Showing papers by "Reza Attarnejad published in 2008"

Application of Differential Transform Method in Free Vibration Analysis of Rotating Non-Prismatic Beams

Abstract: Rotating beams are considerably used in different mechanical and aeronautical installations. In this paper, free vibration of non-prismatic rotating Euler-Bernoulli beams is studied. Dynamic stiffness matrix is evaluated by using differential transform method, a powerful numerical tool in solution of ordinary differential equations, for solving the governing equation of motion. The method is capable of modeling any beam whose cross-sectional area and moment of inertia vary along beam with any two arbitrary functions and any type of cross-section with just one or few elements so that it can be used in most of engineering applications. In order to verify the competency of the method, natural frequencies are obtained for two problems and the effects of rotational speed parameter and taper ratio on natural frequencies are investigated.

Water Hammer Analysis by Characteristic Method

Abstract: Rapid changes in the velocity of fluid in closed conduits generate large pressure, which are transmitted through the system with the speed of sound. When the fluid medium is a liquid the pressure surges and related phenomena are described as water hammer. Water hammer is caused by normal operation of the system, such as valve opening or closure, pump starts and stoppages and by abnormal condition, such as power failure. Problem statement: Water hammer causes the additional pressure in water networks. This pressure maybe defects on pipes and connections. The likely effects of water hammer must be taken into account in the structural design of pipelines and in the design of operating procedures for pumps, valves, etc. Approach: The physical phenomena of water hammer and the mathematical model which provides the basis for design computations are described. Most water hammer analysis involves computer solution by the method of characteristics. In this study water hammer is modelled with this method and effect of valve opening and closure will be surveyed with a program that is used for this purpose and with a numerical example. Results: The more rapid the closure of the valve, the more rapid is the change in momentum and hence, greater is the additional pressure developed. Conclusions/Recommendations: For preventing of water hammer defects, is recommended that valves should be open or closed slowly. Also with using the method of characteristics, we can modelled all pipe networks, and see the affects of water hammer.

Crack Growth in Concrete Gravity Dams Based on Discrete Crack Method

Abstract: Seepage is the most parameter in water management safety and in stable agricultural. This seepage is passed through the cracks that are present to some degree in hydraulic structures. They may exist as basic defects in the constituent materials or may be induced in construction or during service life. To avoid such failure in concrete dams, safety would be an important factor. Over-design carries heavy penalty in terms of excess weight. So the fracture mechanics theory is a principal necessity of evaluating the stability of such crack propagation. For the process of crack propagation analysis in concrete structures, there are two general models: discrete crack and smeared crack. This study surveys the crack propagation in concrete gravity dams based on discrete crack methods. Moreover, we use a program provided specifically for this purpose.