Scientia Iranica
Volume:12 Issue: 4, 2005

The unsteady viscous flow in the vicinity of an axisymmetric stagnation point of an infinitely long rotating circular cylinder is investigated, when the angular velocity varies arbitrarily with time. The free stream is steady and with a constant strain rate of k. An exact solution of the Navier-Stokes equations is derived in this problem. The general self-similar solution is obtained when the angular velocity of the cylinder varies as certain functions. The cylinder may perform different types of motion: It may rotate with constant speed, with exponentially increasing/decreasing angular velocity, with harmonically varying rotation speed or with accelerating/decelerating oscillatory angular speed. For completeness, some sample semi-similar solutions of the unsteady Navier-Stokes equations have been obtained numerically using a finite-difference scheme. These solutions are presented for special cases when the time-dependent is step-function, linear and non-linear, with respect to time. All the solutions above are presented for Reynolds numbers, Re=ka^2/2\upsilon, ranging from 0.1 to 1000, where a is the cylinder radius and \upsilon is the kinematic viscosity of the fluid. Shear stresses corresponding to all cases increase with the Reynolds number. The maximum value of the shear-stress increases with an increase in oscillation frequency and amplitude. An interesting result is obtained, in which a cylinder, spun up from rest with a certain angular velocity function and at a particular value of Reynolds number, is azimuthally stress-free.

Large volumes of potable water are imported into almost all urban areas of Iran at extremely high cost, while considerable volumes of urban stormwater are disposed of out of the cities and wasted, mostly into bodies of saltwater. In this paper, urban stormwater is examined as a potentially valuable and reclaimable resource. A model is introduced in which stormwater runoff is captured and stored behind a small dam of height $H$ with an overflow weir of length $L$ to waste excess flows. At the same time, the stormwater is diverted through a side weir of width W, to be conveyed to suitable recharge grounds for later reclamation and use. A particular watershed, in the arid and rapidly urbanizing city of Bandar Abbas in southern Iran, with 24 years of rainfall records, was chosen and used in the model. The rainfall from nine other arid, semiarid and wet regions were used as input to the same watershed. The results show that the amount of reclaimable water as a percent of total runoff is almost the same for all the regions. This study provides a relationship, which defines the reclaimable water as a function of W,L and H. The relationship may be used for planning urban stormwater reclamation projects. The normal parameters defining arid, semiarid and wet climate are not of significance in this relationship. The relationship, however, may be further refined if one incorporates the number of intense storms as an extra parameter.

In this work, boundary conditions of the T-junctions in engine silencers are modeled by the Constant Pressure Model (CPM) and the Pressure Loss Model (PLM). Initially, the mean flow velocity through the ducts is assumed to be zero. Two Benson CPM and Corberan CPM approaches are employed in perforate silencers simulation. For the silencer with more than one perforated pipe, in which N-branch junctions are formed, it is possible to apply the Benson CPM approach. Finally, when the mean flow velocity through the ducts is non-zero, the shortcoming of the CPM model and the ability of the PLM model in describing the T-junctions are shown."

In this paper, a simple and efficient analytical method, combining elastic large deflection analysis and rigid plastic mechanism analysis, is presented for derivation of the average stress-average strain relationship of plates subject to in-plane longitudinal compression. By imposing equilibrium conditions of forces and bending moments and assuming proper stress and strain distributions in the stiffened plate cross-sections, the average stress-average strain relationship of the stiffened plates is also derived. The algorithm can be easily implemented in methods for the evaluation of ship hull girder strength, as well as in the estimation of the ultimate capacity of offshore structures.

This paper presents a new method for optimization of the dynamic response of structures subjected to seismic excitation. This method is based on the concept of uniform distribution of deformation. In order to obtain the optimum distribution of structural properties, an iterative optimization procedure has been adopted. In this approach, the structural properties are modified so that inefficient material is gradually shifted from strong to weak areas of a structure. This process is continued until a state of uniform deformation is achieved. It is shown that, in general, for a MDOF structure, there exists a specific pattern for distribution of structural properties that results in an optimum seismic performance. It has been shown that the seismic performance of such a structure is optimal and behaves generally better than those designed by conventional methods. The application of the proposed method for optimum seismic design of different structural forms, such as truss-like structures and shear-buildings, is presented. The effects of fundamental period, target ductility demand, damping ratio and seismic excitations on an optimum distribution pattern are investigated.

A new approach, based on a Generalized Regression Neural Network (GRNN), has been proposed to predict the unsteady forces and moments of two different models; a 70\degree swept delta wing in subsonic incompressible flow and a standard fighter model (SDM) in a compressible flow regime, both undergoing sinusoidal pitching motion. Extensive wind tunnel results were used for training the network and verification of the values predicted by this approach. GRNN was trained by the aforementioned experimental data and, subsequently, was used as a prediction tool to determine the unsteady longitudinal forces and moment of the two models under various conditions. Further, it was applied to extend the experimental data beyond the conditions tested in the tunnel. The results are in a good agreement with the experimental findings. This indicates that the present prediction and optimization tool provides sufficient accuracy with a modest amount of experimental data.

3-D wall panels are used in the construction of exterior and interior bearing and non-load bearing walls and floors in all types of construction. The present paper investigates the mechanical characteristics of 3D wall panels under static shear and bending loads, in order to better understand their structural components. The numerical model is loaded in increments to simulate the tests and to allow detection of failure in flexural tests for vertical and horizontal bearing panels and also for direct shear. The load-displacement curves, resulting from finite element analysis, are very similar to those tested specimens. Maximum loads in flexural tests, both for wall and floor panels, are equal to the experimental ultimate loads. The failure mechanism is started, after moving from the elastic zone at the load stage of 700 kg, by tension failure in the lower wythe of the concrete. Then, the crack propagates to the upper layer, at the level of 1200~kg load. The bottom mesh is yielded and, finally, the crushing of concrete causes the instability of the system. The maximum load is reported as 2200 kg. In direct shear analysis, the panel behaves as a cantilever deep beam.

In this paper, a new approach for solving partial differential equations by means of multiple Laplace transforms is developed. The theorem regarding the independence of the final image (final original) on the sequence of realizing the transforms is proved. The diffusion equation with delay is analytically exactly resolved. An algorithm of the solution is given for cases \xi>>\gamma and arbitrary values of parameter \gamma. It has been shown what changes in solution take place for problems of diffusion with a moving boundary. The solution may be used for most problems with a delay argument.

A numerical analysis has been performed to investigate the ground effects on the main parameters of a two-phase unconfined cloud of fuel and air to study its detonability. Equivalence ratio, turbulence intensity, cloud shape and volume, uniformity, temperature gradient and delay time distribution are the most important factors that affect the detonability of a vapor cloud. The effects of the altitude of the dispersing device from the ground on these significant factors have been demonstrated. A modified KIVA-based program has been employed to carry out the computations. A finite volume method is used to solve the equations describing the gas phase. A discrete particle technique is applied to represent the liquid spray and a k-\varepsilon model is used for modeling the gas phase turbulence. Theoretical considerations and comparison with associated experimental values were made for validation. As the injection height increases, the cloud becomes more uniform and the possibility of the pulsing propagation of the detonation wave decreases. When the injection height decreases, the contour of the detonable range rotates faster and delay time decreases. As a trade-off between all effective parameters, in this paper, an optimum for the injection height is introduced

This study is an attempt to quantify the concept of sustainable transportation. The countries are comparatively studied using a pioneer measure for Sustainable Development (SD) and elasticity, that reflects the conformity and harmony of the growths of all sectors with passenger and freight transportation. Firstly, the elasticity of the non-transportation variables, with respect to passenger and freight transportation ones, were developed. Using individual elasticities, composite sustainability indices were suggested. Then, utilizing the Data Envelopment Analysis (DEA) technique, two composite indices, as well as the national variables, are employed to achieve a unique SD efficiency score. Country groupings, based on composite indices, were developed for comparative appraisal. The methodology may be applied to any other time and geographic scope for addressing pertinent issues for the balancing and SD of transportation systems

Different parameters of the asymmetric contact problem between an elastic wedge and a half-plane have been introduced in this paper. These parameters include the distribution of contact pressure and length of contact zones due to frictionless normal loading. For each parameter, the results have been compared with the results of the symmetric problem and numerical solution, which show excellent agreement. The method of approach is a completely analytical method based on singular integral equations. In this method, the boundary conditions of the problem are stated as some singular integrals, and distribution of the contact pressure is specified. Then, with use of the equilibrium equations and the consistency conditions of the singular integral solution, the lengths of the contact zones are specified.

In this paper, the results of limit analyses are presented in dimensionless form for a wide range of geometric parameters, which are practically of interest. Analytical estimation of upper bound and lower bound limit loads of a plate with an elliptical hole under uniaxial loading and with a circular hole under biaxial loading in-plane stress condition are calculated; the analytical results are then compared with finite element calculations and the correlation between them is discussed. The finite element calculations consist of elastic-plastic estimation of limit load and lower and upper bound limit load prediction, using the elastic compensation method. The analytical upper and lower bound estimations and elastic-plastic results are found to be in good agreement with elastic compensation lower bound values, while the elastic compensation upper bound results are found to be overestimated