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Abstract The complex three-dimensional flow characteristics in river bends necessitates the use of a three-dimensional numerical model. The numerical model used in this paper is called SSIIM (Sediment Simulation In Intakes with Multiblock option).The 3D model already has showed satisfactory results in related applications. In this study, SSIIM is applied to study the variation of channel bed under steady flows in a 180 mild curved open-channel with sandy bed. Bed roughness was calibrated to ks  4.5 d90  6.25d50 . Comparison of results showed good agreement between the computed and measured bed topography. The model simulated both the point bars near the inner bank and the scour holes near the outer bank. In addition, the positions of these phenomena are in fair agreement with the measured data. Numerical results show some dependencies on the grid size. In addition, deviations of model from the experimental data as well as uncertainties in numerical modeling are discussed.

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A passive scalar is a property that is affected by the flow field without affecting it. In this paper, first, the governing equations on the turbulent flows are solved and the property of a passive scalar in two dimensions is numerically studied. Having the values of the velocity components, the governing equation on transport of a passive scalar is solved. To compute the turbulent velocity field, the Large Eddy Simulation (LES) method using Smagorinsky subgrid scale is invoked. The flow in a cavity has been the basis to validate the accuracy of the generated computer code. To ensure the compatibility between the flow and the transport of passive scalar fields a similar LES approach is used. As a three-dimensional numerical solution for a turbulent flow fields needs a massive computational time and efforts, therefore a two-dimensional simulation used for a proper saving. Instead, to validate the numerical results, the range of the Reynolds number of the flow is kept within the range of the two-dimensional measurements. Comparison of the numerical results and the experimental measurements in two-dimension reveals the high accuracy of the results and compatibility between the flow and passive scalar fields. Ability of developed scheme to accurately handle transport of a passive scalar is promising to extend LES method into the transport of more species and hence to simulate reacting flows.

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In this article, a special duct is introduced in which, inlet water jet initiates to oscillate after a short time and it causes the velocity and pressure to oscillate regularly. Considering that there is a linear relationship between the inlet jet velocity and its oscillations frequency, the flow rate can be calculated by measuring the pressure frequency. In order to study the flow field inside the current geometry of fluidic oscillator and also to find the optimum location for sensor to detect the pressure oscillation, the unsteady turbulent Navier-Stokes equations are solved by ANSYS CFX software. Having studied the grid independency, capability of K-ε and SST turbulence models for numerical simulation of unsteady flow inside the fluidic oscillator is considered. Then, according to the peak to average ratio (PAR) criterion, the qualities of pressure signals are compared at some points, to distinguish an optimum pressure sensor position. Afterwards, a prototype of fluidic oscillator flow meter is manufactured for the first time in Iran. Using this prototype and inserting the pressure and Piezoelectric sensor at the optimum point, the numerical simulation results are validated by the experimental data. Comparison between the numerical and experimental results shows that the SST model is more suitable for this flow simulation. Finally, by performing experiments in different flows, acquiring and processing pressure signals, the flow meter characteristic diagram (inlet jet oscillations frequency- inlet jet velocity) are extracted.

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Abstract- Gaseous detonation in tubes produces moving pressure-thermal waves. A gaseous detonation consists of a shock wave and a reaction zone that are tightly coupled. The speed, pressure, and temperature of the products of detonation depend on the type and amount of the initial mixture. The maximum pressure of mechanical wave caused by detonation can be as high as 20-30 times the ambient pressure and temperature of gas in detonation may exceed 2000°C. The mechanical shock waves can cause oscillating strains in the tube wall, which can be several times higher than the equivalent static strains. On the other hand, the passage of the heat wave produces thermal stresses in the tube wall. In the current study the resulting mechanical and thermal stresses have been assessed using numerical simulations. In practice, the mechanical and thermal displacements have been computed separately. Finally, the combined effects of mechanical and thermal stresses caused by gaseous detonation have been simulated.

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To prevent unpleasant incidents, preservation high-speed railway vehicle stability has vital importance. For this purpose, the Railway vehicle dynamic is modeled using a 38-DOF includes the longitudinal, lateral and vertical displacements, roll, pitch and yaw angles. A heuristic nonlinear creep model and the elastic rail are used for simulation of the wheel and rail contact. To solve coupled and nonlinear differential equations, Matlab software and Runge Kutta methods are used. In order to study stability, bifurcation analyses are performed. In bifurcation analysis, speed is considered as the bifurcation parameter. These analyses are carried out for different wheel conicity and radius of the curved track. It is revealed that critical hunting speed decreases by increasing the wheel conicity or decreasing the radius of the curved track. Keywords: railway vehicle dynamics, nonlinear creep model, critical hunting speed, numerical simulation, bifurcation analysis Keywords: railway vehicle dynamics, nonlinear creep model, critical hunting speed, numerical simulation, bifurcation analysis

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In this paper a CFD code has been developed to investigate effects of the double inlet on the performance of a Stirling type pulse tube refrigerator. In this respect, set of governing equations have been written in a general form such that all porous and non-porous sections of the system can be modeled. In order to discretize the governing equations, a second order method has been used for time, a second order upwind method for mass, enthalpy flow and temperature in the surfaces of the control volumes and the central differential scheme has been employed for pressure and heat conduction terms. Results show that application of double inlet optimizes the phase shift between velocity and pressure and suitably decreases the fluid temperature along the pulse tube, causing to increase COP of the system. Furthermore, it is observed that a minimum temperature of 56.5 K and COP of 0.0352 @ 80 K is attainable using optimum double inlet; whereas, for a simple refrigerator a minimum temperature of 71.3 K and maximum COP of 0.0227 @ 80 K are concluded.

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In most airbag systems, the gaseous mixture that fills up the airbag is produced by the fast combustion of a propellant in a combustion chamber called inflator. Since the process of gas production in the airbag inflator is a high-temperature combustion process, having a right understanding and precise control over the combustion in the airbag inflator has always been a challenge. In this paper, the numerical study of combustion process in a pyrotechnic inflator was carried out based on a Zero-Dimensional Multi Zones model. The parametric study show that the performance of inflator is more affected by the propellant characteristics such as mass, combustion index, and propellant temperature coefficient and is not significantly influenced by hardware elements of inflator. In order to simulate hybrid pyrotechnic inflator, the initial pressure of gas plenum was increased by 25 to 50 times. As a result, the performance both in combustion chamber and in discharge tank decreased. This lower temperature leads to a higher thermal efficiency.

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In this study, the results of a direct numerical simulation (DNS) of turbulent drag reduction by microfibers in a plane channel flow at a shear Reynolds number of Re = 950 are reported. For this purpose, we make use of a numerical solution of three-dimensional, time-dependent Navier-Stokes equations for the incompressible turbulent flow of a non-Newtonian fluid. The non-Newtonian stress tensor which is required to solve the problem depends on the orientation distribution of the suspended fibers, which is computed by a recently-proposed algebraic closure model. It is shown that the use of this algebraic closure, due to the great reduction in computational efforts, enables us to perform a DNS at high Reynolds numbers. Ultimately, statistical quantities of turbulence (in particular, the mean velocity profile, Reynolds stresses, etc.) are presented and discussed. Variations in the isotropy of the Reynolds stress tensor are explained by the aid of Lumley anisotropy map.

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In this article, effect of axial angle of injection nozzles on the flow field structure in a Low-Pressure vortex tube has been investigated by computational fluid dynamics (CFD) techniques. Numerical results of compressible and turbulent flows are derived by using the standard k-ε turbulence model. The dimensions of studied vortex tubes are kept the same for all models and the performance of machine is studied under 6 different axial angles (β) of nozzles. Achieving to a minimum cold exit temperature is the main goal of this numerical research. Our investigation shows that utilizing this kind of nozzle changes the energy separation and flow characteristic. Considering total pressure of cold flow, a new parameter, ξ is defined and results shows that changing the amount of ξ can affect the cold exit temperature directly. Finally, some results of the CFD models are validated by the available experimental data which show reasonable agreement.

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Blade forging is an intricate process due to complicated geometry, high dimensional accuracy, complicated material flow, varying and sometimes very thin thicknesses. As a powerful tool, numerical simulation is used in different steps of designing process. Mechanical properties (stress-strain curves) of the material and friction factor of contact surface are of the most important inputs of the simulation. Thus, cylinder and ring compression experiments were conducted to obtain these inputs for Al-2024 used in forging of 202 MVA Siemens generator fan blade. Because of high costs of blade forging and suitability of cylinder side-pressing this experiments were used to evaluate the compression tests and simulations. Good accordance was observed between simulation and experiment. Final forging die cavity and then preforms needed to produce a sound part are modeled. Designed preforming steps include extrusion, bending and upsetting. Blade final forging step was simulated in different temperatures of die and workpiece and strain rates and the optimum condition was determined.

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An advance cooling method for buildings is use of radiant cooling system, which is not only economically feasible but also enhances thermal comfort for occupants. In this numerical study the flow and temperature fields in a room equipped with radiant cooling panel, either on the ceiling or on the wall, are studied. Outside summer design temperatures of Tehran and Semnan have been considered and to model the presence of an occupant a cube is placed in the center of the room with its external walls having constant heat flux. The results show that the vertical and horizontal temperature distributions become uniform and the maximum absolute air speed is around 0.2 m/s. The share of radiation heat transfer to the ceiling or the wall cooling panel is at least 58% or 65%, respectively, which increases due to presence of a human model. The net radiation decreases by increasing the panel temperature, but increases by increasing the outside temperature. The wall cooling uses less energy and regarding temperature and velocity distribution provides a better comfort condition

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This paper presents pore scale simulation of turbulent combustion of air/methane mixture in porous media to investigate the effects of multidimensionality and turbulence on the flame within pore scale. A porous medium consisting of a staggered arrangement of square cylinders considered here. Results of turbulent kinetic energy, temperature, flame thickness, flame structure and flame speed are presented and compared at different equivalence ratios. The turbulent kinetic energy increases along the burner because of turbulence created by the solid matrix with a sudden jump at the flame front due to increase of the velocity as a result of thermal expansion. Also, it is shown that at higher equivalence ratios, the effect of turbulence within porous burner is highly significant phenomenon. Due to higher turbulence effects in higher equivalence ratios, the flame thickness increases by increasing the equivalence ratio which is in opposite of the trend observed in laminar flow simulation. Also, it is shown that the dimensionless flame speed and excess temperature is higher at lower equivalence ratios due to lower heat loss to the cold upstream environment of burner. Two dimensional structure of flame in the pores of porous medium is shown in the present study via isotherm lines.

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In this study, factors affecting ballistic performance of fabrics used, including material properties, projectile geometry, boundary conditions, fabric dimensions, multiple plies of fabric armors and friction, has been studied. Ballistic limit was obtained as a criterion of ballistic performance of fabric to identify and compare the effect of the mentioned factor. To obtain the ballistic limit, ballistic tests were performed on the fabric. Also, a finite element model was created using LS-DYNA software and the results of the the simulation of this model show an acceptable agreement between the experimental and numerical analysis. Due to limitation in experimental tests,many of factors affecting performance of armors can be evaluated using this model.

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Airflow over a passenger train has been investigated experimentally and numerically in this research. The experimental model was a 1:26 scale model of a real train including a locomotive with one wagon behind it. A total of 16 pressure tabs for train were employed to measure the air pressure at various points on the model for different air flow velocity. Turbulent, incompressible and 3D model of air flow has been applied in numerical simulation. The numerical results of pressure coefficients were compared with the results obtained by the experimental investigation for the numerical simulation verification. The wagon number affect on the train drag coefficient and air pressure distribution on the symmetry plane of the train have been investigated numerically. The results show that the drag coefficient increases to 1.2336 for a locomotive and 7 wagons behind it but the air flow velocity has not a sensible affect on the drag coefficient. The averaged drag coefficient of each intermediate wagon has been obtained 0.1321.

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In this study, the radial flow turbine of a cooling turbine is investigated numerically and then compared with the experimental results at some operation conditions. Performance characteristics of the compressor are obtained experimentally by measurements of rotor speed and flow parameters. In this investigation, the turbine performance curve is obtained and three dimensional flow field in the turbine is analyzed. The rotor and casting geometry are modeled in BLADE GEN and CATIA softwares respectively. The TURBO GRID software is used for grid generation of rotor while the ANSYS MESH software is applied for grid generation of casting. Finally, 3D numerical solution of fluid flow in the turbine is solved by CFX flow solver. In this approach, compressible flow equations are solved according to the pressure based method with SST turbulence model. To ensure the numerical results, the grid independency is studied. Finally, the performance characteristics of the turbine are obtained numerically which are then compared to the experimental results. The comparison shows good agreement between numerical and experimental results.

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In the present paper a micro-pelton turbine with very small dimension has been studied. This micro turbine was designed for 15 kW output power and was utilized in KhorasanRazavi. To analyze and evaluate the efficiency and effectiveness of physical and geometrical parameters, the turbine flow was simulated using the commercial software Ansys CFX 13 and the simulation results of the performance point were compared and evaluated with experimental results. Because of complexity of simulation and heavy computation, instead of entire turbine, just a part of it containing several buckets was simulated. A 3D transient flow simulation was applied using the SST turbulent model. In order to model two-phase flow, the standard homogeneous free surface model was employed. In the results the effect of rotating speed on the efficiency was investigated. Moreover, the effect of physical parameters: flow rate and head and geometric parameters: the distance from nozzle to the axis of buckets, the number of buckets in constant pitch circle diameter and constant bucket size, the number of buckets in constant pitch circle diameter and variable bucket size and the number of buckets in variable pitch circle diameter and constant bucket size on the performance of a micro-turbine was investigated.

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In this study, using the results of a DNS of drag-reduced turbulent channel flow, vortical flow structures especially in the near-wall region are investigated. For this purpose, a Lagrangian Monte-Carlo method has been used to simulate the spatial orientation of fibers. Namely, the flow field is treated in an Eulerian manner whereas the fiber dynamics is described by a Lagrangian point of view. This method yields the exact solution of the governing equations. Vorticity fluctuations in the channel are studied and it turns out that the level of these fluctuations decreases in the drag-reduced flow. The reason for this reduction is explained using the reduction in velocity gradient fluctuations. Also, the distribution of the angle between the vorticity axis and the wall is studied and it turns out that horseshoe vortices exist in both flows. However, in the drag-reduced flow, they are formed farther away from the wall which indicates a weakening of sweep and ejection mechanism in the vicinity of the wall. This weakening leads to drag reduction. Also, the orientation of vortices in the drag-reduced flow is well ordered.

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In this study, the stochastic field method is developed for the direct numerical simulation of turbulent drag reduction by microfibers. For this purpose, the governing equations without any simplification are discretized on an Eulerian grid. A fifth-order upwind scheme is used for the discretization. A Monte-Carlo method is employed in the conformation space. Then, three-dimensional, time-dependent Navier-Stokes equations for the incompressible flow of a non-Newtonian fluid are numerically solved for a turbulent channel flow. Statistical quantities obtained by the proposed method are compared with those of a Lagrangian method and the high precision of the new method is demonstrated. The main advantage of the new method is its low computational cost.

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A two dimensional numerical study is presented for steady state performance analysis of a catalytic radiant counter-diffusive burner. In these burners, the gaseous fuel enters from the rear of the burner and passes through the insulation and catalyst layers. The oxygen enters the catalyst layer from the burner surface and opposite to the fuel path. The reaction takes place over the catalyst layer. In this paper, the momentum, energy and species conservation equations in porous and non-porous media are solved using the finite element method in the COMSOL software. The simulations are based on proposed corrections on boundary conditions and combustion rate of methane equation. The simulation results compared with experimental measurements published in the literature for the same geometry and conditions which shows a considerable (10%) improvements. It is shown that diffusion of oxygen through the pad limits the catalytic combustion and controls the fuel conversion in the burner.

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In this study, the theoretical design of a vapor ejector used in an air-conditioning system is performed and the designed ejector is then optimized via computational fluid dynamics. Based on the numerical simulations, two geometrical parameters, throat diameter and nozzle position, are optimized. Then, the effects of the operating parameters on the performance of the optimized ejector are investigated numerically. The optimized ejector geometry is used as a variable-geometry ejector by using a spindle in the primary throat and the performance of the system in various spindle positions is studied. The results show the importance of using a analytical design to obtain the overall geometry of the ejector and numerical simulation in order to achieve the optimal ejector performance. The variable-geometry ejector designed based on the proposed method in this study with using solar energy, in conjunction with a cold storage system, might be able to provide the necessary refrigeration for all day long.