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In the presented work, the lagrangian grid-free vortex element method is used for flow simulation in forced shear layer. Growth rate of instability and eddy formation is well simulated. Also, effect of eddy formation, pairing and the interaction among them are studied. In an unforced shear layer the growth rate is linear depending on the biggest instable frequency but in the forced shear layer other frequencies are taken part. The most important frequency is the forced frequency among other frequencies and the instability growth rate is no longer linear. The results show that the instability would accelerate due to increase of velocity ratio. The wavelength effect is well studied on the instability and is in good agreement with the experimental data.

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The Zab river basin is locate in south of west Azarbaijan and north of Kurdistan province with total area equals 3527 km 2 .This river is flowing in the tectonic vally and slope instabilities occur frequently in center of this basin.Annually ,occurs of landslide and other type of mass movements cause of damages for roads , habitats and agriculture lands. This paper has attempted to evaluate and recognize these natural hazards and finally prepare hazard maps by applying Anbalagan method.The method of research in this paper are recognition and verify of landslide in field and determination of cause of their occurrence.The second stage is preparing factor maps and finally drawing slope instability zonation map.

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The semi arid mountain ,due to climatical ,hydrological and topographical properties ,represents vulnerable base for the occurrence of landslides.The deep stream bed and streamflow in long beds ,main triggering factors materials wall of valleys to slides events.In study region ,as a semi arid region ,was occurred many landslides ,because these landslides events changing the hydrological properties of valleys and amount of load sediment.In this study ,by using of geological and topographical maps ,sampling of soil and field work ,analysied the valleys landslides.The result of this study show that, all of failures and landslides are related to rate of drainge density ,specials depended to drainage density of deep beds , related to rate the changing of ratio of Ds/Dd ,also related to changing R .When increases the rate of slope angle ,then increases probably of the landslides occurrence on unconsolidated materials .

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Abstract: Applications of two-phase flow in nuclear power plants, transmission lines, oil and gas have been considered in recent decades. Different models have been introduced that can contribute to the current two-phase flow approach to numerical analysis. Two-fluid model is the most widely used and most accurate model for predicting two-phase flow in channels during different regimes of unstable flow. This study addressed the PFM model Hyperbolicity. Hyperbolicity of this model is the most important for the well-posed condition; otherwise the model is in ill-posed condition and the results are unstable numerically. Hydrodynamic instability of two-phase gas-liquid by using the PFM model is calculated and discussed.

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In this paper, stress gradient theory is used to model the static pull-in instability and size effect of electrostatic nanocantilevers in the presence of electrostatic and dispersion (Casimir/van der Waals) forces. The Differential transformation method (DTM) is employed to solve the nonlinear constitutive equation of the nanostructure as well as numerical methods. The basic engineering design parameters such as critical tip deflection and pull-in voltage of the nanostructure are computed. It is found that in the presence of dispersion forces, both pull-in voltage and deflection of the nanobeam increase with increasing the size effect. Compared to the pull-in voltage, the pullin deflection of the beam is less sensitive to the size effect at sub-micrometer scales. On the other hand, the size effect can increase the pull-in parameters of the nano-actuators only in sub-micrometer scales. The results indicate that the proposed analytical solutions are reliable for simulating nanostructures at sub-micrometer ranges.

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Banks as important financial institutions have drawn attention of economic researchers because their impacts on various economic sectors. Despite positive role which banks play, they cause economic instability due to providing services particularly intermediary ones, so that most of economic researchers consider banks as main cause of current financial crisis. With regard to role of banks in economic instability, this research pays attention to financial intermediary role of commercial banks as their main product on economic instability. In particular, this research tests the impact of commercial bank products on economic instability as the core hypothesis. The results show that commercial banks as financial intermediaries had negative impact on Iran’s Economic stability during 1981-2007.

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The main object of current study is investigation of instability threshold of flow in a gradual expansion from symmetric to asymmetric situations. The expansion ratio is 1:3 and expansion angles are 30, 45, 60 and 90 degree. Discretization of governing equations is performed using finite volume method based on PISO algorithm on a staggered mesh. The CFD code is validated based on the results of sudden expansion reported in previous works. Here, the effects of expansion angle and Reynolds number on flow instability in transition from symmetric situation to two and three asymmetric vortices are investigated and the first and second critical Reynolds numbers are obtained. The bifurcation diagrams of vortices and velocity profile in centerline are plotted for each case and the effects of instability on flow field are discussed based on them. Unlike the previous studies which have been focused on the planar flow in sudden expansions, the flow instability in gradual expansions with different expansion angles is investigated which is the main innovation of current study.

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In this paper, analysis the performance of PI, PI-like fuzzy, and parallel fuzzy P+ fuzzy I controllers for He-Ne lasers frequency stabilization by combination of frequency locking and power balanced methods is presented. He-Ne lasers can be attributed to an unstable system due to the influence of environmental factors on its' frequency. Therefore, the stabilization of He-Ne laser is so important in sensitive applications such as laser interferometers and nanometrology systems. The simulation results of controllers by powerful software MATLAB/SIMULINK-GUI show that parallel fuzzy P+ fuzzy I controller has better stabilization performance and integrated absolute error (IAE) than others. Also, frequency fluctuations of He-Ne laser is about 2×10^-11 by parallel fuzzy P+ fuzzy I controller.    

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One of the natural disasters causing many damages in dry and desert areas of the world and especially in the Yazd region of Iran, is strong winds and storms. This region (Yazd), due to it is geographical situation and climatic conditions, is always subject to strong winds and dust storms. Therefore, understanding the storms and strong winds regime, in order to reduce the destructive effects of this phenomenon, especially in stabilizing the running sands, is necessary. This study was accomplished using the data of horizontal vision, speed and direction of wind, relative humidity, temperature, pressure, cloud and dust event in Yazd and Kerman synoptic stations, as well as the atmosphere data of the above stations, and the synoptic maps during statistical period between 1983-2003. The results of the study showed that more than 77 percent of strong winds in the area blow from 2500 to 3300, and its speed is between 15-29 m/sec during the fluctuation. These strong winds blow in the afternoons. The blow of dominant winds is from North -‌West and West. More than 5 percent of storms and strong winds in this region occur in Apirl and May.‌ In the begining of the hot period of the year and changing of the season, the atmospheric instability and the dust storms increase in the region. The main reason of this phenomena is the intensive winds that occur by the fast changes of the temperature and pressure in this time. Passing of a low pressure system with cold and dry front from the West-North Western along with the presence of trough in the 500 and 850 hpa levels, and also the local instabilities are the most important reasons of these strong winds in this region.

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Forming limit diagram (FLD) shows the formability of metal sheets under different loading conditions before that necking is taken place. In this paper, the application of plastic instability criterion for prediction of necking and also FLD has been investigated. Using Balart’s anisotropic yield function and plastic instability criterion in different strain ratios, limit strains have been calculated, and then the limit strains have been converted to limit stresses. To verify the analytical results, a free bulge setup with the ability of applying the axial feeding has been fabricated. Tubes have been undergone different loading paths and different plane strain conditions have been induced to obtain FLD. FLDs which have been obtained using plastic instability criterion have been compared with experimental results. The results show that swift instability criterion for tubes have the best prediction of FLD in tube hydroforming process.

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Aims: Tehran, as the capital of Iran, is facing many urban problems and challenges to survive as a city. Identifying sustainability indicators have been evaluated by many articles, but less attention has been paid to the instability indicators that fuel the challenges of instability in urban planning; In this regard, the aim of the current research is to identify the challenges of instability in the urban planning of Tehran metropolis.

Methods: This research evaluated the instability indicators in urban planning from a real point of view by rereading the indicators related to urban sustainability.

Findings: The findings show that the main unstable indicators of urban planning are summarized in three economic, social, and environmental indicators. In the economic index, including uncertainty in economic policies, housing prices, non-standard housing, income, employment, unemployment, cost of living, and female heads of households; In the environmental index, including climate changes, heat island, flood, transportation, energy security, noise and air pollution, urban traffic, water quality and quantity, waste disposal quality, urban green infrastructure, environment as a luxury issue and changing approach to the environment; And the social index includes social interaction, access to facilities and services, social injuries, sense of identity and belonging, neighborhood relations, social segregation, people's participation in city affairs and welfare security.

Conclusion: Reframing the challenges of instability in the urban planning of the Tehran metropolis from a practical point of view shows the necessity of attention to urban planning with a special focus on the relationship between citizens and planners.

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In this study, static/dynamic instability and nonlinear vibrations of FG plates resting on elastic foundation under parametric forcing excitation, are investigated. Based on CPT, applying the von-Karman nonlinear strain–displacement relation and the Hamilton’s principle, the governing nonlinear coupled partial differential equations are derived. By considering six vibration modes, the Galerkin’s procedure is used to reduce the equations of motion to nonlinear Mathieu equations. In the absence of elastic foundation, the validity of the formulation for analyzing the static buckling, dynamic instability and nonlinear deflection is accomplished by comparing the results with those of the literature. Then in the presence of the foundation and by deriving the regions of dynamic instability, it is shown that as the parameters of the foundation increases, the natural frequency and the critical buckling load increase and the dynamic instability occurs at higher excitation frequencies. The frequency response equations in the steady-state condition are derived by applying the multiple scales method, and the parametric resonance is analyzed. Then the conditions of existence and stability of nontrivial solutions are discussed. Moreover, the effects of the system parameters, including excitation frequency, amplitude of excitation, foundation parameters and damping, on the nonlinear dynamics of the FG plate are investigated. Also it is shown that the presence of the foundation has a considerable influence on the resonance characteristic curves.

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In this paper, effect of advanced yield criteria including BBC2003, Yld2004 and BBC2008 are studied using Swift hardening law on determination of limit strains based on Marciniak–Kuczynski (M-K), Swift’s diffuse necking and Hill’s localized necking models. These models are used to assess the formability prediction of orthotropic AA2090-T3 and AA6111-T4 sheets. The Portevin-Le Chatelier effect on formability of AA5182-O sheet is also studied. The sensitivity of Marciniak–Kuczynski model to the initial imperfection factor is investigated and the effect of this parameter on prediction of forming limit diagram has been shown. Study of the effect of yield criteria exponent on formability shows the higher exponents which are recommended for FCC metals have higher forming limits in comparison with lower exponents adopted to BCC metals. The effects of strain hardening coefficient and anisotropy are studied too. The results show that the increase of strain hardening exponent improves the formability, while the PLC effect and increase of anisotropy coefficient reduce it. Experimental results show better conformity with M-K model at the right side of FLD and with Hill’s localized necking model at its left side.

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In previous experiments, practice and in some condition of loading, geometry and support configurations, a type of instability has been observed in which tension flange moves laterally which it has been addressed in design codes as “web sidesway buckling”. Web sidesway buckling is a case of instability that it was observed in beam with restrained top flange and no constraint for bottom flange. Primary studies and experiments show that web sidesway buckling is due to both local instability in the web just under loaded zone and global instability of tension flange along length of the girder. In the present paper, a concise study has been carried out on behavior and the mechanism of this instability occurrence in web sidesway and how to evaluate loading capacity of the girders in the light of experiments, then a simple model which is a modification on the existing model has been proposed. Experimental work has been conducted to investigate the effect of tension flange on the load capacity of beams. The specimen's dimensions were adjusted in order to show web sidesway buckling. In addition, the supports configuration was made compatible with this instability. The objectives of the experiment were to obtain; mechanism of instability initiation, deformation pattern, effect of tension flange width and nonlinear deformations underneath the loading point. A closer view in load-displacement behavior of test specimens shows at first, loading accompanies lateral displacement due to imperfection and then rate of displacement reduces. After reaching to maximum load, girder has still capacity for load carrying but with excessive lateral displacement in tension flange. The results of experiments also show that web sidesway buckling generally accompanied by local buckling or crippling of the web under loaded zone. Deformation initiates with lateral movement of tension flange. Then local buckling and yielding occur, and finally, web sidesway buckling develops along the beam length. From this time onwards, load capacity is approximately constant. Furthermore, it can be seen that the rate of lateral displacement is directly dependent on width of tension flange. On the other hand, the results from design equations of design codes for estimating load capacity of girders against web sidesway buckling are too conservative in comparison to the proposed model. The critical load is affected by tension flange clearly, and occurrence of this type of instability is credible in other sections such as T- shape beam but in a lower critical load with respect to the I-shape sections. Also the results from the new model are in good agreement with that of experimental data. Finally, • It seems that only the existence of an area in the web affected by a tension stress field do not cause web sidesway buckling and transformation of tension stress field into compression field determents on initiation of instability.

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In recent decade, modeling the instability of nanostructures has attracted many attentions in nanomechanics. Nanomechanical switches are fundamental building blocks for the design of NEMS applications, such as nanotweezers and nanoscale actuators. One common type of NEMS including nano-bridge in micro mirrors is used. At nano-scales, the decreasing gap between the two electrodes makes surface traction due to molecular interaction such as van der Waals that must be taken into account in the analysis of NEMS. In this study, strain gradient theory has been used to investigate the size dependent pull-in instability of beam-type (NEMS)where is an inherent instability in them. The von-Karman nonlinear strain has been applied to derive the constitutive equation of the system. Effect of intermolecular force have been included in the nonlinear governing equations of the systems. Homotopy perturbation method (HPM) has been employed to solve the nonlinear equations. Effect of intermolecular attraction and the size dependency and the importance of coupling between them on the instability performance i.e. critical deflection and instability voltage have been discussed. According the findings of this research, one can conclude that intermolecular forces decrease pull-in voltage and size effect parameter in nano scale leads to increase of pull-in parameters. Also HPM method can be applied as efficient method to analyze beam type nano structures.

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In present work, models that predict contact angle of a droplet with a solid surface, are considered and compared with each other. Two phases were assumed to be Newtonian, incompressible and immiscible fluids. OpenFOAM software is applied to simulate the two phases interface by using Color function VOF (CF-VOF) method. Different models for contact angle of a droplet as Tanner and Yokoi models are implemented in the OpenFOAM. In addition, the dynamics and statics contact angle models were used to compare with recent models in order to choose the best one. The outcome of study shows, even though the static contact angle model is simple to understand, however, it could be the best model to predict the droplet behavior in a wide range of different conditions. The fluid viscosity effect was also considered in different models of the present study. It concluded that the fluid viscosity affects the type of pattern of droplet impact and as viscosity of fluid increases; more energy is needed to uplift the droplet again from the surface. Kelvin-Helmholtz instability (K-H) was also simulated and explained in details which initiates on the interface of two fluids due to velocity differences of droplet and the surrounded air.

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Dynamic modeling of beams under aerodynamic loading is extremely important in many engineering applications. So the objective of this paper is to present a new approach to model and simulate the time domain response of tapered cantilever beams with airfoil cross section to wind excitation. The extended Hamilton’s principles along with the Euler-Bernoulli assumptions are utilized to derive the Partial Differential Equation (PDE) governing the deflection of the beam. A new finite difference based algorithm is proposed for finding the mode-shapes as well as the natural frequencies of the beam. These mode-shapes are then used in a Galerkin projection procedure to convert the PDE governing the system’s behavior into strongly coupled nonlinear Ordinary Differential Equations (ODEs). The aerodynamic loadings are modeled using the open source code of XFOIL. The blade of an under developed 100KW wind turbine is considered as a case study. The results reveal that even a single mode approximation is accurate enough in predicting the beam’s dynamic exposed to wind excitation. It was also observed that the instability speed of beams with higher modal damping is considerably higher than those with lower modal damping. The knowledge resulting from this effort is expected to enable the analysis, optimization, and synthesis of tapered cantilever beams for improved dynamic performance.