal exchange performance, while keeping pressure losses under control.
Abstract: This work presents a study on a gas–solid cyclone separator used in a complex cement production plant. The main objective of the study is based on the performance evaluation and optimization of the cyclone separator in terms of particle separation and heat transfer efficiencies, while keeping pressure losses under control. The thermal interaction is between two gas–solid mixtures, one at 850°C and the other at 600°C, respectively. The solid phase consists mostly of calcium carbonate subsequently intended to the so-called baking process for the production of clinker and ultimately cement. A first model has been setup using experimental data as boundary conditions to assess the physical model behavior and the CFD solver parameters. After that, five additional models with different geometries have been analyzed to evaluate the influence of the vortex finder (vf) length on the separation efficiency and on the heat exchange performance. By increasing the length of the vf, the results show a global improvement in the separation efficiency of up to 5% if compared to the geometry without the vf. Further, the increase in the vf length determines a monotonic decrease of temperature at the exit but a monotonic increase of pressure losses. In the second part of this work, using one of the previous models with vf, a study of the influence of the particle diameter on the separation efficiency has been performed. The increase of particle diameter causes an increase of the separation and thermal exchange performance, decreasing at the same time the pressure drop. The numerical approach for all the cases is based on implicit unsteady simulations using the Eulerian Multiphase model.
TL;DR: In this paper, the authors present a direct numerical simulation of the Stokes flow in liquid bridges obtained between two cylindrical pillars confined between two horizontal plates, which is obtained under conditions of hydrostatic equilibrium thanks to the Surface Evolver software.
Abstract: Flow within thick liquid films present owing to capillary effects in the pore space is of key importance in many multiphase flow applications in porous media, for example, drying or oil recovery processes. The viscous resistance to the flow is a key parameter for modelling fluid transport in such situations. It is well known for liquid films wetting the corners of tubes of polygonal cross-section. In this latter case, the liquid films shape is simple and can be readily obtained. The situation is much more involved when considering a realistic pore space, as in a packing of spherical particles, for example. In this case, X-ray tomography observations have shown that most of the liquid is confined around contact points between particles at intermediate liquid saturation. Nonetheless, a connectivity of all the liquid bridges throughout the particle packing can exist, allowing liquid transport across the porous medium. The ultimate goal of the present research is to provide the viscous flow resistance for such capillary liquid cluster of complex shape. As a first step in this direction, we present in this paper direct numerical simulation of the Stokes flow in liquid bridges obtained between two cylindrical pillars confined between two horizontal plates. The liquid bridge shape is obtained under conditions of hydrostatic equilibrium thanks to the Surface Evolver software. Then simulations of the viscous flow within the bridge are performed using Comsol Multiphysics ® Creeping flow solver.
TL;DR: In this paper, a 3D finite element model was developed for a guardrail post installed through an asphalt layer, and the model was refined using the experimental results from the test program as well as material testing.
Abstract: The preferred procedure for steel guardrails in the state of Georgia, USA for vehicle impact employs a post-installation machine to drive the posts through a layer of asphalt placed to retard vegetation growth around the system. However, in order to avoid undesirable restraint at the ground line, the AASHTO Roadside Design Guide recommends incorporating leave-outs. Using a leave-out in vegetation barriers is seen as less desirable because of issues including significantly higher expected costs, variability in the placement and spacing of posts, and the need for variable construction scheduling. In lieu of leaveouts, predetermined fracture planes, or “pre-cuts” were installed in the asphalt and evaluated in terms of ground restraint. An experimental program was carried out on an outdoor test site. Posts were installed in pre-cut asphalt and subjected to static loading to provide a better understanding of the behavior of a post restrained with an asphalt layer at the ground line. In parallel with the experimental program, a three dimensional finite element model was developed for a guardrail post installed through an asphalt layer. The model was refined using the experimental results from the test program as well as material testing. Results from the experimental program and finite element analyses indicate that certain precutting configurations lead to significantly less ground restraint as desired.
TL;DR: In this article, the authors presented results on the isothermal laminar flow of a thin fluid layer over a sphere as it exits from a small hole at the top of the sphere.
Abstract: We present results on the isothermal laminar flow of a thin fluid layer over a sphere as it exits from a small hole at the top of the sphere. Such a flow can be observed in a globe fountain. The fluid is taken to be viscous, incompressible and Newtonian while the flow is assumed to possess azimuthal symmetry. The governing Navier-Stokes equations are solved subject to no-slip and impermeability boundary conditions on the surface and the dynamic and kinematic conditions along the free surface. An approximate analytical solution for the steady-state flow has been derived by expanding the flow variables in powers of a small parameter, d, which represents the shallowness parameter. The leading and first-order terms in the series have been determined and the findings demonstrate that for thin flows the approximate solution is indeed accurate. Various results and comparisons are presented and discussed. Lastly, the analysis was also extended to solve the problem of thin flow over a cylinder and the fundamental differences between the flow over a sphere and that over a cylinder have been identified and explained. The technique and the approach adopted can be used to model and understand similar thin flows that occur in other settings.
TL;DR: In the present contribution, three different numerical strategies are compared and it is concluded that the SPH method is the preferred strategy to model the considered scenario, a contact detonation scenario with a concrete target and with a breakthrough.
Abstract: With continuous advancements in computational capacity, it has become possible and feasible to numerically model very complex physical phenomena, for instance, high dynamic loads. Hydrocodes or, in other words, “wave propagation codes” were conceived to model such scenarios. Several numerical discretisations are available in these programs, which require the problem at hand to be modelled in distinct ways and which yield different results. In the present contribution, three different numerical strategies are compared. These employ a coupling of the Euler and the Lagrange scheme, the Euler scheme by itself as well as the Smooth Particle Hydrodynamics (SPH) scheme. Their application in the hydrocode ANSYS Autodyn to a contact detonation scenario with a concrete target and with a breakthrough is described as an example of a high dynamic load. This scenario is of special interest since it is a possible threat to critical infrastructure. The numerical results are compared and contrasted; individual strengths and weaknesses of the three numerical modelling strategies are identified also by validating their numerical results with an experimental one. To the authors’ knowledge, such comparison has not yet been done for contact detonation. It is concluded that the SPH method is the preferred strategy to model the considered scenario.
TL;DR: In this paper, the effects of drop ion concentration and interfacial tension on the coalescence process were studied and it was shown that increasing interfacial ion concentration along with electric field makes it more likely that the drops stabilize after coalescence, as opposed to breaking up.
Abstract: Binary drop electrocoalescence is the process of inducing two drops, suspended in an immiscible fluid, to coalesce in the presence of an external electric field. Electric forces have been known to accelerate the rupture of the interfacial film and enhance drop coalescence but the process has not been well characterized. The effects of the drop ion concentration and interfacial tension on the coalescence process are studied. It is shown that increasing interfacial tension, along with electric field makes it more likely that the drops stabilize after coalescence, as opposed to breaking up. This is due to the relative magnitudes of the drop deformation and charge separation timescales.
TL;DR: In this article, the characteristics of the measurement capacity of an air-coupled ultrasonic sensor influenced by the incident angle were investigated analytically and experimentally, and the optimized incident angle between ultrasound and test pipe was determined.
Abstract: The characteristics of the measurement capacity of air-coupled ultrasonic sensor influenced by the incident angle were investigated analytically and experimentally. The optimized incident angle between ultrasound and test pipe was determined. The air-coupled ultrasonic sensor with this determined incident angle was applied to the flowrate measurement in the aluminium pipe. The measurement results were compared to those obtained by using the electromagnetic flowmeter. Since the measurement results show good linearity, the capacity of the air-coupled ultrasonic flowmeter is revealed.
TL;DR: In this paper, the authors report efforts made on the characterization of semi-dilute, mono-dispersed suspensions of rayon fibres in turbulent water flow using Magnetic Resonance Imaging (MRI), Ultrasound Velocity Profiling (UVP) and Electrical Impedance Tomography (EIT).
Abstract: Experimental comparisons between imaging techniques serve to provide confidence in the validity of each technique for the study of multiphase flow systems. Such cross-validation can establish the limitations of each technique quantitatively. In the present paper, the authors report efforts made on the characterization of semi-dilute, mono-dispersed suspensions of rayon fibres in turbulent water flow using Magnetic Resonance Imaging (MRI), Ultrasound Velocity Profiling (UVP) and Electrical Impedance Tomography (EIT). Increasing flow velocities and fibre concentration were studied using these three experimental techniques. For lower fibre concentrations more uniform distributions were observed and as flow velocity increased fibre agglomerations were found in the centre region of the pipe.
TL;DR: In this paper, the problem of scattering of surface water waves by a thin circular arc shaped porous plate submerged in the deep ocean was studied in terms of a hypersingular integral equation of the second kind in which an unknown function representing the difference of potential function across the curved barrier was solved by a collocation method.
Abstract: In this paper, we study the problem of scattering of surface water waves by a thin circular arc shaped porous plate submerged in the deep ocean. The problem is formulated in terms of a hypersingular integral equation of the second kind in terms of an unknown function representing the difference of potential function across the curved barrier. The hypersingular integral equation is then solved by a collocation method after expanding the unknown function in terms of Chebyshev polynomials of the second kind. Using the solution of the hyper-singular integral equation, the reflection coefficient, transmission coefficient and energy dissipation coefficient are computed and depicted graphically against the wave number. Known results for the rigid curved barrier are recovered. It is observed that the porosity of the barrier reduces the reflection and transmission of the waves and enhances the dissipation of wave energy. The reflection coefficient and dissipation of wave energy decreases as the length of the porous curved barrier increases. Also the reflection coefficient is almost independent of the inertial force coefficient of the material of the porous barrier. However, the inertial force coefficient of the material of the porous barrier enhances transmission and reduces dissipation of wave energy.
TL;DR: In this paper, a high speed camera is used to investigate the behavior of CO2 dry-ice particles in an application of dryice blasting to the defouling of commercial aircraft engine compressors.
Abstract: A high speed camera investigation is presented into the behavior of CO2 dry-ice particles in an application of dry-ice blasting to the defouling of commercial aircraft engine compressors. An image acquisition system is deployed in the compressor section of an aircraft engine and is used to determine the evolution of dry-ice particle size and velocity from the nozzle exit to the entrance to the engine’s high pressure compressor as the engine is cranked. A comparison study between CO2 dry-ice particle laden flows and airflows with single Polyoxymethylene (POM) particles of various diameters is also presented. Measurements are made using a range of blasting system pressures and using sonic and supersonic blasting nozzles. The behavior of large CO2 dry-ice particles (dP ≥ 1 mm) in this discontinuously and inhomogenously laden flow is compared to that of single POM particles under similar flow conditions and is found to behave similarly. The experiments presented turn out to be useful for supporting development of special purpose dry-ice blasting systems.
TL;DR: In this paper, the results of three different trash receptacles: non-blast resistant, blast resistant and blast resistant trash receptacle with blast load redirection were presented, which showed that a considerable effect can be achieved by using blast resistant receptacles, thus reducing the possibility of deaths and injuries.
Abstract: Many terrorist attacks in the last decade around the world have exposed the vulnerability of citizens in public places. Public trash receptacles can be easily abused as well-covered places in which Improvised Explosive Devices (IED) can be simply left and then remotely activated. Therefore, blast resistance and possibility of blast loads redirection are very important characteristics of trash receptacles placed in crowded public areas. This paper presents the results of three different trash receptacles: non-blast resistant, blast resistant and blast resistant trash receptacle with blast load redirection. The results have shown that a considerable effect can be achieved by using blast resistant receptacles, thus reducing the possibility of deaths and injuries. A thickness optimization study was additionally performed, based on the size and geometry of the opening by using a finite element model. Based on the results of the study, some valuable recommendations for design of trash receptacles are also given.
TL;DR: In this article, the authors used finite element simulations of metal-skinned honeycomb panels to identify combinations of skin thickness and core density that are more susceptible to spring back, which can cause the skin to return close to its original position, leaving little indication that damaged core exists.
Abstract: Sandwich panels are commonly used for aerospace structures that require a high-bending stiffness, but the thin facesheets that are bonded to the core can be susceptible to impact damage. It is necessary to be able to identify and assess the severity of the damage, but this can be difficult when dents are not visible on the surface of the skin. This can occur when the dent elastically springs back immediately after impact, and can cause the skin to return close to its original position, leaving little indication that a damaged core exists. Identifying combinations of skin thickness and core density that are more susceptible to spring back can enable better decisions to be made with respect to inspection procedures. Finite element simulations of metal-skinned honeycomb panels indicate that more spring back is expected to occur from panels composed of thicker skins and lower density core.
TL;DR: In this article, an URANS-based simulation strategy using the commercial code AVL Fire for the direct injection (DI) of CNG and the mixture formation, including a discretisation of the full nozzle and cylinder geometry is presented.
Abstract: The use of compressed natural gas (CNG) as a fuel in internal combustion engines brings significant advantages in terms of reduction of CO2-emissions and fuel consumption. Compared to standard gasoline combustion, the CO2-production can be clearly reduced by using a methane-based fuel as it has a beneficial H/C ratio. The high knock resistance of methane allows higher compression ratios so that the thermodynamic efficiency is enhanced. Furthermore, the realization of a stratified mixture formation concept shows great potential to further increase fuel savings due to the reduced throttling losses. In the present work, an URANS-based simulation strategy using the commercial code AVL Fire for the direct injection (DI) of CNG and the mixture formation, including a discretisation of the full nozzle and cylinder geometry is presented. High pressure ratios between the injector and the cylinder lead to a choked flow inside the nozzle. A supersonic region, including shock-occurrence follows as the jet is expanded further downstream the orifice. To successfully resolve the multi-scale flow phenomena the mesh generation process involves the design of a fine hexahedral mesh for the injector, which is merged to the moving cylinder mesh by an arbitrary interface. Turbulence is modelled using the k-ζ-f model. To estimate the grid-induced error of the simulation, a set of calculations was performed on meshes of decreasing cell dimension. Different nozzle geometries are investigated and evaluated with regard to their mixture formation suitability as well as the effect of increasing rail pressure. Variations include an inward opening multi-hole injector and an outward opening annular ring injector. The results show a strong tendency of the gas jets to interact with each other and with the surrounding walls.