پوریا اکبرزاده

One of the unique features of viscoelastic fluids in unsteady flows is the damping oscillatory behavior without using oscillation and external force, the cause of this is the elastic feature of these fluids. In this essay, the power-law preconditioning method of local stress sensors is used for the first time, to solve the numerical solutions of unsteady flows of viscoelastic fluid while passing through two fixed parallel planes. The Maxwell's equations have been used for this simulation. In this method, by adding a false time derivative to the governing equations, the formation of equations become hyperbola (artificial compressibility). By acquiring the preconditioning matrix of these equations that are corrected from the relationship between stress fields, solving unsteady incompressible flow equations as artificial compressible, is possible when using a dual-time algorithm that contains inner and external cordon. For convergence of inner cordon , Vossougi Far’s four-step numerical method is used. For discretization of equations, the first-order finite difference and second-order finite difference are used. The calculations of unsteady flows of viscoelastic fluid for Reynolds numbers, Weissenberg numbers and various passive viscosity ratio, are presented. These results are similar to numerical results. The convergence rate results show that the power-law preconditioned method of local stress sensor for a viscosity ratio of less than 0.5 for high stability, increase the convergence speed and reduce the cost of calculations.

The Markov chain model is one of the statistical-probabilistic approach that can predict the failure status of samples in higher cycles by using experimental results in primitive cycles and also it can be used instantaneously to control fatigue failure of parts in working. In this paper, double lap adhesive joints are subjected to cyclic loading at three different charge levels in the form of tensile load, and the purpose of this work is investigating the fatigue failure process of adhesive joints. In this study, ratcheting changes have been introduced as a failure indicator that shows the growth trend of fatigue failure. It is observed that fatigue damage occurred after 18% growth in the initial ratcheting size. These experimental results are consistent with the data obtained from the Markov chain model. Therefore, this forecasting method can predict the remaining life of double lap adhesive joints based on strain evaluations regardless of their loading history.

In the present study, in order to fabricate AC electroosmotic micropumps, the improvement of geometrical parameters of the 3D electrode, such as width, height, and location of 3D steps on the base electrodes in one pair, the base electrodes size (symmetric or asymmetric), electrodes gap, and also electrical characteristics including voltage and frequency have been investigated. Also, the fluid flow (KCl) in the channel was analyzed. The governing equations of fluid flow and electrical domain have been solved using the finite element method to investigate the effect of electrode geometry on slip velocity, which affects the fluid flow. In order to validate our numerical simulation, this chip is fabricated by photolithography method such as deposition of platinum electrodes, creating 3D steps on the base electrodes using a polymer, and fabrication of a microchannel. Finally, Our results indicate that an optimal design results in a pump with the width (50 µm) and steps height (5 µm) of each electrode and their displacement (30 µm) are capable of generating a high velocity, flow rate, and pressure around 1.77 mm/s, 14.9 ml/min and 74.6 Pa, respectively at a given voltage (2.5 V) and frequency (1 kHz), which qualitatively matches the trend observed in the experiment. This design provides an improvement in electroosmotic pumping.

The increase in fresh-water demand due to the rapid population growth and climate changes with severe droughts highlights the protection of limited fresh-water resources. In arid regions, evaporation accounts for a significant fraction of losses from water reservoirs. Among different methods for suppressing evaporative loss, the use of modular floating elements offers a simple and reliable technique. Despite numerous studies on application of floating elements, performance of this method in the presence of surface flows is not yet addressed comprehensively. Hence, the present study aims to investigate the effect of surface flows on evaporation from covered reservoirs. For this purpose, a 500-liter water reservoir was covered with white and black balls and a water-pump provided surface flows at different rates. The results show that in the presence of surface flows evaporation decreases to a specified rate, called optimal-flow-rate, and then increases with increasing the flow rate from the optimal case. Regardless of surface flow condition, the results indicate that the lowest water evaporation occurs for the coverage with white balls while coverage using a mixture of black and white balls and only with black balls showed higher evaporation rates, respectively (the highest evaporation is also for the uncovered surface). The experimental findings demonstrate that surface flows with appropriate rates can effectively enhance evaporation suppression efficiency of floating elements. Comparison of the modeling results with experimental outputs highlights application of the physically-based energy balance model in estimating surface evaporation for covered and uncovered water surfaces with and without surface flow conditions.

Today energy crisis and inherent environmental pollution caused by increasing consumption of non-renewable resources are one of the most major concerns of many countries including Iran and using of them is one of the most important factors to Earth demolition and climatic changes. Thus, numerous developed and enlarged countries try to extend the application and use of renewable energy resources. In this regard, the solar energy is one of the most important renewable and sustainable energy sources in the world and it mainly can be enumerated as the second largest energy sources in Iran after wind energy. This energy can be exploited in several methods such as flat collectors as one of the simplest and most usable systems that can be used in the buildings and solar chimney power plant. Due to daily and seasonal variations of the sun radiation, the optimal tilt estimation of collectors with maximum efficiency is very important.

This paper is based on a mathematical pattern and climatic data and the optimized tilt of flat collectors in city of Kerman is investigated. Then, by comparing and analyzing results of charts and tables, optimized tilt of flat collector at each month, season and annual are extracted and contrasted. Additionally, the percent of received radiation of each angle of plates in comparison to the optimized Angele are discussed.

In the conclusion section, the optimal angle tilt of collector over the year including spring, summer, autumn and winter seasons besides the optimal angle building heating systems and outdoor swimming pools heating systems are also presented. Finally, efficiency of vertical and horizontal collectors has been investigated with respect to maximizing the efficiency of optimal tilt of collector over the year.

This paper is devoted to numerical simulation of steady and unsteady natural heat transfer convection of nanofluids in an eccentric porous annulus. Governing equations including mass, momentum, and energy conservation are discretized by means of finite difference methods and they are solved by Alternating Direction Implicit (ADI) method and Successive over Relaxation (SOR) method. In the present study, the effect of Rayleigh number, nanoparticle volume fraction (in the range of 0 to 4 percent), Darcy number, porosity coefficient, and eccentricity ratio on average Nusselt number, local Nusselt number, streamlines, and isothermal lines are investigated. The results show that by increasing Rayleigh number, the porosity coefficient, and the nanoparticle volume fraction, the heat transfer rate increases. Reducing the Darcy number reduces the permeability of the porous medium and therefore reduces the heat transfer. In unsteady conditions, by increasing the amplitude of the inner wall temperature fluctuation, (due to the increase of the temperature gradient between the two walls), the average Nusselt number increases, and the frequency of the variation of the average Nusselt number is consistent with the inner wall temperature variation frequency.

In the present study, for the first time, the locally power-law preconditioning method for analyzing steady and unsteady incompressible turbulent flows around airfoils in high Reynolds numbers is utilized. In this method, the governing equations are modified by altering the time derivatives terms. The governing equations are discretized by the numerical method derived from the cell-centered Jameson’s finite volume algorithm. In addition, for solving the unsteady flows, an implicit dual-time procedure and for simulating the turbulent flows, Baldwin and Lomax algebraic model have been employed. The computations are presented for steady and unsteady turbulent flows around NACA0012 and ONERA-A airfoils at various angles of attack and Reynolds number. Results presented in the paper focus on the velocity, pressure and eddy viscosity profiles, distribution of pressure coefficient, lift and drag coefficients and the effect of the power-law preconditioning method on the convergence rate. The numerical solution indicates an acceptable accuracy with the aid of the power-law preconditioning method in both steady and unsteady turbulent flows for high Reynolds numbers. Moreover, using the power-law preconditioning method improves the convergence speed significantly and reduces the iteration number of solution steps and central processing unit time simultaneously in both steady and unsteady flows.

In large hydroelectric power-plants with a power of more than 100 megawatts, vertical generators are used, in which the diameter and the weight of their stator exceeding 8 meters and several hundred tons, respectively. In this structure, which is the location of the magnetic plates and the stator core, there are two rings with radial or oblique arms for mounting bearings of rotating parts of the generator (they are called upper and lower brackets). These arms transfer the forces due to the increase in the length of the ring (due to the increase in temperature), the radial forces applied from the turbine, and the torsion hydraulic torque from the turbine axis to the stator or its foundations. Therefore, accurate prediction of these forces and torque in the design of the structure and the calculation of compressive and bending stresses on the structure or foundation of the arms is of particular importance. In this paper, the effect of using oblique arms on the reduction of compressive stresses and the foundation forces of large vertical generators, is analyzed analytically. The effect of angle, length, number, and arrangement of oblique arm on stress and forces for three different load cases are investigated.

Present study is a numerical solution for incompressible fluid-elastic structure interaction by use of immersed boundary method. The solid phase is an elastic filament hinged on rear of a rigid cylinder in the presence of fluid flow. The purpose of this study is investigation of interaction of flexible filament and rigid cylinder. In the immersed boundary method, solid and fluid domains are solved in separated regions. For solving the flow field and momentum equations, the lattice Boltzmann equations are employed. In present study in contrast to previous studies, the elastic filament is also modeled by a mass-spring network that provides elastic properties like young’s and bending modulus. The mass of filament is discretized on nodes of spring network. These two solution regions are connected by applying suitable boundary conditions at each time step. Results show that the mass, geometry and location of filament behind the rigid cylinder have significant effect on the filament’s stability and drag reduction. Indicating and analyzing a special point behind the cylinder, where the drag is minimum, is the another purpose of this study.

One approach to improve the aerodynamic performance of airfoils and hydrofoils is inject a small amount of energy to the system (such as fluid injection or suction on the surface), to change the lift and drag. In fact, surface suction and blowing of the fluid can improve the pressure distribution and velocity gradient on the airfoil/hydrofoil surfaces and modify the flow separation point. Therefore, in this study the hydrodynamic behavior of turbulent flows over an airfoil exposed to the injection and suction of fluid in a part of its upper surface is discussed. Firstly, the behavior of the airfoil under one position of the injection or suction and then under two positions of injection or suction are investigated. In this simulation, the FLUENT software is used. The aim of this study is to investigate the effect of the power, the number, the position and the angle of blowing or suction on the hydrodynamic performance of the airfoil. The most important results are the reduction of 10 to 50 percent of the drag and the increase of 5 to 10 percent of the lift (for the blowing) and the reduction of 5 to 30 percent of the drag and the increase of 5 to 10 percent of the lift (for the suction) by considering two positions of injection in comparison to one position.

Peristaltic phenomenon is widely used for biologically tissues such as the digestive and excretion of urine systems. Fingered and roller pumps, hoses and internal pumps, pumps for waste management in the nuclear industry are also working on the wavy walls rules. Hence, in this paper, the magnetic hydrodynamic flow of nanofluids inside a curved porous channel, with peristaltic walls and within the internal heat source has been studied. In the present study, the flow is incompressible and the governing equations, including flow, heat and mass transfer are obtained by using an assumption of long wavelength. For solving the equations, the central finite difference approximation algorithm and Keller-box method are utilized. Heat transfer is reduced due to the presence of a magnetic field. Also, increasing the power of the heat source and the Darcy number reduces the heat transfer. Increasing porosity in the environment increases the heat transfer. Increasing the power of the heat source is accompanied by a reduction in velocity in the central line of the channel in the corrugated mode.
In this paper, by using the numerical solution results, the effect of various parameters such as source term, Darcy number and porosity on the velocity, distribution of temperature, the function of the magnetic force, increase the pressure on the wavelength, Nusselt number and also the flow trapping phenomenon has been studied.

In this paper three algorithms of Cyclic-Reduction, Parallel-Cyclic-Reduction and Parallel-Thomas are introduced to solve the Tridiagonal system of equations using GPUs and the effect of coalesced-memory-access and uncoalesced-memory-access to global memory are studied. To assess the ability of these algorithms, as a case-study the simulation of the lid-driven cavity flow have been compared to the results of Runtimes and physical parameters of the classical Thomas algorithm, executed on CPU. The maximum speed-up of these algorithms against CPU runtime is about 4.4x, 5.2x and 38.5x, respectively. Also, approximately a 2x speed-up achieved in coalesced-memory access on GPU.

Todays, in Power Plant Industries, hydraulic coupling are frequently used for transmission of power from electromotors to pumps and fans. However, because of the oil trap in the coupling and continuous operation the properties of the oil change and early destruction of coupling happens. Consequently, this problem can be delayed by precise analysis of thermal and hydrodynamic behavior of coupling and also using a proper cooling system. Thus, in this paper, thermal and hydrodynamic behavior of a mixture of oil and air in a hydraulic coupling, because of the 3D-geometry and complicated flow, is simulated by FLUENT software. The aim of this study is to identify locations of the secondary flows and vortex which have the greatest role in heat-generation. Also, the areas of fluid and body with high temperature will be determined. In this simulation, the internal flow in the coupling is examined in 3D, two-phase, and turbulent conditions. The velocity and temperature distribution of the fluid flow and the temperature distribution on the body of coupling are analyzed. The results indicate that the maximum temperature is occurred at the outside diameter of turbine coupling, increasing the distance between the pump and turbine increases the thermal dissipation around 65KW and increases the maximum and mean temperature of oil, and displacing the fins can reduce the thermal dissipation around 8KW.

Use of adhesively bonded joints in structures has been increased in recent years because of their advantages. In structural applications, fatigue is generally considered to be the most important form of loading in respect to long-term service life. In this paper, experimental tests have been performed in order to examine the fatigue damage process of double lap adhesive joints, composed of E-glass laminates and epoxy adhesive with TiO2 particles, under fatigue loading and then a method was proposed for fatigue life prediction based on initial stiffness. The scatter in results shows that the fatigue life is mostly dependent on initial stiffness. According to this, an exponential equation based on the initial stiffness has been proposed to have an initial estimation of fatigue life of the adhesive joints. In addition to the initial stiffness, stiffness degradation of the joints under fatigue loading influences on their final performance and therefore the initial estimation of fatigue life based on the initial stiffness could be modified. Also to have an online control of the fatigue damage process during the fatigue loading, a damage index using stiffness degradation was introduced.

Nowadays, using oblique arms and columns in the structure of large hydrogenerators has many advantages such as thermal expansion without large mechanical stresses, foundation load reduction, oval air-gap prevention, and etc. Therefore, in this paper the use of oblique columns in the stator-frame of a large hyrogenerator in comparison to a traditional structure (i.e. stator-frame with radial columns) is investigated for the first time by developing a Finite Element inhouse program. The use of this inhouse software reduces the time of modeling of complex structures of large hydroelectric power generators and heavy analysis of commertial softwares significantly, and helps to develop a preliminary design and structural analysis of this type of generators. In this paper, the effect of number, dimensions, and angle of oblique columns on foundation loads and structural deformation in stator-frame are described and the optimum values are obtained. The results of this study show that, the application of oblique column technology has significant advantages in reducing foundation forces, reducing the natural frequencies, and uniform structural deflections.

Aerodynamic study of flows at low Reynolds for special applications such as micro unmanned underwater vehicles, underwater robots and explorers are interested. In this paper, an improved progressive preconditioning method named power-law preconditioning method, for analyzing unsteady laminar flows around hydrofoils is presented. In this method, the 2D Navier-Stokes equations modifies by altering the time derivative terms of the governing equations. The preconditioning matrix is adapted from the velocity flow-field by a power-law relation. The governing equation is integrated with a numerical resolution derived from the cell-centered Jameson’s finite volume algorithm and a dual-time implicit procedure is applied for solution of unsteady flows. The stabilization is achieved via the second- and fourth-order artificial dissipation scheme. Explicit four-step Runge–Kutta time integration is applied to achieve the steady-state condition. The computations are presented for unsteady laminar flows around NACA0012 hydrofoil at various angles of attack and Reynolds number. Results presented in the paper focus on the velocity profiles, lift and drag coefficient and effect of the power-law preconditioning method on convergence speed. The results show satisfactory agreement with numerical works of others and also indicate that using the power-law preconditioner improves the convergence rate and decreases the computational cost, significantly.

Numerical analysis and simulation of industrial lubricants in journal bearings are very important because of their numerous applications in various industries such as power plants, turbomachinery, electrical machinery, shipbuilding, and etc. In these investigations, valuable information such as temperature distribution of pads and oil, heat and friction losses, load capacity, and etc. are extracted which are used by the designers and constructors to improve the performance of bearings. In this paper, a numerical three-dimensional thermo-hydrodynamic code has been developed to simulate the steady condition of tilting-pad journal bearings without restrictions on their size, especially length of bearings. In this program, Reynolds equations for oil flow in the gap between the shaft and the bearing pads are solved by using a numerical finite difference method with a successive over-relaxation scheme. In this simulation, for closing the solution to the real conditions, oil viscosity changes with temperature and the deformation of the pads is also taken into account. To assess the effect of the physical properties of oil-bearing on hydrodynamic behavior of bearings, several important and widely used industrial bearing oils have been selected and the results are presented in this paper. Friction loss, the maximum temperature of the pads, oil flow rate, the minimum thickness of the oil Film and the pads tilting angle are the main presented results.

Numerical analysis and simulation of cavitating flows due to appearance and its application in the maritime industry، water turbomachinery، hydrofoils، underwater vehicles، etc. have specific importance. For this reason in this research، the effect of blowing on hydrodynamic behavior of cavitating flows over hydrofoils has been investigated. Jameson''s finite volume method and power-law preconditioning method with single-phase cavitation model (Barotropic model) have been used to the analyzing of cavitating flow. The stabilization of solution has been achieved with help of the second and fourth-order dissipation term. Explicit four step Runge-Kutta method has been used to achieve the steady state condition. As regards the cavitation often occurs at high Reynolds number، to facilitate the simulation the inviscid flow equations are considered. For apply the blowing from hydrofoil surface، a jet has been placed on hydrofoil’s upper surface. The parameters of jet location، blowing velocity ratio، blowing angle and width of jet are investigated and simulation has been performed for two different cavitation numbers. The numerical results show that the power-law precondition increases the convergence speed significantly. Blowing reduces the cavity length، lift and pressure drag coefficients compared to no blowing case. Also the increase of blowing velocity ratio، blowing angle and width of jet، decrease the cavity length، lift and pressure drag coefficients.

Effect of boundary layer and its local separation on lift and drag coefficients، especially in the analysis of hydrodynamic behavior of hydrofoils is considered as an interesting subject for fluid mechanics researchers. Boundary layer control methods to increase the lift coefficient and reduce the drag coefficient، are very common. Aerodynamic study of flows at low Reynolds to special applications such as micro unmanned underwater vehicles، underwater robots and explorers are interested. For this reason in this study، the effect of fluid blowing and suction through upper surface of hydrofoils on flow control، lift and drag coefficients for flow under Re =500 and Re=2000 are investigated. Jameson’s finite volume method and power-law preconditioning method for analyzing viscous incompressible flows are presented. To control the boundary layer a jet with a width of 2. 5% of chord length is placed on hydrofoil’s upper surface and results for different blowing (suction) parameters are introduced. Results show that، blowing far from leading edge at low blowing angel and perpendicular suction far from leading edge increase the lift coefficient. Also blowing with law velocity ratio and suction with large velocity ratio، has the better impact on increasing lift coefficient.