AMIRKABIR
Year:2001 | Volume:12 | Issue:47|Papers Count:9

Aerodynamic heating calculation of a hypersonic body is normally performed during the critical part of its flight trajectory. This requires solution of inviscid flow field around the hypersonic body and most crucially around its nose, for several times. In this paper, using Thin-Shock-Layer theory, three- dimensional Euler equations are transferred to a shock-oriented coordinate, and by implementing appropriate approximations, an inverse method is applied for the calculation of flow field between the shock wave and the body surface. Based on the nose shape of a hypersonic body flying at M ∞ a three-dimensional shock geometry is first estimated. Using explicit formulations obtained from the inverse method, inviscid flow field behind the shock wave is numerically calculated. From this calculation the resulted surface with zero stream function corresponds to a nose that has produced the estimated shock wave. Based on the error between this nose and the real one, the 3D shock shape is repeatedly changed until the calculated nose matches the real one. Using this engineering approximate method, which is very fast, all of the flow variables can be determined in the solution domain.  An excellent agreement is observed between the results obtained in this paper and those calculated by others or extracted from experiment. Since the method is very fast it for preliminary design, or parametric study of vehicle aerodynamics and thermal protection at hypersonic flows. Such a fast method is also desirable for making initial conditions suitable for CFD codes.

An approach for dynamic simulation of ball-end milling of die surfaces is presented in this paper. It has. the capability of estimating the static and dynamic cutting forces and tool deflections for various cutting conditions. A commercially available geometric engine is used to represent the cutting edge, cutter and updated part. The contact face between cutter and updated part is determined from the solid model of the updated part and cutter location. To determine cutting edge engagement for each tool rotational step, the intersections between the cutting edge with boundary of the contact face are determined. The instantaneous chip thickness is computed by summing up the static chip thickness, the tool deflection and the undulations left from the previous tooth. The geometric information and are used in an orthogonal cutting data base of the ball-end miling process, from which the dynamic forces are obtained. For calculating the ploughing forces, Wu's model is extended to the ball-end miling process. The total forces, including the cutting and ploughing forces are applied to the structural vibratory model of the system and the dynamic deflections at the tool tip are predicted. The developed model is verified through experiments performed for several workpiece geometries and different cutting onditions.

In order to justify the operating limits at actual nuclear plants, development of a design ,limit is a necessity. This paper describes the techniques involved in development of critical, heat flux correlation (regression model) and its design limit for safety purposes of nuclear water reactors. The procedure involves all conservative statistical modelings. The method uses the basic relationship between critical heat flux and local equilibrium quality which are ,linearly dependent on each other. The technique that is required to optimize the correlation is a non-linear regression process. Detection of outliers, collinearity check, and biasing toward a variable are considered. Checks for equality of standard variations, means, normality, and design limit of subgroups of samples and / or whole correlation data are also considered.

A set of benchmark problems and solutions have been developed and applied to verify the accuracy of computer package used for static and dynamic analysis and design of nuclear power piping systems. The problems range from simple to complex configurations which are assumed to experiences a linear elastic behavior. The dynamic loads consist of harmonic excitation, impact loading and uniform support motion response spectrum which is compared to independent support motion response spectra. Thermal expansion loading is employed too. An investigation on flexibility and stress intensification factors is also provided. The benchmark problems are applied to the Sim flex II computer code as a test problem. This package is licensed "as is" [3] by the supplier and may be used to analyze nuclear power piping systems. The results show that Sim flex II is a good program for analyzing and designing of commercial piping systems and works well within the range of its capabilities. For analyzing the systems such as the nuclear power plant piping systems, this program may be used as a pilot program to make good estimations. A more accurate results may be provided by general programs which have more options and capabilities in analysis but is more difficult to handle and therefore higher risk of mistake being committed by the user.

Despite the fact that balloon angioplasty has been used to enlarge the stenosed arteries for several years, the influence of balloon dilatation on the atherosclerotic plaque is not completely understood. So far several theories have been proposed. but nowadays, the major mechanism of balloon angioplasty is postulated to be plaque fracture, with immediate formation of fissures within the atherosclerotic lesion, which provides channels for blood .flow. On the other hand, fracture of the surface of an atherosclerotic plaque is known as a common cause of the progression of atherosclerosis.In this study we supposed that plaque rupture is a disadvantageous outcome of the balloon inflation and the influence of balloon angioplasty is due to varying the elastic property of the atherosclerotic plaque. We also supposed that atherosclerotic tissue will become more extensible because its collagen fibers will tear under tension due to balloon inflation. Based on these mechanical properties. some finite element models were used and analyzed under balloon pressure using ANSYS 5.4. Finally, it has been shown that under normal blood pressure, the atherosclerotic artery after angioplasty is wider than before the procedure.

Compressible flows for unsteady, inviscid and viscous cases have been studied. Important features of flows such as formation of shock waves across the flow in an unsteady flow as well as interaction of shock waves with boundary layers and their effects on the flow around the blades have been analyzed.Jameson control volume approach was used to spetially integrate the flow equations and the fourth order Ruge-Kutta method was used for time integration. The obtained finite difference equations were used to simulate invisicid and viscous flows in VKI cascades and the effects of viscosity, angle of attack, balde pitches and back pressure on the flow were obtained. It was shown that when the flow was assumed inviscid, the error on the distribution of pressure on the blades were about ten percent. Finally, unsteady flow were simulated and formation of shock waves and their motions were analyzed.

This study used three-dimensional finite element analysis to investigate the stress distribution in a mandibular fixed prosthesis with six implants. There are three different materials for implants (titanium alloy, pure titanium, chromium-cobalt alloy) and four different materials for frame (titanium alloys, chromium-cobalt alloys, palladium-silver alloys and gold alloy) that used in twelve models by different combination of materials. A total of 150 N vertical load is distributed over the teeth in the finite element model. The result shows that the concentrated stress in the screw will increase the risk of prosthesis failure. We obtained the best model with Cr-Co alloys for both implant and frame or for second choice Cr-Co alloys for frame and titanium alloy for implant.

It is well-known that the most of frames are under lateral loads along their members as well as nodal loads. But the majority of researchers who work on nonlinear analysis consider only nodal loads in their study. In second order analysis, the stress of nodal loads cannot simply a be added to stress of lateral loads. In this paper by principle of energy and finite element method we present an element with its stiffness while non linearity effects of non symmetrical lateral loads are considered. Finally some numerical examples have been solved by prepared a computer program by using the above described element.

An analytical model of penetration analysis of ogival projectiles into metallic target has been presented by Dikshit & Sundararajan (1992). That model under condition of negligible deformation of projectile has given good results, but in the real case of current projectile with considerable deformation due to impact, it overestimates the depth of penetration. In this paper; the plastic work done on the projectile during the penetration process has been evaluated and as a considerable dissipating energy added to the other three dissipating energies involoved in the initial model of Dikshit & Sundararajan. Besides, a series of ballisic tests using 7.62 and 5. 56mm projectiles and steel plates have been done and the experimental results compared with analytical solutions.This Comparison shows that the modified model of this paper gives appropriate estimation for depth of penetration and may be used for further parametric studies and designing of metallic targets.