Year:2007 | Volume:41 | Issue:4 (106)|Papers Count:16

In this paper the eccentric forward extrusion of sections has been simulated and analyzed using FEM Abaqus-explicit. The results have been compared to those obtained from upper bound theorem and experimental works. Close agreements was observed between the FEM and experimental work and as compared with the upper bound data more realistic and better predictions were made by the authors' method. The sections considered in this work were circular and square shaped cross sections. Process parameters such as die length, friction factor and percentage of eccentricity have been investigated and their influence on the relative extrusion pressure and the curvature of the extruded profiles has been illustrated. The effect of die profile has also been studied and shown that a 10 percent reduction in the extrusion pressure could be achieved by using the optimum die profile.

Effect of altitude is discussed in the unsteady separation of multi stage rockets. Axisymmetric, unsteady and turbulent Navier stokes equations are solved numerically. The governing equations are split into a hyperbolic in viscid part and a parabolic diffusion part. The hyperbolic part is solved by an explicit second-order time and space of Godunov-type scheme. Moving mesh and moving boundary algorithm are used in the numerical simulation. Separation of missile staging is simulated at 10, 20, 30 and 40 kilometers altitudes, and 10 Mach number. The flow physics of injection of a secondary supersonic jet into a hypersonic turbulent primary core over a missile configuration is studied numerically. The injection occurs opposite to the free-stream direction. It is shown that the intense interactions between the jet flow and the main free-stream have a noticeable effect on the aerodynamic loads. High momentum of the jet injected to the main flow of rocket causes interaction of shock waves and consequently it changes flow pattern. The aerodynamic forces can be changed significantly by the intense jet flow interactions. The results show that drag forces of head and body of the rocket are irregularly periodical. Amplitude of drag force has been increased by altitude. At primary time of separation, rate of displacement of head and body has incredible behavior in various altitudes too.

An experimental and numerical study of free convection heat transfer from a channel consisting of a vertical sinusoidal wavy surface and a vertical flat plate has been carried out. The vertical wavy surface was maintained at a constant temperature, while the flat plate is adiabatic. A Mach-Zehnder Interferometer was used to determine the local heat transfer coefficients of sinusoidal wavy surface. FLUENT code was used for numerical simulation. The numerical results are in good agreement with experimental data. The amplitude-wavelength ratio, a, in this investigation is kept constant at a =0.05. The effects of Rayleigh number Ral and wall spacing are investigated as well. Experiments were carried out for eight different Rayleigh numbers and thirteen different wall spacing. Results indicate that the frequency of the local heat transfer rate is the same as that of the J wavy surface. The average heat transfer coefficient increases as the Rayleigh number, Ral increases. For each Rayleigh number there is optimum wall spacing where the heat transfer rate from the wavy sinusoidal surface reaches its maximum value. This optimum wall spacing depends on Rayleigh number and decreases with increasing Rayleigh number.

Capability of the Proper Orthogonal Decomposition (POD) method in extraction of the coherent structures from a spatio-temporal chaotic field is assessed in this paper. As the chaotic field, an ensemble of 40 snapshots, obtained from Direct Numerical Simulation (DNS) of the Kuramoto-Sivashinsky (KS) equation, has been used. Contrary to the usual methods, where the ergodicity of the field is needed, the ensemble members are generated directly by disturbing the initial conditions in a random fashion. The POD eigenvalues and eigenmodes are extracted, using the POD/SVD technique and an ensemble averaging process is performed on the resulting modes, which are used in reconstruction of the field. The resulted mean field is compared with the Fourier mean field as well as the original (instantaneous) field. The results strictly have shown presence of non-coherent parts in the POD reconstructed field, which can be interpreted as the POD weakness in the filtering of the random part of the field. On the other hand, the ensemble averaged POD modes had obvious superiority, in comparison with the mean Fourier modes, in tracing of the mean behavior of the field and the mean temporal gradients. As a consequence, use of the ensemble averaging in the POD modes can be suggested, at least in some spatio-temporal the fields with dominant coherent structures.

An analytical model for the prediction of drillpipe fatigue life has been proposed. The model utilizes a physically based critical plane approach. This approach uses two damage parameters: one is based on normal strain amplitude and the other is based on shear strain amplitude. Included in both parameters, the effect of the normal stress on a specific critical plane has been considered. Meanwhile, stress concentration and fatigue strength reduction factors are also taken into account in this analysis. A drillstring is used to transmit rotation to the drill bit and provide fluid circulation in hole bottom while drilling. As these functions are being performed, the drillpipe experiences a combination of cyclic loads that include bending, torque, axial force and pressure. This situation is very common in directional drilling operations.Axial force and pressure can be assumed to be non-cyclic for short intervals. However, a torque load, like bending, is cyclic in nature. Due to common slip/stick effect, drill pipes experience small torque fluctuations with high frequency. Also, torque fluctuations occur due to making connections. The torque changes from zero to a relatively constant value soon after rotation resumes. In a directional well with high friction, torque can be significant factor and may cause long term fatigue damage. Since drillpipes are generally thin-walled tubulars, it is assumed that they are in a plane stress condition. An analytical model is developed in this paper to calculate the stresses and strains that drillpipes experience during drilling operations. Then a proposed analytical model has been applied to predict drillpipe fatigue life. Two computer programs were also accomplished and used. The first one allows the calculation of the loads, and stresses along the drill string. The second one allows the prediction of the fatigue damage in several planes along the drill string for a certain drilled interval. It has been shown that there is a good agreement between test results and the fatigue model predictions. It is also shown that torque and stress concentrations are very important and must be considered in drill pipe fatigue analysis. The drillpipe fatigue model is compared to the present API approach. It is observed that the drillpipe fatigue model is more conservative than the present API approach. This is because the API model does not account for torque.

An extensive experimental study has been performed to investigate the wind tunnel sidewall effect and the boundary layer suction on a half model pressure distribution. A three-dimensional half wing was made according to its full span model. The wing was tested at angles of attack ranging from a= -5 to 25 degree and at Reynolds numbers of 0.5*106, 1.0*106 and 1.3*106 in a subsonic wind tunnel. Experimental results such as static pressure distribution, lift coefficient, and wake are compared with the corresponding 2-D results from that of similar model. Wind tunnel sidewall boundary layer suction effects on the surface static pressure distribution have been studied at various conditions has been studied. The results show that the flow over the portion of the model affected by the sidewall pressure separates at an angle of attack which is smaller than the corresponding angle for other portion of the wing. This phenomenon yields an increase in the drag coefficient and a noticeable decrease in the value of the lift coefficient. However, when suction was applied to the sidewall, separation was delayed to higher angles, and the corresponding aerodynamic loads varied significantly.

In this paper, transfinite element method is used to analyze the two dimensional thermoelasticity problems. A comparison is made between the thermoelastic analysis results of the classical theory and theories with one or two relaxation times (i.e. L-S and G-L theories), for the half space problem. Governing equations are transformed to Laplace domain and then, node variables are calculated by the finite element method. Results are transformed to physical domain by the Laplace inverse transform. Finally, results of the three theories are compared and discussed. The obtained results reveal that the tensional stresses predicted by the classical theory for points located before the stress wavefront are higher whereas the compressive stresses predicted by L-S and G-L theories are higher for points located after the stress wavefront. Furthermore, the temperature predicted by the modem theories is higher than that of by the classical theory at the wavefornt.

A new unified fatigue life model based on the energy method is developed for unidirectional polymer composite laminates subjected to constant amplitude, tension tension or compression-compression fatigue loading. The model is based on static failure criterion presented by Sandhu and substantially is normalized to static strength in fiber, matrix and shear directions. So that all stress components are responsible for fatigue failure. The proposed model is capable for predicting fatigue life of unidirectional composite laminates over the range of positive stress ratios in various fiber orientation angles. By using this new model all data points obtained from various stress ratios and fiber orientation angles are merged into a single master curve. Inputs for the proposed model are ultimate strengths in material directions under tension and compression monotonic loadings and one set of fatigue test data to prescribe the material constants.In order to verify the new fatigue model, the model is applied to the available experimental data to show its capability for predicting the fatigue lives. Two different sets of experimental data available in the literature for different materials, carbon/epoxy and E-glass/ epoxy, of agreement unidirectional plies are selected and examined. The best with the experimental data is obtained by using least square method. Obtained results by the new fatigue model are in good agreements with the experimental data and confirm that the model becomes a unified strength based model to eliminate the effects of positive stress ratios as well as the fiber orientation on the off-axis fatigue behavior of unidirectional composite materials subjected to constant amplitude cyclic loading.

In this paper, the response of hybrid composite laminate plate reinforced by the smart wires (shape memory or the SMA wires) subjected to low velocity impact is studied. The smart wires are embedded within the layers of the composite laminate. The effect of the smart wires on contact force history, and deflection of these structures is investigated. The first-order shear deformation theory as well as the Fourier series method is used to solve the governing equations of the composite plate analytically. The interaction between the impactor and the plate is modeled with the help of two degree-of-freedom system, consisting of springs-masses. The results obtained from this research were compared with some of the published papers and the accuracy of the present model is validated. The results indicated that some of the parameters like the orientation and the through thickness location of the smart wires will reduce the deflection of the structure, hence would increase the impact resistance, and the damage tolerance limit of the structure.

In this paper conjugated heat transfer in thermal entrance region through the sinusoidal wavy channel has been investigated. The fluid flow is assumed to be laminar, steady state, incompressible, and hydrodynamically fully developed. A constant heat flux is assumed to be applied on the outer edge of the channel wall. In this study the governing equations including continuity, momentum and energy are solved numerically by a finite volume method (SIMPLE). The flow field for different Reynolds numbers has been obtained using this flow field, pressure loss and skin friction coefficient has been calculated. Also temperature field in both solid and fluid for wide range of effective parameters in conjugated heat transfer such as Peclet number, solid-fluid conductivity ratio and solid thicknesses have been investigated. From the obtained numerical results for thermal field effects of conjugated heat transfer characteristics on fluid mean bulk temperature, solid-fluid interface temperature, solid-fluid interface heat flux and Nusselt number have been calculated. The obtained results have been compared with available numerical and experimental data and a good agreement is achieved.

In this paper, a mathematical model for symmetrical multi-layer sheet rolling, in which the layers are unbounded before rolling, by using the upper bound method and stream function theorem is proposed. Using this model, we can investigate the plastic deformation behavior of sheets at the roll gap during rolling. Effect of various rolling conditions such as initial and final thickness and flow stress of sheets, friction factors, rolling velocity and etc. on the rolling power and force, the thickness reduction of each layer, the relative length of plastic region in each layer and etc. are discussed. The velocity field derived from the newly proposed stream function can automatically satisfy the volume constancy and velocity boundary conditions within the roll gap. The optimized velocity fields are obtained through the minimization of total power, which is expressed by the function of five pseudo-independent parameters, during the plastic deformation. The analytical predictions from the proposed model were compared with the analytical and experimental results of other investigators and a good agreement is shown. Present model is applicable for simulating and online control applications of the rolling process of multilayer sheets.

Control Systems such as Anti Lock Brake Systems (ABS) and Traction Control Systems (TCS) are vastly used in most vehicles as to enhance the safety of the systems. These systems have many virtues on controlling the dynamics of vehicles, but they only control the longitudinal dynamics of the vehicle directly and the lateral dynamics of vehicle is not controlled because they do not receive any feedback from the lateral dynamics parameters. Therefore they cannot recognize the critical situations. In the preceding essay, by implementing the rotational velocity sensors and lateral accelerometers in addition to the ABS sensors, a developed control system is designed in which lateral motion is controlled in addition to the longitudinal motion. In order to derive the control laws and measuring system's indefinite parameters, a 7-DOF model was used. The designed control system has two layers, which the upper layer is based on PID and the lower layer was designed upon the Sliding Mode Control (SMC). For measuring parameters, the least square with exponential forgetting was used. In order to verify the control system a 14-DOF model with a nonlinear tire model was used.

In this research, a model for optimal PM planning based on reliability is developed and solved for multi-component systems. In the proposed model, the type of PM actions for each inspection period is determined in such manner that the total weighted related costs are minimized while a minimum required system reliability is maintained. The planning horizon is divided into some inspections intervals of equal size. In the beginning of each interval, with respect to the system constraints, one of the following PM actions is suggested for each component: 1) inspection and minimal service, 2) preventive repair and 3) preventive replacement. Each of these activities consumes different resources and has different effect on the system reliability. The PM costs include, repair cost, replacement cost, system downtime cost, and random failure cost. In the optimal PM schedule, the PM actions are determined so that a minimum required reliability is obtained with minimum total PM cost. Since the proposed model has a complex structure, Tabu Search and Simulated Annealing are employed to provide quick solutions. The efficiency of these techniques has been demonstrated by solving a PM - scheduling problem for a system with 14 components.

Dimensional accuracy in machined parts depends on the precision of spindle, which is highly affected by applied forces, itself. This precision of spindle becomes more serious when it is used for a period of long times. Therefore, stress and strain analysis of spindle is very important in the behavior and preservation of its precision. In this paper, the forces applied to the spindle of a turning machine are calculated and analyzed. Afterwards, the spindle is simulated with MSc Visual Nastran software and analyzed due to the application of boundary conditions. According to the spindle's stress and strain, the modal analysis has been done and the natural frequencies of the spindle are determined. The results show that, the front  bearing of the spindle is the critical section and its stress and strain will be increased with feed and depth of cut. When these feed rates and depths of cut become less, the created stresses at the least speed of spindle are more in comparison with the other speeds.