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A Study on Microstructural and Mechanical Properties of a Stir Cast Al (SiC-Mg-TiFe) Composite
Full text linkDevelopment of metal matrix composite is becoming widespread in most engineering applications where excellent mechanical properties are required. Mechanical and microstructural properties of aluminium reinforced with silicon carbide was investigated. Ingot of aluminium was melted in a furnace at temperature ranging between 650-700 ℃. Ferrotitanium and silicon carbide were preheated in a muffle furnace before addition to molten aluminium in a crucible furnace. Fixed proportions of magnesium, ferrotitanium and varying proportions of silicon carbide were utilized as reinforcements. Stirring was carried out manually for a minimum of 10 mins after the addition of each weight percent of silicon carbide. Resulting as-cast samples were sectioned for various mechanical and microstructural analysis. Microstructural studies from optical microscopy and scanning electron microscopy (SEM) showed the dispersion of reinforcements in the aluminium matrix. Mechanical properties which includes hardness and tensile strength of fabricated composites were observed to increase, while XRD analysis showed various phases formed from reaction between the matrix and reinforcements
Effect of Richardson Number on Unsteady Mixed Convection in a Square Cavity Partially Heated From Below
Full text linkThe objective of the present study is to analyze the laminar mixed convection in a square cavity with moving cooled vertical sidewalls. A constant flux heat source with relative length l is placed in the center of the lower wall while all the other horizontal sides of the cavity are considered adiabatic. The numerical method is based on a finite difference technique where the spatial partial derivatives appearing in the governing equations are discretized using a high order scheme, and time advance is dealt with by a fourth order Runge Kutta method. The Richardson number (Ri), which represents the relative importance of the natural and forced convection, is chosen as the bifurcation parameter. The effect of this non-dimensional number on the behavior of the fluid flow and the heat transfer is analyzed. Although the geometry and boundary conditions concerning the velocity and the temperature are symmetrical with respect to the vertical axis passing through the center of the cavity, the results show the existence of symmetric and asymmetric flow structures, varying according to the considered value of the Richardson number
A Preconditioned JFNK Algorithm Applied to Unsteady Incompressible Flow and Fluid Structure Interaction Problems
No full textDespite the advances in computer power and numerical algorithms over the last decades, solutions to unsteady flow problems remain computing time intensive.
In previous work [Lucas, P.,Bijl, H., and Zuijlen, A.H. van(2010)], we have shown that a Jacobian-free Newton-Krylov (JFNK) algorithm, preconditioned with an approximate factorization of the Jacobian which approximately matches the target residual operator, enables a speed up of a factor of 10 compared to nonlinear multigrid (NMG) for two-dimensional, large Reynolds number, unsteady flow computations. Furthermore, in [Lucas, P., Zuijlen, A.H. van, and Bijl, H. (2010)] we show that this algorithm also greatly outperforms NMG for parameter studies into the maximum aspect ratio, grid density and physical time step: speeds ups, up to a factor of 25 are achieved.
The goal of this paper is to demonstrate the wider applicability of the preconditioned JFNK algorithm by studying incompressible flow and an incompressible fluid structure-interaction (FSI) case. It is shown that the preconditioned JFNK algorithm is able to tackle the stiffness induced by the low Mach regime, making it possible to apply a compressible flow solver to nearly incompressible flow. Furthermore, it is shown that the preconditioned JFNK algorithm can be readily applied to FSI problems
Numerical Analysis of Concrete Composites at the Mesoscale Based on 3D Reconstruction Technology of X-ray CT Images
No full textA numerical analysis of concrete composites at the mesoscale based on three-dimensional (3D) reconstruction technology of X-ray computed tomography (CT) images is presented in this paper. For X-ray CT images of concrete, morphology processing was used to recover complete image information, including borders, and the median filtering method was applied to eliminate potential impurities in the images. The final X-ray CT images obtained after processing for a concrete section were composed of three-value pixels that indicated aggregate particles, mortar matrix and air voids, and the 3D structure of the concrete specimen was reconstructed using the volume data method. The mapping mesh method was used to identify element material attributes, and a four-point bending-tension fracture process for the specimen was performed. The numerical results show that the load-displacement curve and failure crack location from the reconstructed model were similar to those from the experiment. The damage development for each phase of material obtained from the numerical results indicates that crack initiation occurs in the interface transition zone (ITZ); one macro crack is formed at final failure, and it is caused by the growth and propagation of the micro cracks
Darcy-Stokes Equations with Finite Difference and Natural Boundary Element Coupling Method
No full textNumerical method is applied to investigate the Darcy-Stokes equations, which is governing the steady incompressible Stokes flow past a circular cavity in a porous medium. The free fluid flow is modeled by the incompressible Stokes equations, and the flow in the porous medium is imposed by Darcy equations. Based on domain decomposition method with D-N alternating iteration algorithm, the coupling method of finite difference method and natural boundary element method is studied for the coupling Darcy-Stokes equations under a certain pressure difference. The numerical results indicate that the finite difference and natural boundary element coupling method is efficient and convenient for the Darcy-Stokes problem of the steady-state parallel flow with a void space
Investigation on the Singularities of Some Singular Integrals
No full textIn a boundary element method, the treatment of all the possible singular integrals is very important for the correctness and accuracy of the solutions. Generally, the directional derivative of a weakly singular integral is computed by an integral in the sense of Cauchy principal value if the directional derivative of the weakly singular integral kernel is strongly singular or in the sense of Hadamard finite part integral if it is hypersingular. In this paper, we try to discover how the strongly singular and hypersingular integrals are generated and propose an idea to avoid the appearance of such kind of strongly singular and hypersingular integrals. This idea is termed as the 'exact derivation' of the directional derivative of a weakly singular integral. Using some simple examples, we proof that the directional derivative of a weakly singular integral found by this idea can still be a weakly singular integral. That is none strongly or hypersingular integrals are generated in such a process. Therefore, Cauchy principal value and Hadamard finite part integral are not indispensable
Stagnation Point Flow Over a Permeable Stretching/Shrinking Sheet with Chemical Reaction and Heat Source/Sink
Full text linkThe present study considers the magnetohydrodynamic (MHD) stagnation point flow with chemical reaction effect over a permeable stretching/shrinking sheet. The partial differential equations are reduced to a set of ordinary differential equations using a similarity transformation. The transformed equations are then solved numerically by employing the bvp4c function available in the MATLAB software. The numerical results illustrate the effects of several parameters on the skin friction coefficient, local Nusselt number and the local Sherwood number. Dual solutions are obtained for a certain range of parameters. The temporal stability analysis is carried out to determine which one of these solutions is stable and thus physically reliable in a long run
Probabilistic Performance-Based Optimum Seismic Design Framework: Illustration and Validation
Full text linkIn the field of earthquake engineering, the advent of the performance-based design philosophy, together with the highly uncertain nature of earthquake ground excitations to structures, has brought probabilistic performance-based design to the forefront of seismic design. In order to design structures that explicitly satisfy probabilistic performance criteria, a probabilistic performance-based optimum seismic design (PPBOSD) framework is proposed in this paper by extending the state-of-the-art performance-based earthquake engineering (PBEE) methodology. PBEE is traditionally used for risk evaluation of existing or newly designed structural systems, thus referred to herein as forward PBEE analysis. In contrast, its use for design purposes is limited because design is essentially a more challenging inverse problem. To address this challenge, a decision-making layer is wrapped around the forward PBEE analysis procedure for computer-aided optimum structural design/retrofit accounting for various sources of uncertainty. In this paper, the framework is illustrated and validated using a proof-of-concept problem, namely tuning a simplified nonlinear inelastic single-degree-of-freedom (SDOF) model of a bridge to achieve a target probabilistic loss hazard curve. For this purpose, first the forward PBEE analysis is presented in conjunction with the multilayer Monte Carlo simulation method to estimate the total loss hazard curve efficiently, followed by a sensitivity study to investigate the effects of system (design) parameters on the probabilistic seismic performance of the bridge. The proposed PPBOSD framework is validated by successfully tuning the system parameters of the structure rated for a target probabilistic seismic loss hazard curve. The PPBOSD framework provides a tool that is essential to develop, calibrate and validate simplified probabilistic performance-based design procedures
Experiment and Simulation for Controlling Propagation Direction of Hydrofracture By Multi-Boreholes Hydraulic Fracturing
Full text linkHydraulic fracturing has been applied to enhance CBM production and prevent gas dynamical hazard in underground coal mines in China. However, affected by in situ stress orientation, hydrofracture can hardly continuously propagate within coal seam but may easily extend to the adjacent roof-floor strata, causing ineffective permeability enhancement in coal seam and increasing the risk of gas transfinite during mining coal. Thus, it is very necessary to artificially control the propagation direction of hydrofracture and make it well-aligned in large scale in coal seam. In this study, a method for controlling propagation direction of hydrofracture by multi-boreholes is investigated by theoretical analysis, laboratory experiment and numerical simulation. And this is followed by an on-site test in an underground coal mine to verify this method. Firstly, stress intensity factor at the hydrofracture tip is analyzed where pore pressure is taken into consideration. Results show that the pore pressure is able to increase the stress intensity factor and reduce hydrofracture propagation pressure. Based on this, a method of hydraulic fracturing using multi-boreholes to control hydrofracture direction is proposed. Afterwards, laboratory experiments are conducted to explore the impact of pore pressure on hydrofracture propagation. The experimental results agree with the theoretical analysis very well. Later on, a series of numerical simulations are performed to examine the influence of principal stress difference, the angle between assistance drillholes and the maximum principal stress, and the fluid pressure of the assistance drillholes on hydrofracture propagation. Finally, an on-site test in an underground coalmine is practiced where this proposed method is used to enhance the CBM production. Results show the scope of the hydro-fracture resulting from the multi-boreholes hydraulic fracturing method increases 2.7 times compared with that of conventional hydraulic fracturing. And gas production rate also increases 4.1 times compared with that of conventional hydraulic fracturing and 12.3 times compared with direct borehole extraction without fracturing
Location Privacy in Device-Dependent Location-Based Services: Challenges and Solution
Full text linkWith the evolution of location-based services (LBS), a new type of LBS has already gain a lot of attention and implementation, we name this kind of LBS as the Device-Dependent LBS (DLBS). In DLBS, the service provider (SP) will not only send the information according to the user’s location, more significant, he also provides a service device which will be carried by the user. DLBS has been successfully practised in some of the large cities around the world, for example, the shared bicycle in Beijing and London. In this paper, we, for the first time, blow the whistle of the new location privacy challenges caused by DLBS, since the service device is enabled to perform the localization without the permission of the user. To conquer these threats, we design a service architecture along with a credit system between DLBS provider and the user. The credit system tie together the DLBS device usability with the curious behaviour upon user’s location privacy, DLBS provider has to sacrifice their revenue in order to gain extra location information of their device. We make the simulation of our proposed scheme and the result convince its effectiveness