Scientia Iranica
Volume:25 Issue: 3, May - June 2018

Cold expansion is an effective technique of inducing favorable compressive residual stresses around fastener holes essentially vital for improving fatigue performances of aircrafts. The benefit is caused by the magnitude and distribution of the compressive stress field. Stress gradients are entirely contingent on type of the cold expansion, local geometry of the hole and characteristics of the metallic structure. During cyclic loading however, initial residual stresses do not remain stable. In the present work, specimens with 4% split-sleeve cold expanded and reamed holes were cycled at the fatigue limit for short periods. Recent method of Combined Step Drilling-Fourier Series Solution ‘ATÖzdemir Method’ was employed to appraise the continual redistributions of residual hoop stresses at the side of the hole subjected to cyclic load. Some results were compared with that of diffraction methods and Artificial Neural Network (ANN) modeling where, close similarities in stress distributions were confirmed. It is clear that during early fatigue, material nearby the hole is dynamically hardening; in response compressive residual stresses along the bore are gradually increasing till the onset of cracking. Short fatigue cracks mostly initiate from the mandrel entrance side, where subsequently residual hoop stresses begin to relax considerably.

The effects of zinc dialkyl dithiophosphate (ZDDP) concentration on tribological behavior of refined, bleached and deodorized (RBD) palm kernel were analyzed using a pin-on-disk tribotester. RBD palm kernel was selected as base lubricant due to its availability, low price, renewability and environmental friendly. Commercial mineral oil (SAE 40) was used for comparison. The tests were performed at 2.5 m/s of sliding speed under 9.8 N applied load for 60 minutes and temperature at 25 ( 2 ). Worn surface of the pin was measured by optical microscope to understand the wear mechanism involved. Experimental results showed that the addition of 5wt% of ZDDP has reduced the coefficient of friction to 21% compared with SAE 40. However, the anti-wear property was still lesser, approximately at 23%. Besides, the tribological behavior of RBD palm kernel was improved with ZDDP additive concentration above 1wt% where the best performance was at 5wt%. At this concentration level, ZDDP additive was able to protect the surface against wear and oxidation, and at the same time reduces coefficient of friction, wear rate and surface roughness values.

Rings are widely used in mechanical equipment and their fitness may damage under severe vibration. In these structures functionally graded rings can be used to optimize the resistance, energy consumption and better fitness. Due to their complexity, finite element analysis may be implemented using special elements. Enhancement of accuracy and minimization of time consumption play an important role in the analysis of these rings.
In this study a new cylindrical superelement for the FGM rings is designed and implemented for the vibration analysis of the rings. The power-law distribution is used for modeling of the FGM rings in the thickness direction. Natural frequencies and mode shapes are obtained and results are compared with simple cases obtained from analytical solution and conventional elements. Findings indicate with a few newly designed superelements, comparable results for simple cases in the reported literature can be achieved.

In the present work, a numerical method to solve the problem of free convec- tion in enclosures with complex geometries is developed. This development is executed by the combination of projection and Galerkin nite element methods. Nine-node (quadratic) quadrilateral elements are used to generate the grid for the eld of the problem. The results show that the convergence of this method is acceptable, while there is no necessity to use upwind schemes. Increasing the numbers of nodes and decreasing the time increment yield a more accurate solution. The advantages of this numerical method are the ability to model any complex geometry and no necessity to use upwind schemes.

In this paper, the robot-assisted interventions for a pair of fraternal twins with autism, one of whom is high-functioning and the other low-functioning is presented. Since many genetic and environmental factors were the same for both participants, the effect of individual and group robotic games on these two children with high- and low-functioning autism were compared. The results indicated improvement in joint attention skills for both subjects. It was further observed that robot group games had the potential to improve social skills in the subject with high-functioning autism; and lower the amount of stereotyped and detrimental behaviors in the participant with low-functioning autism. Both participant’s communication with each other improved, and their mother claimed she observed her children playing a meaningful game together at home for the first time since their birth.

This article describes the heat and mass transfer as well as the entropy generation in an unsteady double-diffusive natural convection in the presence of an external magnetic eld, Soret and Dufour e ects. The analysis uses a two- dimensional skewed enclosure, where the inclined walls make a skew angle  with x-axis. The governing equations in the physical domain are transformed into an orthogonal computational domain by co-ordinate transformations, and then are solved using a nite volume method based on SIMPLE algorithm. The computations are carried out in the six skewed enclosures with skew angles  = 15, 30, 45, 60, 75, and 90, for a wide range of the Hartman number, Lewis number, buoyancy ratio, and Dufour coecient, while Soret coecient is kept constant at 0.25 in most of the study. Results show that the fluid flow, heat and mass transfer, as well as the entropy generation are sensible to some extent to the skew angle variation. Meanwhile, the Lewis number and buoyancy ratio have an aiding and opposing action, respectively, on suppression eff ect of Lorentz force against convective heat and mass transfer. It is also shown that the average entropy generation is an increasing function of Lewis number, buoyancy ratio, and Dufour coecient, while its a decreasing function of Hartman number.

This paper developed a new mathematical model to investigate the heat transfer as well as wick thickness of a heat pipe. The model was set up by conservative equations of continuity, momentum and energy in the thermal boundary layer. Using the similarity variable, the governing equations have been changed to a set of ordinary differential equations and they were solved numerically by the forth order Runge-Kutta method. The flow variables such as velocity components, wick thickness and Nusselt number were obtained. The results show that the Nusselt number is proportional to the root square root of the Darcy-modified Rayleigh number and that the distance from the edge of the condenser surface. Furthermore, the thickness of the wick material depends on the Jakob number and proportional to the heat transfer between the wall and the liquid film.

An attempt has been made to explore the effect of thermal radiation on electrically conducting, viscous and incompressible magneto-nanofluids free convective flow in the presence of an inclined magnetic field. Fluid flow is persuaded due to an accelerated movement of an infinite vertical ramped temperature plate. The water based nanofluids with the nanoparticles of alumina (Al2O3), copper (Cu) and titanium oxide (TiO2) have been accounted. In order to model the problem mathematically, the model of nanoparticle volume fraction has been employed. The exact solution of the mathematical model in closed form has been obtained analytically by making use of Laplace transform technique. The amalgamated form expressions are obtained for nanofluid velocity, nanofluid temperature, skin friction and Nusselt number, in both ramped and isothermal conditions. The consequence of various physical parameters affecting the nanofluid velocity and nanofluid temperature have examined by means of various graphs whereas, the numerical values of skin friction and Nusselt number have been reported by different tables. The numerical results have been compared for both ramped and isothermal conditions. It has been noticed that both the nanofluid velocity and temperature are smaller in magnitude in the case of ramped temperature plate than that of isothermal plate.

The raw milk is an important basic material for many food products. Fresh milk must be cooled immediately after milking to keep high quality and process ability. In this work the overall heat transfer coefficients of the milk cooling vessels used for this purpose have been studied experimentally. This study is aimed to determine the overall heat transfer coefficients of cooling vessels which have different types and capacities without the freezing and churning to avoid separation of milk's fat. Vessels are used ranged as follows: 300-500-1000-1500-1850-2000 litres for verticals and 2000-2500-3000-4000-5000-6000 litres for horizontals. It is found that the theoretical calculations are in satisfactory agreement with the experimental data. As a result of investigation, the overall heat transfer coefficient can be written as related with power intensity for horizontal vessels. It is a certain constant value for vertical ones.

In friction stir spot welding (FSSW) process the welding parameters (the tool rotational speed, the tool plunge depth and the stirring time) effect the nugget formation in high density polyethylene (HDPE) sheets. The size and the microstructure of the nugget determine the resistance of the joint to outer forces. The optimization of these parameters are vital to obtain high quality welds. Feed forward back propagation artificial neural network models are developed for the optimization of the FSSW parameters for HDPE sheets. Input variables of these models are the tool rotation speed (rpm), the plunge depth (mm) and the stirring time (s) affect to lap-shear fracture load (N) output. Prediction performance of 6 models in different specifications are compared. These models differ in terms of the training dataset used (80%-100%) and the number of neurons (5-10-20) in hidden layer. Best prediction performances are obtained with using 20 neurons in hidden layer in both training dataset. There is a good agreement between developed models’ predictions and the experimental data.

In today's globalizing world, which is also called the information age, information and information technologies are becoming increasingly important for businesses, and they have become an indispensable part of economic and social life. Nowadays, it is impossible to think about a business that is far from information technology, and also it is important not only to find information but also to find the fastest and highly reliable information. The important thing is to use information technologies effectively and efficiently. Therefore, it is expected that the effective usage of information technology will have significant positive effects on business performance. The aim of this study is to examine and analyze the impact of the intensive usage of information technologies on business performance in the supply chain process. A sequential, multi-method approach integrating both structural equation modeling (SEM) and neural network analysis was employed in this research. The information technology usage performance network was formed by using the SEM model, and the ANN model was used to predict a relationship between information technology usage levels and business performance by using these network outputs. Furthermore, the validity and reliability tests of the relevant model data were performed.

Nonlinear filtering techniques are used to fuse the global positioning system (GPS) and the inertial navigation system (INS) together to provide a robust and reliable navigation system with a performance superior to that of either INS and GPS alone. Prominent nonlinear estimators in this field are the Kalman Filters (KF) and Particle Filters (PF). The main objective of this research is the comparative study of the well-established filtering methods of EKF, UKF and the PF based on EKF and UKF in INS-GPS integrated navigation system. Different features of INS-GPS integrated navigation methods in the state estimation, bias estimation and bias/scale factor estimation are investigated by using these four filtering algorithms. Both ground-vehicle experimental test and flight simulation test have been utilized to evaluate the filters performance

A new approach for numerical solution of sinusoidal steady state Maxwell's equations is developed. This approach is based on Yee's method, and can be applied on unstructured grids. A case problem with available analytical solution is solved by the method and the results show a good agreement with analytical solution. This method can be improved to be applicable for general unsteady problems.

Bubble coalescence is an important stage of foaming process. A goal of foaming is to produce numerous, uniform-size bubbles. Therefore suppression of bubble coalescence is desirable during foaming process. For stationary bubbles, if their distance be less than a critical gap they will coalesce. Actually in this case, attractive forces attract the outer surfaces to touch each other and form a growing gas bridge which merge the bubbles finally. For bigger distance, the attractive forces cannot make a bridge and coalescence will not happen. In this study the dynamics of bubble coalescence is modeled using a diffuse-interface LBM. Then critical gap of bubble coalescence is defined as the maximum distance between the stationary bubbles where the coalescence will happen. Sensibility of critical gap is obtained with respect to critical properties of material, bubble size, viscosity of gas and liquid, density ratio, surface tension, temperature and interface thickness. The results show that, interface thickness is the only factor that controls the critical gap. In the other word, in the case of stationary bubbles, by a precise estimation of interface thickness, the coalescence can be predicted. Critical gap is a useful parameter in foaming where the maximum number of bubbles is desirable.

In principal, a proper analysis of the dynamic response of a structure can provide general indicators of its operational conditions. When the dynamic response changes due to a variation of the physical properties of a structure, then one may conclude that some kind of damages has occurred. This paper presents investigation of the robustness and comparison of four simple methodologies to both identify and quantify the damages in structures, based on the use of Frequency Response Functions (FRF) signals, Principal Component Analysis technique (PCA) and Transmissibility. A steel beam with constant rectangular cross-section is used to compare the proposed approaches. At first, nine damaged scenarios are created and for each of them numerical examples are discussed; a database of FRFs is measured using modal testing. Then, PCA theory is applied to the FRF matrix and global damage detection and quantification indices are defined by using the first 3 Principal Components; Hotelling's T-squared distribution is also applied and by using transmissibility two other indicators, Transmissibility Damage Indicator and Weighted Damage Indicator, are computed for the assessment of damage. The reported examples show that all proposed methods are able to detect and quantify damages at the initial stage.