Scientia Iranica
Volume:11 Issue: 1, 2004

Collapse, defined as the additional deformation of compacted soils when wetted, is believed to be responsible for damage to buildings resting on compacted fills, as well as failure in embankments and earth dams. In this paper, three different types of neural networks, namely, conventional Back-Propagation Neural Network (BPNN), Recurrent Neural Network (RNN) and Generalized Regression Neural Network (GRNN) are employed as computational tools to predict the amount of collapse and to investigate the influence of various parameters on the collapse potential. To arrive at this goal, 192 series of a single oedometer test were carried out on three soils with different initial conditions and inundated at different applied pressures. The test results were used to prepare the necessary database for training the neural network. Similar test results available in literature were also included in the database to arrive at a total of 330 sets of data. A comparison of the network prediction for collapse potential with some available models shows the superiority of the network in terms of the accuracy of prediction. Moreover, by analyzing the network connection weights, the relative importance of different parameters on collapse potential was assessed. Based on this analysis, for a given soil type, the initial dry unit weight, \gamma_d, is the most important factor influencing collapse potential

Using Helmholtz''s decomposition theorem, the laws of balance of linear and angular momenta are restated as surface integrals over the closed surface of an arbitrary subregion in a continuum. Newton''s law of action and reaction and Cauchy''s theorem for stress and couple-stress are proved as corollaries of these surface balance laws.

This study is concerned with the general motion of a guided flexible launch vehicle idealized as a non-uniform beam under continuous thrust action. The governing equations of motion are derived following the Lagrangian approach and generalized coordinates. The rigid motion consists of the conventional vehicle velocities (rotational and translative), whereas the elastic motion, introduced through modal substitution, represents the vehicle local lateral and transverse displacements relative to a mean body axis system. A complete simulation routine has been developed, which allows for investigation of the influence of various vibrational forcing functions, local stiffness changes and the Inertial Measurement Unit (IMU) displacements on the vehicle trajectory and the required control action histories.

Euler conservation equations, ideal gas state equations and simplified chemical kinetics models were used to simulate two-dimensional straight and baffled shock tubes. In a straight channel, detonation waves were initiated by a strong shock wave and allowed to travel down the channel to reach a CJ wave condition. It has been shown that a two-step reaction, kinetics model with an induction time delay, resulted in a physically plausible transient solution. The one-step kinetics model solution is only valid at the limit of a steady state CJ wave condition and should not be used for transient problems. The two-step kinetics model was then used to simulate a detonation initiation in a baffled shock tube. It was shown that the presence of multiple baffles in a channel could result in an initiation of detonation, in cases where the temperature jump across the traveling initial compression wave and the presence of a single baffle are not sufficient to initiate a detonation. Furthermore, it is shown that in the absence of any viscous mechanisms, shock reflection from the second baffle created a moving Mach stem between the baffles. The coalescence and focusing of pressure waves behind this Mach stem resulted in the creation of a hot spot leading to a detonation wave.

In this paper, results of a preliminary study on the feasibility of using an Active Tendon Control (ATC) mechanism for frame structures subject to earthquakes is presented. So far, the ATC mechanism has mainly been considered as a means for installation on structures to mitigate their response under severe loading. In this study, it is desired to evaluate the possibility of using the ATC mechanism to serve as the main means for the stability of frames against earthquakes. Hence, a methodology is presented for the integrated design of frames with ATC mechanisms, which is tested numerically. A number of five-, ten-, fifteen- and twenty-story steel frames are used for this purpose. To this end, first, each of the frames is designed in accordance with the Uniform Building Code of Practice (UBC). Then, the same structure is re-designed for its dead load only, but equipped with a number of ATC mechanisms that help the structure withstand earthquake loading, so that its overall behavior is similar to the UBC frame. This results in a reduction of the cross-sectional dimensions and weight of the columns at the expense of providing the required hydraulic actuator(s), the sensory system, the controller chip(s) and the prestressed tendons. The ATC frame so designed is, then, considered to be equivalent or comparable to the UBC frame. Furthermore, the behavior of the ATC and UBC designs are compared.

In this paper, a 3-D, unsteady vortex lattice model to compute aerodynamic coefficients, using time domain eigenmode analysis, is presented. A computationally efficient technique for constructing a reduced order model of unsteady flow about a low aspect ratio wing, modeled as a cantilever plate of constant thickness, is presented. Analysis demonstrates that limit cycle oscillations of the order of the plate thickness are possible. The eigenmodes of the system, which may be considered as aerodynamic states, are computed and, subsequently, used to construct a computationally efficient, reduced order model of an unsteady flowfield. Only a handful of the most dominant eigenmodes are retained in the reduced order model. The effect of the remaining eigenmodes is included approximately, using a static correction technique. An advantage of the present method is that, once the eigenmode information has been computed, the reduced order model can be constructed for any number of arbitrary modes of wing motion very inexpensively. The method is particularly well suited for use in the active control of aeroelastic phenomena, as well as in standard aeroelastic analysis for flutter or gust response. Finally, a numerical example is presented that demonstrates the accuracy and computational efficiency of the present method.

In this investigation for Expandable Pattern Casting (EPC) simulation, an algorithm was developed to calculate the gas pressure of the evaporated foam during mould filling. Also, the effect of backpressure evaporative foam on filling behavior was modeled with a new experimental function by adding a Volume Of Fraction (VOF) function. The simulation of molten flow and track free surfaces is based on the (SOLution Algorithm) SOLA-VOF numerical technique. To simulate the three-dimensional incompressible flow of melt in EPC moulds, the pressure boundary conditions, heat transfer and foam gas pressure effect were modified. In order to verify the computational results of simulation melt flow in EPC casting, a thin grey iron plate was poured into a transparent foam mould. Mould filling and foam depolymerization were recorded with a 16mm high-speed camera. The comparison of experimental and simulation results of the sequence filling of EPC casting showed a good consistency, which confirms the accuracy of the model.

In this paper, heat and mass transfer phenomena occurring simultaneously in falling film generator of absorption chillers have been studied. The analysis is based on the laminar flow of an Li/Br solution over a horizontal single tube and tube bundle having a constant tube wall temperature. The effect of boiling has been ignored. An extensive numerical code is provided to calculate the heat transfer coefficient and the rate of evaporation. A parametric study is performed on the coefficient of heat transfer and the evaporation flux of the refrigerant. Dimensionless correlations are obtained to calculate the heat transfer coefficient on the horizontal tube and tube bundle. The comparison between numerical and analytical results with the existing numerical and experimental data verify the validation of the present model.

In this paper, a framework for operation of hydropower reservoirs in Iran is discussed. A time decomposition approach, which breaks down the operation optimization problem to long, mid and short-term planning stages, is developed. Stochastic dynamic programming models are developed for long-term and mid-term optimization and a deterministic dynamic programming model is developed for short-term optimization of two hydropower-reservoirs (a parallel system). An economic analysis of the benefits and cost of the operation are incorporated in all three stages of planning by developing economic cost functions. The developed algorithm is applied to the Karoon and Dez river-reservoir systems in the southwest of Iran. The results of this study have shown the significant value of the developed models and their relational framework in providing more flexibility and adaptability in using methods and tools for decision-making in real world situations.

In this paper, the temperature of a moving surface is determined with a moving, finite element-based inverse method. In order to overcome the ill-condition of moving inverse problems, three different conventional regularization methods are used: Levenberg, Marquardt and Modified Levenberg. The moving mesh is generated employing the transfinite mapping technique. The proposed algorithms are used in the estimation of surface temperature on a moving boundary in the burning process of a homogenous solid fuel. The measurements obtained inside the solid media are used to circumvent problems associated with the sensor and the receding surface. As the surface recedes, the sensors are swept over by the thermal penetration depth. The produced oscillations occurring at certain intervals in the solution are a phenomenon associated with this process. It is shown that regularization delays convergence and, therefore, the use of normal analysis is sufficient. The method can be used successfully for a wide range of thermal diffusivity coefficients.

In this paper, the hydraulic characteristics of a sharp crested triangular side weir have been experimentally studied. It was found that the DeMarchi coefficient of discharge for a sharp crested triangular side weir in subcritical flow is related to the main channel Froude number, the apex angle of weir and ratio of weir height to upstream depth of flow. Suitable equations for discharge coefficient are also obtained.

This paper implements a derivative free optimization method called `Genetic Algorithm'' to estimate the parameters of a four-wheel, three degrees of freedom vehicle handling model. At first the model is developed containing a non-linear tire model called `Fiala''. Then, an error function is defined and the `Genetic Algorithm'' optimization method is introduced and applied to minimize the error. Finally, verification of parameter estimation is checked.

In this paper, an unsteady, two-dimensional solver is developed, based on Van Leer''s flux splitting algorithm, in conjunction with the ``Monotonic Upstream Scheme for Conservation Laws (MUSCL)'' limiters for improving the order of accuracy. For a minimum usage of computer memory and faster convergence, the two-layer Baldwin-Lomax turbulence model is implemented for a viscous solution. Three test cases are prepared to validate the solver. The computed results are compared with experimental data and the good agreement of the compared results validates the solver. Finally, the solver is used for the flow through a multi-blade stage of an axial compressor in its design condition. The solutions of inviscid and viscous flows are prepared and the computed results are compared with each other, to show the accuracy of an inviscid approach, with respect to the viscous flow at the design operating point. The comparison shows that the viscous approach is more acceptable.

The closed-form solution of a three-dimensional line-of-sight guidance with a moving tracker is derived for an ideal case, in which a pursuer is always on the instantaneous line between the target tracker and the target. The solution can be applied to both surface-to-air and air-to-surface applications. Some significant characteristics, such as intercept time, cumulative velocity increment, initial condition for interception, and the effect of acceleration limit, are also obtained and discussed. In addition, the equivalent effective navigation ratio for the line-of-sight guidance is introduced. Finally, solutions for a special case of maneuvering target are presented.

In this paper, a finite volume scheme is used to discretize flow in porous media. A numerical method for treating advection-dominated contaminant transport for flow of groundwater is described. This system combines the advantages of numerical discretization and the finite volume method (like local mass conservation). The equations are discretized using a finite volume approach. The resulting nonlinear differential system is integrated in time using a solver. The conservation of energy (convection-diffusion) equation is solved using a special method for reducing the oscillations induced by the standard finite volume method. Consequently, a mathematical model for multi-component flow transport in an anisotropic media is presented, which couples the equations for multi-component diffusion and Darcy''s law for flow in a porous medium. Furthermore, application of an integrated matlab system in several studies has been provided. The integrated matlab system is based on open data formats and standards and may be used for many other application areas, especially where modeling in 2D and 3D is involved. Numerical simulations are performed to validate the model and investigate the effect. The final purpose of this paper is to discuss and compare the difference between the finite volume scheme for uniform and for unstructured grids, which is shown to be less than 0.2 percent.

In this paper, a systematic procedure for design of a combustion chamber pressure control system is presented. The procedure is applied to a typical liquid propellant engine and the performance of the resulting new control system is compared with that of the present one. In this research, the nonlinear and dynamic mathematical model of the engine, which includes both soft and hard nonlinearities, is used. The systematic controller design procedure is based on describing function models of the engine coupled with the factorization theory.