In industry, flow instabilities are important in journal bearings at high rotational speeds. For inviscid flows between two rotating coaxial cylinders, an instability criterion was introduced by Taylor-Couette. However, viscosity may play as stabilizer or destabilize in such flows. In this work, stability theories were reviewed for two coaxial rotating cylinders. Then, flow instability patterns, velocity, pressure, and vorticity fields were simulated using Fluent. In this method, the fluid flow was assumed laminar and by gradually increasing the relative rotational speeds between two coaxial cylinders, the critical Taylor number was calculated and compared with Taylor analytical solution. By increasing Taylor number, it is observed that vortices become stronger and instabilities appear in the form of cellular square patterns. The characteristics of these cellular square patterns were examined.

Reviewing economic literature indicates that economists were emphasizing on economic factors as causes of economic instabilities until 1970s. But since then, political economists emerged who attributed economic instability to factors other than economic ones.Offering theories such as political business cycle, they tried to prove that political factors lead to economic instabilities. They offered opportunistic political business cycles and partisan political business cycles that respectively, attributed the economic instabilities to election, and to replacement of political parties. To elaborate this literature, the present article first focuses on these two kinds of political business cycles and tries to explain through which mechanisms political factors cause instabilities in economic sphere. It considers political business cycles as a sign of "bringing the politics back in economics" an idea which is an obsolete thought, dating back to the past two centuries. Finally, the article indicates that "bringing the politics back in economics" can question those economists who insist in separation of economics from politics, and emphasizes interdisciplinary studies developed in the recent decades.

In this article, aerodynamic stability of J79 compressor was analytically investigated. For this purpose, conservation equations of mass, momentum and energy were applied with several assumptions .By accomplishment of equations related matrix as well as investigating its eigenvalues, the effect of parameters on aerodynamic stability were assessed. Due to the complexity of the equations used in the matrix, the equations codes in MATLAB and the desired matrix have been provided. By applying the geometry and flow characteristics of J79 engine on the matrix and extract the relevant diagrams, the effect of some parameters on the aerodynamic stability of the engine is evaluated. According to the results, by reducing the mass flow rate through the compressor, the system tends to be working in an unstable condition. By increasing the ratio of upstream to downstream compressor duct length, the aerodynamic stability of the system will increase. Increasing the plenum volume will yield to a considerable reduction in stability. But by increasing the volume of compressor, the system stability will increase. Enhancing the combustion chamber temperature and also increasing the compressor flow area have favorable effects on system stability. The results of this study compared with previous outcome. Significant achievements were observed between both aforementioned results Relatively good alignment between the results has also been observed.

This work deals with effects of asymmetric stiffness on the dynamic behaviour of the rotor system. The analysis is presented through an extended Lagrangian Hamiltonian mechanics on the asymmetric rotor system, where symmetries are broken in terms of the rotor stiffness. The complete dynamics of asymmetries of rotor system is investigated with a case study. In this work, a mathematical model is developed considering symmetry breaking of a finite rotor due to stiffness. The natural frequency and amplitude of the rotor are obtained analytically through extended Lagrangian formulation. The asymmetries in rotor are also modeled through bond graph modeling technique for the computational analysis. The simulation result shows a considerable agreement with the analytical results. The limiting dynamics of rotor is shown and analyzed through simulation.

A correlation-based transition model has been developed by Langtry and Menter for modern computational fluid dynamics codes, which is widely used for transition prediction in the field of turbomachinery and aircraft. Langtry’s transition model could simulate bypass, laminar separation and streamwsie Tollmien–Schlichting wave transition. Even so, this model has no ability to predict the transition due to crossflow instabilities in three dimensional boundary layer. In this paper, a new correlation-based transport equation for the transition due to crossflow instabilities has been established based on the experiment data and self-similar equations. The new transport equation is introduced to describe the crosswise displacement thickness Reynolds number growth in boundary layer. This new equation is added to Langtry’s intermittency factor equation to improve the ability of predicting transition. Finally, coupling of these transport equations and Shear Stress Transport (SST) turbulence model completes the new improved transition turbulence model.Comparisons of predictions using the new model with wind tunnel experiments of NLF (2) -0415 infinite swept wing and 6: 1 inclined prolate spheroid validate the predictive qualities of the new correlation based transport equation.

Iran is a country whose political set up has long historical background and historically, the initial efforts to divide the national territory dates back to Achaemenid era. Despite the fact that adherence to administrative divisions and political stability from the aspects of continuity of sovereign government and nation-state at different level have always been considered as the final goal in the planning system, here question arises that why the pattern of administrative divisions especially in the contemporary era have failed in accessing to this goal? Hypothetically, it has been assumed that with reference to the failure of this pattern, one of the factors which has not been fully taken into account is the formation modern internal political divisions as well as none execution the fundamental legal principles that was started under the aegis of central government with the coup-d’etat of 1920 by Reza Khan. Actually, fostering his authoritarian way that was contrary to the constitutionalism had fulfilled the basis for political development in the previous era, lead to empower national and cultural movements and collapse of the first and second autocracy, instance of which can be seen in the Kurdistan and Azerbaijan crises of 1920s. This research aims to study the relationships between the pattern of administrative divisions and political instabilities between two constitutional revolutions and Islamic revolution, using literature survey and content analysis methods.

A realistic problem of an explosion-generated relativistic plasma is talked about with respect to the instabilities developed in such systems. For this, the dispersion equation is derived analytically and solved numerically for typical values of physical quantities. Our calculations reveal that two types of instabilities occur in the said plasma if the dust particles and relativistic effects of ions and electrons are considered. Both types of the instabilities are high-frequency instabilities, which carry growth rates of different magnitudes. In view of the magnitudes, the instability having higher/lower growth rate is called as high-frequency higher/lower growth rate instability. The relativistic effects of ions support the growth of these instabilities, whereas those of electrons suppress the growth of the instabilities. The waves propagating with larger phase velocity are found to grow at higher rates. There exists a critical value of the drift velocity of dust particles, above which another instability starts growing but with significantly lower growth rate.

The transonic turbulent two-dimensional airflow over a symmetric flat-sided double wedge is studied numerically. Solutions of the Reynolds-averaged Navier-Stokes equations are obtained with ANSYS-18. 2 CFX finite-volume solver of second order accuracy on a fine mesh. The solutions demonstrate an extreme sensitivity of the flow field and lift coefficient to variation of the angle of attack α or free-stream Mach number M∞. Non-unique flow regimes and hysteresis in certain bands of α and M∞ are identified. Interaction of shock waves and local supersonic regions is discussed. The study confirms a concept of shock wave instability due to a coalescence/rupture of supersonic regions. In addition to the instability of shock wave locations, the numerical simulation shows a buffet onset, i. e., self-exciting oscillations due to instability of a boundary layer separation at the rear of wedge. Curious flow regimes with positive lift at negative angles α and, vice versa, with negative lift at positive angles α, are pointed out. A piecewise continuous dependence of the lift coefficient on two free-stream parameters, α and M∞, is discussed.

In the past 10-15 years the need for the development of specifically engineered .structures of polymers has resulted in the proliferation of products manufactured using various coating and multilayer extrusion technologies. Initializing these technologies, great advantages can be achieved by combining polymers of widely dissimilar solid-state structures and properties into unique multilayer blends and composites. However, the manufacture of these products is not without problems. Many processes are currently hindered due to the occurrence of irregular interfaces and undesirable layer distribution. These irregularities are due to the different rheological properties of the material used and can significantly deteriorate the mechanical or optical properties of the product. Hence, to establish guidelines for the overall processing of multilayer coating and extrusion processes, a better understanding of interfacial instability and deformation of multilayer flow of polymeric fluid is required. In this research, an experimental apparatus has been developed for observing interfacial stability and deformation of multilayer pressure driven channel flows. This apparatus has been used to directly observe the deformation of the interface and growth of interfacial waves in order to enhance our understanding of the effect of viscoelastic forces on stability and deformation of multilayer viscoelastic flow. Due to synergetic effects of combining individual polymers, multilayer films and conjugated fibers have been progressively gaining interest in the past decade. In the manufacture of multilayer products, it is a well-known fact that the multilayer extrusion process is more economical than the conventional laminating process. These products are generally superior to their single component counterparts because of the combining materials with different properties into a single structure. This effect is readily observed in the food packing industry where packing films contain multiple layers, each of which contribute a specific barrier, mechanical or optical property to the resultant film. Interfacial instabilities that manifest themselves in form of traveling waves at the interface are the limiting factor in production of multilayer plastic structures. These interfacial waves result in a significant deterioration of the final products properties of interest (i.e. barrier, mechanical, optical, etc.), hence to better design and control these processes a comprehensive understanding of the interfacial instability phenomenon is required. This would facilitate the selection of materials with suitable rheological properties, proper design of co extrusion die and proper selection of layer ratio within the structure.