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Volume 13, Issue 2, 2006
Civil and Mechanical Engineering


Buckling of Discretely Stringer-Stiffened Composite Cylindrical Shells under Combined Axial Compression and External Pressure
 
        M.Z. Kabir (PhD.)
  • D. Poorveis [PhD.]

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In this paper, the static buckling of especially orthotropic stringer-stiffened composite cylindrical shells subjected to combined axial compression and external pressure is investigated, based on geometrical non-linear analysis with considering pre-buckling deformations. The kinematic relation of shells is based on the Donnell non-linear theory and First Order Shear Deformation (FOSD) is adopted for both shell and stiffeners. Displacements, rotations and interacting forces are expressed in terms of Fourier series expansions as independent approximate solution functions. Unknown coefficients of shell and stringers are related by satisfying continuity conditions of displacements at their contact areas using Lagrange multipliers. The non-linear equilibrium equations are obtained using the Ritz method. The effects of sensitivity parameters, e.g., shell lay-ups, different numbers of stringers in the circumference, location of stiffeners (outside vs. inside) and the discrete versus smeared approach on interaction buckling curves are considered. Results indicate remarkable differences between outside and inside stringer stiffened cylinder buckling loads and also illustrate the fundamental role of shell stacking sequences and stiffened shell geometry on the applicability range of the smeared stiffener approach.


Large Eddy Simulation of Separated Flow over a Wall-Mounted Cube
 
        M. Rahnama (PhD.)
  • M. Farhadi [PhD.]

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Large eddy simulation of flow over a wall-mounted cube in a channel was performed at a Reynolds number of 40000. The structure function modeling of the subgrid-scale stress terms was used with three slightly varying versions of its selective type. The convective terms were discretized using a QUICK scheme, along with a relatively coarse grid. A series of time-averaged velocities and turbulent stresses were computed and compared with experimental data to examine the performance of these models. The structure function model yielded deficient mean flow structure and turbulence statistics compared with the selective structure function. While none of these models could reproduce experimental results exactly, the results of time-averaged streamline plots and turbulent kinetic energy for one of the selective structure function models showed less discrepancy with experimental data compared with other models. It was shown that implementation of a wall function does not improve the results considerably and, in general, with a coarse grid resolution, it is possible to obtain some reasonable results as compared to the experiment.


Experiments in Near-Field of Turbulent Jets into a Crossflow
 
        M. Taeibi-Rahni (PhD.)
  • K. Aavani [PhD.]
  • M.R. Soltani [PhD.]

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Low-speed wind tunnel experiments were conducted to examine the effects of jet exit behavior on the near-field characteristics of jets in crossflow. To better understand this problem, a row of six square jets were perpendicularly injected into the main turbulent flow. The jet-to-crossflow velocity ratios examined were 0.25, 0.5 and 1.0, while the jet spacing to jet diameter was 3.0. No significant temperature differences between the jet and the crossflow were introduced. The analysis of the vertical structure of the transverse jets, including focusing on the jet shear layer and the vorticity dynamics of the exiting jets, is complicated. The vorticity around the circumference of the jets was tracked to identify its relative contributions to the nascent streamwise vortices, which evolve eventually into kidney vortices downstream. The mean velocities and the six turbulent stresses were measured using a dual-sensor probe (X-array wire). Comparisons between the present work in the measurement sections with previous experimental data show reasonably good agreement. In this paper, the flow statistics are reported in the form of vector plots, contours and X-Y graphs, showing the velocity vectors, turbulence intensities and Reynolds stresses.


Application of Rough Set Theory as a New Approach to Simplify Dams Location
 
        M. Arabani (PhD.)
  • M.A. Lashteh Neshaei [PhD.]

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Recently, advanced methods have been developed for selection of suitable sites for different types of dam. Apart from location, selection of the proper type of dam for a given site is of the greatest importance for engineers. Although these methods are being developed and some new approaches, like GIS techniques, are being used currently, all of these methods are mostly dependent on engineering decision making and a need for high costs. In this study, a new approach in the determination of dam locations is proposed. This method is based on the rough set mathematical theory that is a new approach presented by a well-known mathematician, Pawlak, in 1991. The information obtained from a practical case, the Karkhe project, which is one of the greatest dam projects throughout the middle-east, is used to present the procedure of this method. This dam was investigated in 1994 by famous consulting engineers and 30 places for the construction of this dam were considered. On the basis of the existing data for these places for siting of the Karkhe dam and based on the data required for the rough set method, finding a location for the most suitable site for this dam, using this new method, is undertaken and compared to the practical results obtained from site investigations. The results of this study are completely correlated with practical methods, which were performed by consulting engineers. The results also indicate that using this method for the project results in a total saving of 70% in costs.


Investigation of Compressible Viscous Flows over Open Cavities Including the Effects of Shear Layer Thickness and Multiple Deck Structure on Interaction with the Trailing Edge
 
        A.B. Rahimi (PhD.)

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The problem considered in this paper is that of compressible viscous flow over an open rectangular cavity, including the effects of shear layer thickness and triple deck structure on interaction with the trailing edge. This analysis unifies analytical cavity studies and provides a wave-number correction for the method of Tam and Block [1], which studies the case of a compressible inviscid flow with a constant shear layer thickness spanning an open cavity. Here, basic equations for a two-dimensional compressible viscous flow are derived. The effect of non-parallelism of the mean flow is introduced. This weakly non-parallel mean flow is perturbed to obtain the governing equations for the shear layer. The inverse of the Reynolds number, here related to the weak effect of non-parallelism of the mean flow, serves as a perturbation parameter. The shear layer is divided into a region of inviscid flow away from the trailing edge and a region where viscous effects are important, near the trailing edge, as a typical boundary layer problem. The viscous region is analyzed by proper scaling of the independent variables. Distinguished limits are obtained by balancing the terms in the set of the governing equations. A multiple deck structure, containing three distinct regions, occurs, each region with a different scaling. Normal modes for the flow properties are introduced to predict an eigenvalue problem, which governs the wave number/frequency relationship in each deck, as well as the inviscid region. These eigenvalue problems are derived using the Fredholm Alternative and are solved numerically by use of a fourth-order Runge-Kutta method. The method of asymptotic expansions is used to match these wave numbers to those among the multiple deck structure, as well as to the one for the inviscid region. This wave number, uniformly valid throughout the region, is used as a correction to the one derived by Tam and Block. This study proves that by considering the effect of non-parallelism of the mean flow, a lower wave-number/frequency is produced at any spanwise location for a given excitation frequency. Predicted discrete tone frequencies, based on this corrected value of wave-number, produce a closer agreement with the experimental results.


Entrapped Air in Long Water Tunnels During Transition from a Pressurized to Free-Surface Flow Regime
 
        A.R. Kabiri Samani (PhD.)
  • S.M. Borghei [PhD.]
  • M.H. Saidi [PhD.]

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Air-water two-phase flow usually occurs during a sudden rise in water level at a tunnel or during the falling of the water level at an upstream reservoir while entering the conduit. When this happens, different flow patterns are generated, due to the hydraulics of flow and fluid properties. An analytical/numerical model, based on the assumption of a rigid incompressible water column and a compressible air bubble, is derived, to simulate pressure fluctuation, void fraction, air/water flow rate and water velocity in a closed conduit, including water depth at the upper reservoir, due to air bubbles becoming trapped in the water, for the highest possible number of flow patterns. It is a comprehensive model, which can generate different hydraulic situations in closed conduits such as tunnels and culverts, based on a hydraulic approach. The boundary conditions are a system of algebraic or/and simple differential equations. The steady solution of the governing differential equations is, generally, performed as the initial data. The frequency of pressure fluctuation and air/water flow rate predicted by the model is in close agreement with the results of the experiments and the numerical model referred to in the literature. Hence, the present model, which is simply derived due to one-dimensional assumptions, shows itself to be a good tool for predicting the characteristics of a two-phase flow.


Dynamic Compaction Method in Physical Model Tests
 
        F. Jafarzadeh (PhD.)

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In this paper, physical modeling tests are conducted to study some aspects of the dynamic compaction method. The loose models are prepared with dry sand. Four cylindrical tampers with different weight and cross section areas are used for compaction of the models and dropped from various heights. Strain gage type total stress measuring sensors are placed at different levels inside the model to measure the transferred vertical stresses due to collision of the tamper with the surface of the models. The typical stress time histories are presented. Improvement depth and crater depth and diameter created on the model surface are studied, among the various parameters affecting the phenomena. Using the results of the tests, a relation is proposed for the improvement depth which has good agreement with the measured site results.


Modeling Truck Accident Severity on Two-Lane Rural Highways
 
        H. Nassiri (PhD.)
  • A. Edrissi [PhD.]

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Truck accidents are an issue of concern due to their severity. Logit modeling and Neural Network modeling are performed to investigate factors such as vehicle, roadway, environment and driver characteristics that can potentially contribute to the severity of truck accidents. The objective of this study is to present models that can predict the severity of truck accidents and to identify the important factors causing these accidents. Comparison between neural networks and logit modeling are made using vehicle crash data on two-lane rural highways in Iran. A variety of variables related to roadways, vehicles, environment and drivers, such as, driver fatigue, head-on collision and lack of vehicle control, are found to have a significant bearing on the severity of truck accidents. Also, investigating the marginal effects of variables showed the same variables to be significant. The results of the comparison between the logit and neural network model indicated that they both show similar patterns regarding the effects of different variables causing truck accidents, with the logit model providing better results.


An Investigation into the Influence of Deformable Dies on the Springback of Circular Plates
 
        M. Kadkhodayan (PhD.)

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The technique of Sheet Stamping by Deformable Forming Tools (SSDFT) has been recognized as a promising alternative to control the springback of sheet blanks. This paper deals with the springback of elastic-plastic circular plates on deformable dies subjected to transverse external forces. Different significant parameters are examined within the framework of the present investigation. For instance, the influence of the friction coefficient between the workpiece and elastomer on the equivalent plastic strain and, therefore, on the springback, for different boundary conditions, are investigated in detail. Moreover, the effects of the thickness of the workpiece on the springback, for both deformable and conventional dies, are discussed.


Free Vibration Analysis of Gantry Type Coordinate Measuring Machines
 
        M.T. Ahmadian (PhD.)
  • G.R. Vossoughi [PhD.]
  • S. Ramezani [PhD.]

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Coordinate Measuring Machines (CMMs) are designed for precision inspection of complex industrial products. Even though CMMs have high accuracy in electrical systems, mechanical structures prevent very high accuracy in these machines. Mechanical accuracy of CMMs depends on both static and dynamic sources of error. In automated CMMs, one of the dynamic error sources is the vibration of probes, due to inertia forces resulting from parts deceleration. Modeling of a gantry type CMM, based on the Timoshenko beam theory, is developed and the natural frequencies of the CMM system, at different positions of the probe, are calculated. Findings from the analytical and finite elements method indicate high accuracy and good agreement between the results.