Computational Methods in Engineering

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In this study, the missile staging process by implementing a side-injected jet is simulated numerically. The problem is considered to be axisymmetric and the thin shear layer approximation of Navier-Stokes equations along with an algebraic turbulence model is used in a quasi-static form for the calculations. The free stream corresponds to a very high altitude flight condition with a Mach number of 10 and an injected jet pressure ratio of about 63000. An explicit Godunov-type scheme is used in the calculations, which is second-order in time and space. Computations are performed on the attached and separated geometry for a range of distances between the body and the warhead. The intense interactions between the jet flow and the main free-stream and its overall influences on the warhead aerodynamic loading are finally demonstrated. Keywords: Missile Staging, Jet Interaction flow, TVD Scheme, Riemann problem

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The importance of the equipment and secondary systems in seismic design and performance evaluation is well recognized and has been the subject of many studies. In all of these studies, earthquake is considered as a single component, and in most of them the primary system is considered as shear building. Most attention has been concentrated on the response of secondary system and its response spectrum. In this paper, the transfer function for absolute acceleration of the secondary system is obtained. The squared modulus of transfer function relates the power spectral density function of the input (excitation) to the output (response), which is useful in the study of the various dynamic parameters of the system. In addition to transfer function, the autocorrelation and power spectral density function of absolute acceleration of the secondary system are obtained. Earthquake is considered as a multi-component system and the necessary formulation is developed for the calculation of these functions as well as the critical angle with and without interaction between the two systems. The damping of the system is considered as proportional in the decoupled analysis, and nonproportional in the coupled analysis. The formulation developed has been illustrated by considering a ten-story torsional builing. Various parameters such as eccentricity, correlation between components, tuning interaction and nonproportional damping are studied. Results show that eliminating the effect of multicomponentness of earthquake can cause large errors especially at large eccentricities. Keywords: transfer function, Random vibration, secondary systems, critical angle, interaction, nonproportional damping

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The objective of this study is to design a robust direct model reference adaptive controller (DMRAC) for a nonlinear cardiovascular model over a range of plant parameters representing a variety of physical conditions. The direct adaptive controllers used in thisd study require the plant to be almost strictly positive real (ASPR) that is, for a plant to be controlled there must exist a feedback gain such that the resulting closed loop system is strictly positive real. We designed a new compensator so that the system composed of the cardiovascular plant and the compensator satisfy the ASPR condition. Numerous studies in the past have considered a small range of gain variations of the cardiovascular system. In most cases, the controller was designed based on variations in either time delay or plant gains. Many of these workers treated the cardiovascular system as a single-input single output (SISo) plant in which the control output was Mean Arterial Pressure (MAO). We treated the cardiovascular system as a multi-input multi-output (MIMO) plant in which both the MAP and Cardiac Output (CO) are simultaneously controlled. In this study, a new linear model is presented that provides a better approximation thanthe one the original linear model does. By doing so and utilizing the DMRAC algorithm, we could satisfy the stability conditions for the nonlinear model while satisfactory responses obtained under every possible condition for the cardiovascular nonlinear model.

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There are a number of parameters influencing the dynamic and seismic response of bridges. Of these, two important parameters warranting special notice include: the properties of the neoperenes in the state of connection between girders and columns and the shear stiffness of underlying soil in the level of bridge substructure’s connectivity to the ground. In this paper, the effects of these two parameters on the dynamic and seismic response of Ghadir Bridge in Isfahan are investigated. The main conclusions drawn from these investigations include: the sensitivity of the bridge’s lateral modes of vibration to the horizontal shear stiffness of the neoperenes and the substantial effects of the soil’s shear rigidity on the longitudinal modes. Based on the findings, it is recommended tha a thorough geotechnical site investigation of the soil be conducted and the properties of the underlying soil be accurately established in order to correctly identify the dynamic behaviour of a bridge.

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Jacketing of reinforced concrete columns is a common and useful strengthening method. This method substantially improves mechanical properties of the column, such as flexural strength as well as shear and ductility. In this paper, the behavior of confined reinforced concrete columns are investigated. The results indicate that the method is more effective for slender columns in the region of their failure zone.

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This paper is concerned with elastic local buckling of rectangular plates subjected to intermediate and end inplane loads. Since closed form solution for buckling analysis of plates with different end conditions and subjected to intermediate loads is complicated, numerical methods are more useful. Because of restrictions on the two finite strip methods, the longitudinal B3 spline expressions combined with conventional transverse shape functions are used as displacement functions. This method is computationally more efficient than the finite element method, more flexible in boundary treatment, and more accurate in dealing with point forces and axial loads than the conventional finite strip method. Local buckling coefficients are presented for plates under intermediate and end inplane loads which are useful for design of steel walls or plates that support intermediate floors/loads.

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Application of pile-raft foundations, which are known as “compound foundations”, is a suitable alternative in the case of heavy load structures. The interaction behavior of pile raft foundations makes these systems very complex to analyze. Different methods have been proposed to determine the bearing capacity of piled raft systems and distribution of loads between the components, i.e. pile group and mat. These methods are generally categorized into computer-based and conventional methods. In most of these methods, the bearing capacity of the mat, which is often a great portion of the total capacity, is neglected. Also, some model parameters used in these methods, as well as pile group or raft stiffness, cannot be determined by routine tests or calculations. In this study, a number of recent analytical methods of piled raft system are presented. A new method is then proposed which is based on settlement analysis of piled raft foundation and distribution of load between pile group and mat foundation, which regards the interaction of compound systems as an equivalent block foundation. In this approach, settlement is computed based on the concept of neutral plane according to which relative settlement of soil and pile group become the same. Two practical case studies are implemented for validation of the method. The comparison demonstrates favorable results for the proposed method.

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Investigation of vertical vibrations of a railway turnout is important in designing track components under moving loads of trains. In this paper, the turnout is simulated by a linear finite element model with modal damping. A section of the turnout has a length of 36 sleeper spans surrounding the crossing. Rails and sleepers are modeled with uniform Rayleigh- Timoshenko beam elements. The rails are connected via railpads (linear springs) to the sleepers, which rest on an elastic foundation. The rolling stocks are discrete systems of masses, springs, and dampers. By passing the trains at a constant speed, only vertical dynamics (including roll and pitch motions) is studied. The wheel-rail contact is modeled using a non-linear Hertzian spring. The train-track interaction problem is solved numerically by using an extended state space vector approach in conjunction with modal superposition for the turnout. The results show that the rail discontinuity at the frog leads to an increase in the wheel-rail contact force. Both smooth and irregular transitions of the wheels from the wing rail to the crossing nose have been examined for varying speeds of the vehicle. Under perfect conditions, the wheels will change quite smoothly from rolling on the wing rail to rolling on the nose. The impact at the crossing will then be small, giving a maximum wheel-rail contact force which is only 30--50 per cent larger than the static contact force. For uneven transitions, the severity of the impact loading at the crossing depends strongly on the train speed. The increase in the contact force, as compared with the static force, is in the order of 100 per cent at 70 km/h and 200 per cent at 150 km/h.

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This study shows how to create different types of crack and discontinuities by using isogeometric analysis approach (IGA) and extended finite element method (XFEM). In this contribution, two unique features of isogeometric analysis approach are utilized to create discontinuous zones. Discontinuities consist of crack and cohesive zone. In isogeometric analysis method NURBS is used to approximate both geometry and primary variable. NURBS can create quadratic shapes exactly. Also, stress intensity factors are calculated in the vicinity of the crack tips for two dimensional problems and are compared with corresponding analytical and numerical counterparts. Extended finite element method is the other numerical method which is used in this work. The enrichment procedure is utilized in extended finite element method to create discontinuities. The well-known path independent J-integral approach is used in order to calculate the stress intensity factors. Also, in mixed mode situation, the interaction integral (M-integral) is considered to calculate the stress intensity factors. Results show that isogeometric analysis method has desirable accuracy as it uses lower degree of freedoms and consequently lower computational efforts than extended finite element method. In addition, creating the internal cohesive zone as one of the most important issues in computational fracture mechanics is feasible due to the special features of isogeometric analysis. The present study demonstrates the capability of isogeometric analysis parametric space to control the inter-element continuity and create the cohesive zone.

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In this paper, nonlinear modal interactions caused by one-to-three internal resonance in a beam-mass-spring-damper system are investigated based on nonlinear system identification. For this purpose, the equations governing the transverse vibrations of the beam and mass are analyzed via the multiple scale method and the vibration response of the system under primary resonance is extracted. Then, the frequency behavior of the vibration response is studied by Fourier and Morlet wavelet transforms. In order to perform the nonparametric identification of the time response, mono-frequency intrinsic mode functions are derived by the advanced empirical mode decomposition. In this approach, masking signals are utilized in order to avoid mode mixing caused by modal interaction. After analyzing the frequency behavior of each mode function, slow flow dynamics of the system is established and intrinsic modal oscillators for reconstructing the corresponding intrinsic mode are extracted. Finally, by analyzing the beating phenomenon in a simple one-degree-of-freedom system, it is shown that the internal resonance causes beating only under the circumstance that the slope of the logarithmic amplitude of oscillator force is nonzero. The results, therefore, show that depending on the periodic, pseudo-periodic, and chaotic behavior of the response, modal interactions might be stationary or non-stationary. Moreover, the chaotic behavior occurs mostly in the vibration mode excited by the internal resonance mechanism

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According to the declining trend of fossil fuel resources and the need to use renewable energies, appropriate research should be conducted for technical and functional studies in this regard. Therefore, in this research, a tubular PEM fuel  cell as a suitable energy source with three-dimensional geometry has been numerically simulated and investigated. For a comprehensive study, the equations of continuity, momentum, energy, stress-strain, and fluid-solid-heat interaction at steady state are defined, coupled together, and then solved by a finite element numerical code. Assuming the cell voltage changes from 0.95 to 0.4 volts, the passage of compressible fuel and air through the channels and porous media of the electrode and catalyst, and also about 6 degrees increase in the average cell temperature, causes approximately 35 nm displacement in different parts. These displacements, due to fluid-solid-heat interactions, cause thermal and mechanical stresses. The maximum stress is about 3500 kN/m²  in the electrolyte due to its displacement limit (average displacement 12.8 nm). Then the relation of voltage variation with current density, stress, fuel flow rate, displacement and fuel cell temperature was shown. Also the results showed that the assumption of fluid-solid-heat interaction reduces the fuel cell power density by about 3%. Finally, the effect of different parameters such as fuel and air channel radius, electronic and ionic conductivity were investigated. For example, at a voltage of 0.4 volt, 20 percent reduction in the radius of air or fuel channels, or 100 percent increase in the electron or ionic conductivity, increases the electrical current density by about 2.17, 0.05, 3.69, and 40 percent, respectively.