Scientia Iranica
Volume:18 Issue: 6, 2011

The specific heat ratio used in heat release calculations plays a crucial role in the determination of combustion parameters. In this study, the effects of assumed specific heat ratio on the heat release analysis of engine pressure data are studied in a spark ignition engine, using natural gas and gasoline fuels. The experiments were carried out with the spark timing adjusted to the maximum brake torque timing, at an equivalence ratio of ϕ=1 and a speed engine of. The combustion parameters are obtained from the heat release rate, which obtained from the first law of thermodynamics during a cycle. The results show that the combustion parameters have high sensitivity to the variation and first derivative of the specific heat ratio. The results also show that the influence of the first derivative of the specific heat ratio on the combustion parameters for natural gas operation is higher than that for gasoline operation. Moreover, the first derivative of the specific heat ratio for determination of combustion parameters should not be ignored.

There are different approaches to Predict the nonlinear moment–rotation relationship and evaluate internal loads and muscle forces of the human cervical spine. In this study a geometrically accurate, nonlinear finite element model of C0–C7 was developed using CT images of the human cervical spine. This model was used to derive the moment–rotation responses of the cervical spine, under physiological moments of 0.33, 0.5, 1.0, 1.5 and 2.0 Nm for flexion/extension in the sagittal plane, lateral bending in the frontal plane and axial rotation. Moreover, the results from the finite element model were used to calculate muscle forces that contribute in equilibrium of the head during rotations in the sagittal and frontal planes. To achieve this, a biomechanical model and the optimization algorithm were used to determine the relationship between required muscle forces and neck angle for the quasi-static condition. Finally, muscle forces were exerted on the finite element model to calculate internal forces. The results showed an excessive increase in internal loads by increasing the angle of rotation in all directions. In conclusion, this study provides evidence of higher cervical spine internal loads in non-neutral head postures, which can be a risk factor for neck pain and arthritis.

The nonlinear equation of motion for pre-stretched Euler–Bernoulli beams is derived. The effect of pre-tension and pre-compression in Euler–Bernoulli beams is studied. It is shown that compressive strain affects the bending stiffness much more than tensile strain. Based on the derived equation, the dynamic model of bimorph piezo-actuated beams, which is accurate, yet simple, is then developed. Afterwards, the critical voltage, which makes the piezo-actuated microbeam unstable, is numerically investigated. It is shown that the strain of the centerline is proportional to the beam’s aspect ratio squared. Results show that the more the aspect ratio, the less the critical voltage. In addition, it is illustrated that compared to the cantilever microbeam, the doubly-clamped microbeam is more sensitive to the changes in the aspect ratio.

The dissipative particle dynamics method is an efficient method for studying the hydrodynamics of complex fluids. One of the most challenging aspects of this method appears when the solid walls exist. The solid walls disturb the homogeneity of the fluid near the wall and cause some spurious fluctuations. Thus, in recent years a large amount of effort has been devoted to solve this shortcoming. Fortunately the mentioned problem has almost been solved for the simple walls such as flat walls, circular cylinders, spheres, etc. However no systematic model has addressed the complex walls. It should be noted that almost all of the walls we deal with in practical problems such as MEMS devices, polymer and drug containers and so on have complex geometries.In the present paper, we present a systematic method for the dissipative particle dynamics simulation of complex walls based on the representation of the complex wall by means of a triangular grid. We demonstrate the validity of our model for the flow past over a circular cylinder and then we do a simulation for the flow over an airfoil. The obtained results show that this method facilitates the simulation of each arbitrary complex wall while the spurious fluctuations of density and temperature are diminished effectively near the wall.

A new design for a valveless micropumping device has been proposed that integrates two existing pumping technologies, namely, the wall induced traveling wave and the obstacle-type valveless micropump. The liquid in the microchannel is transported by generating a traveling wave on the channel, while the placing of two asymmetric trapezoid obstacles, along the centerline of the channel inlet and outlet, leads to a significant (up to seven times) increase of the net flow rate of the device. The effectiveness of this innovative design has been proved through a verified three-dimensional finite element model. Fluid–Structure Interaction (FSI) analysis is performed in the framework of an Arbitrary Lagrangian–Eulerian (ALE) method.

In this study, the effects of residual stresses on crack behavior are investigated in a large disk shaped model. The external load and crack-tip geometry constraint in the model are controlled by a remote boundary condition, which is a function of K and T-stress. A crack opening residual stress field, similar to a longitudinal residual stress field in butt welded plates, is introduced into the model using an initial strain field.For calculating the modified, path independent, J-integral in the specimens with residual stresses, a post processor for finite element results was developed by computer programming. To carry out a systematic investigation into the effect of residual stresses on crack behavior, the constraint parameters, Q and R, for different combinations of residual stresses and external loads, were calculated, and the interaction between residual stresses and external loads for different models was studied. It has been shown that the crack driving forces and the crack-tip stress field were significantly influenced by residual stress

A series of wind tunnel tests are performed to examine the flow field over a swept wing under various conditions. The wing has a laminar flow airfoil section, similar to those of the NACA 6-series. Static pressure distributions over the upper surface of the wing, in both chordwise and spanwise directions, are measured at different angles of attack. The data is employed to predict the transition point at each chordwise section. The skewness parameter of the pressure data shows that this factor drops to zero in the transition region. A comparison of the calculated transition point on the wing surface with that obtained from the 2D computational method shows reasonable agreement over a portion of the model. The power spectral density calculated from the total pressure data of the boundary layer, over the wing surface, at several locations shows the instability modes.

The aim of this study is to analyze the hydrodynamics of gas mixtures invoking a recently proposed multifluid model. The model consists of a separate equation set for one component species of the system and an equation set for average quantities of the mixture. Thereby, it provides details of the flow fields for each of the constituents separately. The new model also computes transport coefficients from some kinetic relations without the requirement of being input externally. Moreover, it automatically describes diffusion processes excluding the use of any coefficients for ordinary, pressure and thermal diffusion, which are generally required during Navier–Stokes computation of gas mixture flows.In the present paper, the model that was applied with hard-sphere molecules is extended to include more realistic molecular interaction descriptions (i.e. Maxwell repulsive potential and Lennard-Jones 12–6 potential). Moreover, the contribution of external forces is incorporated in the multifluid balance equations. Afterwards, the resulting equations are solved for some gas mixture problems in the context of a converging-diverging nozzle, and the importance of the molecular interaction description on the hydrodynamics of the mixtures is analyzed.

This study concerns an evaluation of pulsatile flow and arterial wall behavior in a real world model of a stenosed abdominal aorta and iliac arteries. Two different geometries of a healthy and severly stenosed abdominal aorta, extracted from CT scan images and simulations of fluid flow and tissue interaction (Fluid Solid Interaction), are carried out. The blood is taken as incompressible, non-Newtonian, and the arterial wall tissue is treated as isotropic, elastic material with uniform mechanical properties. The results of using two models with rigid and flexible walls are presented and compared. The computed pressure at the abdominal aorta for a flexible healthy wall is consistent with measured values in vivo conditions. Results show that the computed pressure is lower by 15% for the flexible model, as compared to the rigid and complaint models. Although results obtained from the FSI study of pulsatile healthy and stenosed models show a similar trend for Wall Shear Stress (WSS) patterns, considerable differences in magnitude exist. It is shown that for the cases presented here, the effects of wall flexibility and actual stenosed geometry on the flow performance of veins are noticeable.

This paper presents a nonlinear model to illustrate the effect of contact stress in the vibration behavior of mechanical components, especially rotating systems. The problem is considered in the case of the vertical vibration of a sphere on a plate. The Hertzian contact theory is used to obtain the relationship between contact force and the deflection of the mass center of the sphere. Modeling the system by a mass and a nonlinear spring, the vibration equation of the mass center of the sphere is derived. The method of Lindstedt–Poincaré is implemented to solve the equation of motion, and obtain vibration characteristics under a compressive preload. The dependency of frequency on several parameters, such as initial applied force, initial amplitude of oscillation and the diameter of the sphere, is distinguished. Results show that increasing the initial applied force or the diameter of the ball raises the frequency, while increasing oscillation amplitude has an inverse effect. Finally, the accuracy and convergence of the solution are illustrated by comparison between different orders of approximation. Also, results are in good agreement with those extracted from numerical modeling.

In this paper, the vibrational behavior of functionally graded cylindrical shells with intermediate ring supports is studied. Theoretical formulation is established based on Sanders’ thin shell theory. The governing equations of motion are derived, using an energy functional and by applying the Ritz method. Using an appropriate set of displacement functions, the energy equations lead to an eigenvalue problem whose roots are the natural frequencies of vibration. Material properties are assumed to be graded in the thickness direction, according to the power-law volume fraction function. A functionally graded cylindrical shell, made up of a mixture of ceramic and metal, is considered. The influence of some commonly used boundary conditions and the effect of changes in shell geometrical parameters and variations in ring support position on vibration characteristics are studied. The results obtained for a number of particular cases show good agreement with those available in the open literature.

Solving Saint-Venant equations by the finite element method needs long CPU time (even for a short time). Moreover, if the channel length is fairly large, the system resulted by discretization is not directly solvable, and one should use iterative methods. Therefore, total error is error resulted by discretization and by solving the system by iterative methods. In this paper, we apply three adaptive finite element methods for solving Saint-Venant equations to obtain, first, a better approximated solution and, second, a significant decrease in CPU time and computational complexity. In the first method, a coarse grid is considered and, by partitioning a few intervals, the problem is solved on this new coarse grid. In the second method, the problem is solved for the first few moments and, then, using the regression method, the solution is obtained in the following time. In the third method, like the second method, the problem is solved for the first few moments and, then, the approximated solution is predicted for following times. Finally, two numerical examples for supercritical and subcritical flows are given to support our results.