JOURNAL OF SOLID AND FLUID MECHANICS
Year:2015 | Volume:5 | Issue:2|Papers Count:20

In this paper, a new method for extracting sliding surface has been presented about a linear system with parametric uncertainties with known bound. In the proposed method, common signal of first order sliding mode control has been applied to the system and then the sliding surface is extracted as a virtual controller with the aim of minimizing a cost function. The main advantage of the proposed method is the possibility of setting the distance of the state trajectory from the sliding surface setting one of the design parameters. Thus, It is possible to near the ultimate goal of sliding mode control, i.e. staying the state trajectory on the sliding surface, without increasing the degree of sliding mode control, while the amount of used energy will be controlled, too. In order to show the efficiency of the method, a quarter car active suspension system has been chosen and the proposed method in the selection of position subsystem sliding surface is used.

A mobile manipulator robot is a complex system due to properties such as coupling between the manipulator and mobile chassis, holonomic and nonholonomic constraints, multivariable and nonlinear dynamics. The control of robot faces the external disturbance, parametric uncertainty and unmodeled dynamics. Therefore, the use of an adaptive fuzzy system is suggested for its capability in overcoming uncertainties and approximating of nonlinear functions based on the universal approximation theorem. However, the tracking error does not converge asymptotically to zero due to the approximation error of fuzzy system. This paper presents a novel adaptive fuzzy control for a mobile manipulator robot. The novelty of paper is compensating the approximation error of fuzzy system for asymptotic convergence in tracking the desired trajectory in the presence of uncertainties. For this purpose, the closed loop system in the error space converges to a linear system with poles having negative real parts. The control design consists of two parts; the kinematic control and dynamic control. Advantages of the proposed design are the simplicity and very good performance in tracking of the desired trajectory in the presence of uncertainties. The stability of control system and convergence to the desired trajectory are proven by the Lyapunov method. The simulation results show the superiority of the proposed control over a robust adaptive control.

In this study, an experimental investigation of repeated low velocity impact is performed on GLARE using drop weight testing machine. After the first impact on the plate, the second impact energy decreases and stays constant until the last impact. Damage due to repeated impact is investigated using visual inspection and C-scan. Three categories namely, no damage, small damage and serious damage are happened as a result of the first impact on the plate. The number of impacts required for the penetration is increased with decrease of the second impact energy. The threshold limit energy is defined as the maximum impact energy in repeated impact after the first impact which causes no damage on the specimen. For the successive impact energy larger than the threshold limit energy, the number of impacts becomes important, while for the successive impacts energies smaller than threshold limit energy, the number of impact has inconsiderable effect on damage occurrence.

In this article, a spectral finite element (SFE) formulation and its solution are described for free and force vibrations of cracked Euler-Bernoulli beam. The formulation based on SFE algorithm includes deriving partial differential equations of motion, spectral displacement field, dynamic shape functions, and dynamic stiffness matrix. Frequency-domain dynamic shape functions are derived from an exact solution of governing wave equations. The cracked beam with an open crack is modeled as two segments connected by a massless rotational spring at the crack position and frequency-domain dynamic stiffness matrix for cracked Euler-Bernoulli beam is extracted. By considering free vibration of the cracked beam, its natural frequencies are derived for different boundary conditions. In the SFE model, It is possible to represent the whole length of beam only by two spectral elements, while it may not be possible to do that in finite element (FE) model, for reaching the same order of accuracy. The accuracy of results obtained from SFE formulation is compared with that of either FE method or analytical formulations. The SFE results display remarkable superiority with respect to those of FE, for reducing the number of elements as well as increasing numerical accuracy.

MAFVRO is one of the out-put only modal analysis methods which is able to determine the modal parameters of a vibration system through the free vibration responses. It is worth to note that most of the vibration systems are subjected to random excitations. Therefore, in such cases, the traditional MAFVRO cannot extract the modal parameters. In this study, the mentioned method is developed to randomly excited vibration systems. Then, the modified MAFVRO is applied to a randomly excited vibration system. The random excitation has been considered as a set of white noises which are applied to all masses of a discrete system. Then by employing the developed method, the estimated results have been compared with those obtained through the structural eigenvalue problem. Comparing the results reveals that the developed method can give the natural frequencies and mode shapes of a randomly excited system with a good accuracy, but it estimates the damping ratios lower than their exact values. In addition, the effect of the noise on the accuracy of the estimated modal parameters have been investigated.

Nowadays, damage diagnosis of the engineering structures, applying vibration signal for early damage identifying during normal operation of the structure or after some disasters such as earthquake are very important from academic and technology viewpoints. Among different methods, proposed for damage diagnosis of the structures, the methods, based on FE model updating using modal analysis features are very important because of its practicability. In this paper one of the new model updating methods, which is lately proposed and proved to be accurate and fast, are applied for designing a damage diagnosis method. As a case study, a simulated and experimental beam, has been considered with artificial damage for studying the proposed methods’ abilities and the noise effects on the proposed method. The results show that the proposed method are more accurate and fast than previous methods.

In this paper, thermo mechanical buckling of functionally graded plates (FG Plates) with circular cutout and subjected to combined thermal and mechanical loads are investigated by Finite Element Method (FEM). Unlike other studies in which the plate is subjected to only one type of loading at once, in present study it was assumed that mechanical and thermal loads are applied simultaneously. The material properties are assumed to vary across plate thickness according to power law distribution of the volume fraction of constituents. The plate formulation is based on first order shear deformation theory (FSDT) and element stiffness matrices are derived based on principle of minimum potential energy. A flexible mesh generation algorithm is prepared in which the mesh density around the hole can be controlled easily. After validating the results of developed finite element code with those available in the literature the effect of boundary conditions in edges of plate and cut out, plate aspect ratio and cut out size on thermo mechanical buckling behavior of FG plates are studied thoroughly and stability boundary graphs are presented. Finally useful conclusions are presented.

In present research, the numerical and experimental investigation of progressive ductile damage has been conducted in free bulging of 304 stainless steel seamed tube using oil pressure. In the numerical section, using the Lagrangian finite element method with ABAQUS/Explicit solver, the seam weld simulated as a thin strip, contains non-homogeneity factors of strength and formability. Forming Limit Diagram (FLD) criterion was used as a measure of damage initiation, as well as effective plastic displacement factor with linear approach in order to model damage evolution. In the experimental section, tensile test of tube has been conducted to attain mechanical properties and also tube free bulging has been perform. Maximum bulging diameter of tube and critical pressure were recorded at the moment of bursting. In numerical analysis, the effect of material non-homogeneity factor on critical pressure, also the outcome of effective plastic displacement on the maximum bulging diameter investigated and compared with experiments. Finally, the optimum values of non-homogeneity factor and effective plastic displacement obtained respectively equal to 0.9 and 0.05. Using these factors, the accuracy of numerical prediction for tube diameter and oil pressure at bursing moment were more than 99%.

In this paper, designing optimal PID controller using modified particle swarm optimization is presented. The advantage of this new method compared to conventional methods of controller design is that it is not limited to a certain class of systems. In designing phase, sum of rising time, settling time, overshoot and integral of squared-error are minimized. There kind of particle swarm optimization algorithms such as ePSO, mPSO and sPSO are compared with other methods of optimization including Ant Colony. The results clearly show how superior the new proposed method is to the other methods. The proposed method differs from the other optimization algorithms in such a way that, the proposed algorithm does not need a velocity equation. The position of the particle is updated directly by extrapolating the current particle position with the global best particle position obtained so far.

On hot days, due to the high power consumption of the compressor, using the refrigeration cycles cause to increase in energy consumption. According to lack of fossil fuels, refrigeration consumption reduction is very valuable. In this paper, simulation and design of a refrigeration cycle are presented that ejector and separator added to a conventional compression cycle. These changes, can reduce the power consumption of refrigerator. Comparison results show with the addition of ejector to conventional compression cycle, depending on the type of fluid and the temperature of the condenser and evaporator, lead to energy saving up to 35%. Also, whatever temperature difference between evaporator and condenser increases, or at the same temperature difference between the condenser and the evaporator, as lower evaporator temperature, coefficient performance of ejection-compression cooler is better than conventional refrigerant. In comparision between working fluids, Ammonia has the minimum coefficient of performance improvement and one reason for increased compression ratio is that this fluid has the lowest increasing compression ratio. The result show that R290 and R134a have the highest improvement coefficient of performance because the operation of these fluids associated with the highest increasing compression ratio.

Low swirl burner provides an effective and low cost solution for stable lean premixed combustion. Several studies have been conducted on the performance of these burners in different pressures, temperatures, capacities, mixture velocities and equivalence ratios. The main design parameters of low swirl burners are the swirl number and the recess length. The objective of this paper is the investigation of recess length effects on the burner performance. A rig test has been established and utilized to study the effects of different equivalence ratios and recess lengths on the temperature distribution and flame regime of low swirl burners. Results revealed that increasing the recess length causes the increase in axial bulk velocity and hence the lifted flame would be stable in wider range of equivalence ratios. Observations also show the significant effect of the specific divergent flame regime of low swirl burners on result in uniform temperature distribution inside the combustion chamber and lower NOx production

Cement rotary kilns are extensively used to change raw material into clinker. Complex phenomenon is observed in cement rotary kilns resulting from conduction, convection and radiation heat transfer, interactions between bed materials and hot gas flow and kiln rotation. Therefore, the modeling of cement rotary kiln have difficulty due to non-linear and stiff set of equations. Regards to over prediction of the maximum inner temperature of the kiln in Spang model; a one dimensional, steady state model is developed based on Spang model to investigate the operation of a cement kiln. In the present work, temperature distribution of hot gases is predicted using two-step methane kinetic to calculate the heat of combustion. Since the amount of CO2 emission in cement kiln processes is very important, CO2 emission generated by both bed material reactions and combustion process are calculated and compared. Modeling results showed that approximately half of CO2 emission is in result of combustion process and other one is from bed material reactions.

In this study, the laminar conjugate natural convection inside a porous enclosure with two solid walls is studied numerically using the lattice Boltzmann method. The Porous media is simulated at the representative elementary volume scale. The Darcy–Brinkman–Forchheimer model is used to model the porous media in the range of 10-4<Da<10-1 for Darcy, 103<Ra<106 for Rayleigh, 0.4<Î<0.9 for porosity, 0.1<tr<0.4 for wall thickness ratio and 1<Ar<100 for thermal diffusion ratio number at pr=1. The influence of porous media is considered by introducing the porosity to the equilibrium distribution function and by adding a force term to the evolution equation. The effects of these parameters on heat transfer are investigated by an average Nusselt number. The results with respect to changes in heat transfer regime from conduction to convection show that any increase in Darcy, Rayleigh, porosity and thermal diffusion ratio number cause an increase in heat transfer coefficient. On the other hand, any increase in wall thickness ratio causing weakening of heat transfer coefficient.

In this paper, the flow and temperature fields affected by electric field applied to the fine wire are numerically investigated for the incompressible, turbulent, and steady flow over a backward-facing step. The numerical modeling is based for solving electric, flow, and energy equations with the finite volume approach. The computed results are firstly compared with the experimental data in case of flat plate and the results agree very well. Then, the effect of different parameters such as the applied voltage, Reynolds number, and the emitting electrode position on the heat transfer coefficient and pressure drop is evaluated. The numerical results show that the heat transfer coefficient with the presence of electric field increases with the applied voltage but decreases when the Reynolds number are augmented. Moreover, reduction of distance between the emitting electrode and the step edge can significantly effect on the heat transfer enhancement and variations in pressure drop.

In this study, the motion of solid particles and fluid flow pattern around square cylinders is simulated. Two dimension Lattice Boltzmann method (LBM) is used to solve momentum and energy equation. To achieve this aim, first, the isothermal and nonisothermal fluid flow around obstacle is simulated by LBM; then, transport of the particles are evaluated while the equation of motion is employed. In this context, Lagrangian method is applied for simulating solid particles where the effect of particles on the flow is ignored. According to the obtained results, simulating the isothermal flow around the circular cylinder shows that with increasing Reynolds number decreased frequency of the flow. Also, investigating on nonisothermal flow around obstacle shows that with increasing blockage ratio, Nusselt number was increased. The results shows that with increasing Reynolds number deposition of small particles are increased, but deposition of large particles at low Reynolds number are better. Thermophoresis force is affected on particle smaller than 1μm and capture efficiency of particles was increased. Our results are good agreement with previous theoretical predictions and experimental observation.

In this paper a rarefied binary gas mixture flow of Argon and Helium inside a rotating cylinder with constant angular velocity and constant wall temperature is investigated using the direct simulation of Monte - Carlo (DSMC) method. The number of different molecules were used, to study the dependence of the solution to number molecule model. The results show that increasing the number of molecules model is more accurate, On the other hand increase the simulation time. Also for modeling the intermolecular collision, the variable soft sphere (VSS) and the variable hard sphere (VHS) were investigated in present work and the results were compared with each other. The results show that the variable soft sphere model (VSS) compared to the variable hard sphere model (VHS) for the temperature of the mixture near the cylinder walls more carefully. The analytical solution is also presented and compared with the current simulation results.

In this paper, high velocity compressible flows in gas networks have been simulated in order to calculate the amount of released gas. The exact determination of the released gas in the networks needs to calculate the minor losses along with the existing compressibility effects. First, a new definition based on the physics of high velocity compressible flows has been proposed to calculate the minor losses. Then, different types of the three-dimensional flow through T-type junctions are simulated using Fluent Package. Based on the obtained results, an appropriate relation is proposed to calculate minor loss coefficients as function of junction inlet Mach number. The proposed relation has much simpler form than those proposed by the others and it can be more easily applied to develop the gas networks analysis softwares. Finally, the amount of released gas through a branched network is calculated using a new algorithm based on our proposed minor loss equation. The very good agreement between the numerical code and Fluent results shows the accuracy and efficiency of the proposed minor loss relation and applied numerical algorithm.

Unlike a single building, determining the performance of two or more buildings next to each other under the effect of wind is very complex due to the inference effect. One of the latest efforts in investigating the interference effect is determining the interference factors and pressure coefficients of a group of tall buildings using Computational Fluid Dynamics (CFD). In this paper, the pressure coefficients and interference factors of a group of tall buildings with different heights placed at different distances due to various wind directions have been studied. The effect of buildings distances and direction of wind load on two groups of four tall buildings on the average pressure coefficients of the buildings in the group has been investigated. For the selected distances, in over 50% of cases by increasing the distance between the tall buildings the average pressure coefficients on buildings increase. In order to show the wind interference effects, the interference factors, i.e. the ratio of pressure coefficients on a building in a group of tall buildings to those of a single building, are calculated and compared. Interference factors show that the maximum and minimum pressure coefficients on a building in a group of tall buildings are up to three times of those of a single building.

In this paper, a combined cycle power plant (CCPP) was optimized based on three criteria. The first criterion which is for heat recovery steam generator (HRSG) considers the increase of the whole cycle exergy efficiency as an objective function. The exergy analysis has revealed that drum is a high temperature sensitive part of HRSG with high exergy destruction. Therefore, the second optimization criterion was based on the drum saturated temperature which caused the exergy destruction reduction of this component. The third optimization criterion was based on the cost reduction, the increase of the whole cycle exergy efficiency and the decrease of CO2 emission. To validate the results, the Damavand CCPP data has been used and the results have shown that the whole cycle optimization criterion yields better results in comparison with the other optimization criteria and it causes cost reduction (capital, environmental impacts and exergy destruction costs) as well as the increase of exergy efficiency. The value of CCPP decision parameters is highly dependent on the ambient temperature. Therefore, it is not possible to apply the same value for CCPP at various temperatures. The Genetic algorithm improved the cycle optimized parameters with respect two objectives of CO2 emission and power plant costs reduction.

The natural convection heat transfer in a square enclosure depends on the geometry of the enclosure, the amount and the type of heating and cooling on the hot and cold walls, the fluid proprieties and the inclination angle of the enclosure. In this paper, the natural convection in a square enclosure with two hot and cold vertical walls and two adiabatic horizontal walls is investigated. This vertical enclosure by rotating 90o changes into a horizontal enclosure. The aim of this study is to investigate the time dependent behavior of the flow and the heat transfer through it affected by the inclination angle. The lattice Boltzmann method for numerical simulation of the fluid flow and the heat transfer is used. The problem is solved by five values of rotation times for Rayleigh number 105. Streamlines, temperature distribution and the amount of heat transfer in every time obtained. The results show that by the fast rotation major changes in the temperature distribution and the heat transfer occurs after the stopping of rotation. But in the slow rotation the amount of heat transfer is very close to the steady state in any time at the same situation.