M. Hassanzadeh , S. Kashani ,
Volume 19, Issue 1 (1-2019)
Abstract
In this paper, extended complex variables method (ECVM) is presented in fluid flow problems for the first and second-order sensitivity analysis. The finite element method is used to solve the Navier-Stokes equations, and the complex variables method is implemented to it. In the complex variables method, a complex step that only includes the imaginary part is used, but in its development, it uses a complex step that includes both the imaginary part and the real part to achieve higher performance. In the first-order sensitivity calculation, the results are not dependent on the step size, but in the second-order sensitivity, the results of the sensitivity depending on the step size and inevitably the developed formulas should be used to obtain higher accuracy. The proposed method is first validated for a problem with a closed-form solution, and the convergence rate is investigated and, then, applied to a uniform flow past a cylindrical cylinder and, finally, the results are compared by finite difference method. The results show that the range of accuracy for second-order sensitivity in the extended complex variable method is doubled compared to the complex variable method and it can be reduced to 10-12. It means that the effectiveness of the proposed method has increased. The introduced method is applicable to a wide range of problems with simple and complex parameters.
M. Garshasbi, M.m. Jafari, H. Parhizkar,
Volume 19, Issue 2 (2-2019)
Abstract
Today, the effects of three-dimensional flow near the blade and wing tip in the turbomachinery industry, such as rotor helicopters, turbine, as well as wings optimization in the airline industry, for safe flight with high maneuverability, are the focus of the industry in this area. Stall can be considered an influential phenomenon in this field. In the present study, the flow separation control was investigated by a vortex generator on a wing of a radar invader UAV, including a Naca64a210 airfoil with a 5° washout angle at the wing tip and integrated wings and attached to the body with a 47° sweep angle in the subsonic flow. The turbulent flow was solved by the kw-sst method for attack angles ranging from 5-20° and speeds of 30 and 60 m/sec. The results show a good fit with numerical and experimental results so that the pressure distribution curves indicate the growth of pressure in the vortex generating regions and also the areas near the tip of the wing, which results in the flow remain in the wing surface in these areas. Therefore, by examining the pitching moment and velocity contours, it can be seen that the flow separation from the 15° angle of attack, has been delayed to 20°, and also the ability to control the separation of flow along with the growth of velocities has been achieved.
M. Hadi Doolabi, M. Bakhtiari Far, S.h. Sadati,
Volume 19, Issue 3 (3-2019)
Abstract
When a flying vehicle is approaching a watery or earthy surface, the flow pattern around it is changed that is called the ground or surface effect. In this study, the phenomenon of ground effect and its effects on aerodynamic coefficients and flow pattern around NACA0012 and LH37 airfoils are numerically investigated. The analysis is done for statically and dynamically airfoils with plunging motion at subsonic incompressible flow regime. The Navier-Stocks governing equations are used with k-𝜔 SST turbulence model. At first the effects of ground effect on lift coefficient of airfoils are studied in various distance from surface, statically. Then at each position of airfoils from the surface the lift coefficient behavior of airfoils at sinusoidal plunging motion with the specified amplitude and frequency is investigated. the statically results show that the lift coefficient of airfoils and pressure distribution over them are changed when they approach the surface with respect to far from it, which is seen as decreasing to a certain height and then increasing it. Dynamically analyzes also indicate a change in the oscillation amplitude of the lift coefficient and the existence of a phase difference at the points of achievement of minimum and maximum lift, when the airfoils are an approach to the surface. The streamlines also showed the changes in flow field patterns around the airfoils, when they approach the surface.
M.r. Moharreri, H. Ajam,
Volume 19, Issue 3 (3-2019)
Abstract
Ejectors are as widely used as in food industries to refrigeration cycles and power plants. Since condensers of steam power plants are operated in vacuum conditions, there is a continuous air leakage, which results in metal corrosion and reduction in efficiency. Therefore, ejectors are used in these systems to remove the air. Over time, leakage increases, which requires more efficiency of ejector. Entrainment ratio (ER) is defined as the main criterion for ejector efficiency and leads to better performance if increased and also depends considerably on geometry of ejector. The aim of this research is to increase efficiency of ejector of Touss Power Plant by simultaneously changing nozzle exit position (NXP) and converging angle of mixing chamber. The main geometry of ejector was simulated by FLUENT and primary results were validated with experimental and computational data. Then, different geometries with simultaneous change in NXP and converging angle of mixing chamber were selected in the first step of Taguchi method and simulated by FLUENT. Geometries of the second step of Taguchi method were selected and designed based on the results of signal-to-noise ratio for the above-mentioned parameters and the values of entrainment ratio in the first step. An identical approach was followed for the third step. Final results showed 34% increase in entrainment ratio and also revealed that there is an optimum value for NXP and converging angle of the mixing chamber around which the value of entrainment ratio is maximum.
M. Shirzadeh Germi, H. Eimani Kalehsar,
Volume 19, Issue 3 (3-2019)
Abstract
The application of computational fluid dynamics is being developed in recent years in order to evaluate the numerical impact of wind damage on high-rise buildings due to the increasing computing power of computers. With regard to the turbulent downturns around flexible, slender and long-winded buildings with relatively high Reynolds numbers, the study of aeroelastic behavior of tall buildings is essential. In this paper, the turbulent wind flow is simulated numerically with four different velocities around the high standard CAARC building. Large Eddy Simulation has been used to solve the turbulence effect in solving fluid flow equations and the response of tall buildings to wind forces is determined by solving the differential equation of motion. A two-way coupling method is used to transfer data between two areas of fluid and structural solution in each step of time. According to the results of the numerical simulation, the pressure coefficients, streamlines and instantaneous pressure field around the tall building are in good agreement with the common characteristics of the flow around the airborne objects. The critical speed corresponding to the lock in phenomenon in this problem is calculated using a Strouhal number equal to 100m/s. Also, the history of displacement of the roof of the building in the direction of the wind and perpendicular to its length have been extracted for different wind velocities and the mean and their standard deviations respectively have been calculated. The continuous increase in the range of the fluctuations of the building under the wind blowing at 100m/s is observed. This point indicates the efficiency and capability of the numerical process in detecting aeroelastic instability with a predicted speed.
F. Niknejad , N. Fatouraee , M. Nabaei ,
Volume 19, Issue 3 (3-2019)
Abstract
Coronary arteries play a vital role in heart nutrition, and if they get stenosis, they will be at risk of developing a heart attack. Coronary artery disease is a progressive disease that is caused by the accumulation of fat particles on the wall of the arteries, leading to thickening of the wall and the formation of layers of plaque on the wall of the arteries and ultimately causing stenosis. In the present study, in order to obtain the effect of percentage and position of stenosis on the pattern of flow and WALL SHEAR STRESS distribution, followed by the progression of atherosclerotic plaques, left coronary artery and its main branches, the anterior and anterior artery, in different conditions according to Medina classification, 50 and 75%, and three different positions of lesion locations based on their distance from carina relative to the center of the branching were modeled. According to the results, WALL SHEAR STRESS and flow ratio and the percentage of inflow into the lateral branch decreased with increasing percentage of stenosis. For example, in Medina type (1.1.1), in 50% diameter stenosis, the flow ratio was 41% of the main branch and it was 37% in 75% diameter stenosis. WALL SHEAR STRESS values are less than 1, even 0.5 Pascal and in critical range in 75% diameter stenosis. Increasing the spacing of the plaque from the center of the branch, the WALL SHEAR STRESS and lateral branch flow ratio increase, and the likelihood of the expansion of the plaque decreases. Based on the development of stenosis severity, modal type (1.0.1) has the highest probability of developing atherosclerotic plaques and total vein occlusion compared to other types of medina.
M. Ghafarian Eidgahi Moghadam, M.m. Shahmardan , M. Norouzi,
Volume 19, Issue 4 (4-2019)
Abstract
Magneto-rheological damper is one of the most widely used mechanical equipment, which absorbs mechanical shocks by use of magnetic fluid and electrical coil in its structure. In this paper, for the first time, dissipative particle dynamics as a mesoscopic scale modeling method was used to simulate a magneto-rheological damper and its magnetic fluid. Data from 3 categories including magnetic fluids with brand names 122-EG, 132-DJ, and 140-CG have been used and effect of their physical properties on power of damping force have been investigated. Results of modeling show that by increasing shear rate of fluid, shear stress is first increased and, then, it is applied to a constant value, which results in a greater shear stress by applying a stronger magnetic field. It is also observed that, with increasing both maximum piston velocity and strength of magnetic field, maximum power of damping force increased, which in 140-CG is higher than the other fluids. Results of sensitivity analysis show that weight of magnetic particles and strength of dissipative forces have the greatest effect on damping force, in such a way that by increasing weight of magnetic particles and decreasing the dissipative force of particles, accumulation of magnetic particles decrease, so, increasing quality of damping. It was also found that 122-EG is more suitable than other types of magnetic fluids in forming standard magnetic particle chains, and provides a more favorable viscosity distribution for damping.
N. Parsa Mofrad, M.m. Tavakol ,
Volume 19, Issue 4 (4-2019)
Abstract
In this paper, the effect of a mannequin location with an integrated respiratory system in a ventilated room on the flow field and particle dispersion was evaluated, using numerical simulations. Dispersion and deposition of particles inside the respiratory system and inside the room have been investigated, using a Lagrangian approach. The respiratory system contains the nasal airway, nasopharynx, oropharynx, and larynx, trachea, which has been generated from CT scan images and installed on a 3D mannequin model. The evaluation conditions varied as standing and sleeping mannequins form in a room that includes 2 input dampers and 4 output dampers. For simulation of the flow field, the ANSYS FLUENT software- version 17.2 with the 4-equation SST transition turbulence model have been used. Simulations have been performed for 3 different respiration flow rates and 4 different particle diameters. Results indicate higher deposition on the sleeping model rather than standing model due to gravitational effect. The total aspiration of particles inside the respiratory system was less than 0.4%. In addition, the nasal cavity captures large particles, while for small particles, higher deposition occurs in the lower parts of the respiratory tract.
E. Nematollahi , M. Sefid,
Volume 19, Issue 4 (4-2019)
Abstract
Passive micro-mixers have simpler manufacturing in comparison with active micro-mixers and only require energy for flow pumping. In the present study, non-Newtonian fluids and non-Newtonian power-law fluid’s mixing behavior in passive micro-mixers have been studied. Simulation has been performed, using computational fluid dynamics commecrical code of Ansys fluent and two different approaches of two-component mixing have investigated. The first approach studies fluid’s mixing behavior by changing flow behavior index and flow consistency index in 5 different 3D geometries as multiple T-micromixer with aligned and non-aligned inputs in one and two plane, respectively, multiple T-micromixer, double T-micromixer, and T-micromixer, while the second approach studies mixing behavior by changing flow behavior index while flow consistency index is constant in two multiple 3D geometries with non-aligned inputs. In all studies, water was used as Newtonian fluid and carboxymethyl cellulose solution was used as non-Newtonian fluid. The studied range of Reynolds number was 1 to 100. In both approaches, the results for mixing index and pressure drop for power-law index according to criterion are reverse of each other; it means that in the first approach, with increasing power-law index, the mixing index increased and the pressure drop decreased and in second approach, this procedure is reversed. But, procedure of non-dimensional fully developed velocities in two approaches investigated is similar in comparison to geometries with non-aligned inputs.
O. Yousefi , M. Azhdary Moghaddam, N. Keikhaie ,
Volume 19, Issue 4 (4-2019)
Abstract
Many steel structures are damaged due to environmental factors such as accidental loads, exhaustion, rust, and phenomena such as cavitation and time passes. Dams’ bottom outlets are one of the important components of these structures that are subject to numerous hydraulic problems such as cavitation vibration, which causes damage and needs repair. One of the novelties for refining is the use of Carbon Fiber Reinforced Polymer (CFRP). In this paper, the effect of CFRP on gate strengthening under cavitation vibration load and the effect of damage on maximum vibration by using ABAQUS were studied. In order to observe the effects of failure on the maximum vibration of the outlet, two damages were applied to the front or back of the gate. Finally, the damaged gates were reinforced with two layers of CFRP. The outcomes showed that damage resulted in maximum vibration increase and polymer fibers has a significant effect on reducing vibrations and stresses caused by cavitation pressure.
H.r. Talesh Bahrami, H. Parhizkar, S. Ghasemlooy,
Volume 19, Issue 5 (5-2019)
Abstract
one of the key issues in the design of high-speed modern devices such as giant aircraft and high-speed trains. In this regard, it is to design these devices in such a way to have at least aerodynamic noise. The cylinder, as a bluff body, is widely used in the design of various devices, such as a landing gear. Therefore, the reduction of cylinder noise can be widely used. In the present study, numerical solution is used to present a method for reducing the noise generated by flow on the cylinder. This is done by flow suction from the grooves the cylinder. Acoustic numerical calculations were performed, using LightHill's acoustic analog approach in the form of wave equations of Ffowcs-Williams & Hawkings model. The numerical solution is performed in the three-dimensional unsteady form, using the large eddy simulation turbulence model. The characteristics of the grooves, such as their dimensions and distance the generated acoustic noise have been studied. The results show that the active control method presented in this paper is an effective and yet simple way to control noise. The cylinder used in the present study produces a noise of about 110 dB at a speed of 250 km/h. According to the results, it can be said that by optimally arranging the number of slots and creating a proper flow suction, its sound level can be reduced to about 60 dB.
M.a. Badri, F. Sabetghadam,
Volume 19, Issue 5 (5-2019)
Abstract
In the present paper, a new penalization method is proposed for implementation of the rigid surfaces on the Navier-Stokes equations in the vorticity-stream function formulation. In this method, a rigid body is considered as a region in the fluid flow, where the time is stopped. Therefore, by stopping the fluid particles, this region plays the role of a rigid body. In this regard, a new transformation is introduced and applied to the governing equations and a set of modified equations are obtained. Then, in the modified equations, the time dilation of the solid region is approached to infinity, while the time dilation of the fluid region remains In the article, the physical and mathematical properties of modified equations are investigated and satisfaction of the no-slip and no-penetration conditions are justified. Then, a suitable numerical algorithm is presented for solving the modified equations. In the proposed algorithm, the modified equation is time integrated via the Crank–Nicolson method, and the spatial discretization with the second-order finite differencing on a uniform Cartesian grid. The method is applied to the fluid flow around a square obstacle placed in a channel, the sudden flow perpendicular to a thin flat plate, and the flow around a circular cylinder. The results show that the no-slip and no-penetration conditions are satisfied accurately, while the flow fields are also high level of accuracy.
V. Sadri, H. Soltani,
Volume 19, Issue 5 (5-2019)
Abstract
In this research, taking into account the pressure drop of the streams, a simple and useful method is presented for finding the proper path of hot and cold streams inside shell-tube heat exchangers in the synthesis of heat exchangers networks (HENs). Generally, the HENs synthesis by mathematical programming leads to the problems which are answered by Mixed Integer Non Linear Programming (MINLP) methods. Optimization of such formulations results convergence difficulties due to the existence of both continuous and integer variables. In this study, instead of solving simultaneously integer and continuous variables, the genetic algorithm was used to find optimal HEN structure (integer variables). To find optimal values for continuous variables of the network, by categorizing this type of variables into two groups and using Quasi Linear Programming (QLP) instead of the nonlinear programming model (NLP), the complexity of the NLP model solution is also greatly reduced. The optimal values of continuous and integer variables are obtained with respect to a common objective function that reaches the minimum annual cost of the HEN. The comparison of the proposed method with the references shows that this method has the ability to reduce the cost of pumping flows to about 0.76%.
F. Hamedi, H. Moqtaderi,
Volume 19, Issue 5 (5-2019)
Abstract
Heat transfer phenomenon and location prediction as well as disc-type transformer windings have attracted many researches in recent years. The motivation is based on noticeable effects of these issues on transformers endurance, reliability, and functionality. This paper focuses on developing a sufficiently accurate CFD model to carry out studies and address some guidelines for disc-type transformer windings with zigzag cooling path with a reasonable resource . The discs composed from copper wires and paper insulators wrapped around them. Accounting for this inhomogeneity by zone distinction in CFD model results in many computational subdomains in very small size, which makes model development and mesh generation difficult and also computational costs, very high. In this paper, using definition functions, a method is introduced that for different material properties with no need to resolve solution subdomains. dependency of thermo-physical properties such as conductivity, , and density have been taken care of. Results show that using , model development, and also solution time noticeably reduced without any considerable in numerical results. Furthermore, using Ansys Workbench capabilities for , i.e. geometry reconstruction and mesh generation, effects of several parameters on transformer cooling condition have been investigated. Finally, some guidelines for such transformers design have been addressed.
E. Davarpanah, A.r. Teymourtash ,
Volume 19, Issue 5 (5-2019)
Abstract
Applying numerical methods for predicting cake formation and development in cross-flow membrane filtration has been an area of research. The solutions, which are mainly based on the development of zero, one, or two-dimensional methods for estimating filtration parameters, have always suffered from an obvious need for some calibration steps. In this paper, an independent two-way solving method is presented to determine the time variation of the geometry of the cross-flow filtration cake, so that by simultaneously solving the flow through the lattice Boltzmann (LB), it is possible to solve the convection-diffusion equation, using another mesoscopic method (LB-CA) in a two way coupling manner between flow changes and cake growth. Applying LB-CA provides it for all kinds of internal and external forces effects on particles trajectories to be explicitly taken into account. The proposed model was validated against both of theory of Romero and Davis and some experimental results. Moreover, the model was used to determine external effects which are arisen from static imposition of a DC electric field, on cross-flow filtration outcomes. The calculated results exhibits considerable improvements in flux decline curve and removing of fouling in some areas along the membrane length, as DC voltage rises. Also, optimal conditions with considering the electric poles’ size as an optimization parameter shows that with considering the maximum improvement in the flux curve as the target parameter, the electric poles’ size has an optimal value.
A.h. Babaei, R. Aghaei Togh, M.h. Nobakhti, M.j. Montazeri,
Volume 19, Issue 5 (5-2019)
Abstract
In the high-pressure gas-turbines, with hot-flowing gas through the stator channels with a high mass-flow rate, even slight variation in the blade geometry will have significant effects on the downstream flow-field. These minor changes can be compared to corrosion rates. The first occurrence of this corrosion is the non-uniformity of flow in the stator-rotor axial distance. This non-uniform flow, due to the complex pattern of vortices, prevents the complete transfer of fluid energy to the rotor and greatly reduces the turbine performance. In this research, a high-pressure turbine is considered to be at high risk of corrosion. The main goal is to predict these variations due to corrosion. Firstly, a 3D numerical analysis of the turbine initial model was conducted to accurately observe the flow field and the results were validated by the existing experimental results. Then, in order to investigate the effects of corrosion on the turbin performance, the blades geometrical changes were applied in stator blade profile and the flow distribution was analyzed. Results show that the highest corrosion risk is at the trailing-edge of the blades. Due to reduction in the stator inlet-outlet area ratio, the axial-velocity is reduced. But simultaneously, with increasing the stator channels outlet area, the mass-flow rate is increased by 7.31%. Therefore, the turbine undergoes to an off-design condition. The flow pattern will be more complicated in the rotor's entrance, and corrosion will develop rapidly due to temperature rise as the flow separates from the rotor blades.
H. Alisadeghi , H. Safipour , H. Rezaiefard ,
Volume 19, Issue 5 (5-2019)
Abstract
An airfoil that is heaving and pitching simultaneously may extract energy from an oncoming flow, acting as a turbine. The extracting energy from a flow is possible if the effective parameter in performance of turbine is selected properly. In this study, the theoretical performance of an oscillating twin-wing wind generator is investigated through unsteady two-dimensional laminar-flow simulations, using the commercial computational fluid dynamics code FLUENT. Computations By examining various geometric, motor, and slippery parameters and investigating the effect of each of these parameters, we present a mapping of power-extraction efficiency in the frequency and pitching amplitude domain for a NACA 0015 airfoil at a Reynolds number of 41000. Results of a parametric study show that motion-related parameters such as heaving amplitude and frequency have a strong effect on airfoil performances, whereas geometry parameters turn out to play a secondary role. A power extraction efficiency of 49% is reached by twin-wing parallel configuration. This configuration improve the efficiency by around 7% as compared to the single foil configuration.
M. Zahedzadeh , F. Ommi ,
Volume 19, Issue 5 (5-2019)
Abstract
Fuel-air mixing is one of the challenging issues in supersonic velocities that is mostly used in scramjet engine combustors. Sufficient mixing between the supersonic airstream and the fuel jet is critical for designing of scramjet engines, and this is due to the very short residence timescale for the mixture in supersonic flows. Various studies and investigations have been conducted on enhancing the fuel-air mixture. One way to improve fuel-air mixture is to employ step before the injection point, so a low-speed recirculation zone is created before the injection point and causes to improve fuel-air mixture. Employing step causes to increase stagnation pressure loss and we should compromise between mixing efficiency and stagnation pressure loss. In this paper, the effects of step on Gaseous sonic transverse injection in supersonic crossflow are investigated numerically. Two-dimensional Reynolds Averaged Navier-Stokes equations and k-ω sst turbulence model and the perfect gas equation have been solved, using Fluent software. The results of the numerical solution are compared and validated with available experimental data. Numerical results showed good agreement with the experimental values. Then, the effects of varying step heights and distance of step from injection point on Mach disc height and stagnation pressure loss are considered numerically.
S. Jamshidifard , M. Shirvani, N. Kasiri Bidhendi , S. Movahedirad ,
Volume 19, Issue 5 (5-2019)
Abstract
In this paper, black powder of the separation from air flow by a helical one-channel dust concentrator have been experimentally studied and the efficiency and pressure drop have been investigated by Computational fluid dynamics (CFD) simulations in different operating conditions. Experimental set-up is a helical one-channel including 29 branches for exporting diluted stream out. It also has two suction devices at the ends of channels in order to provide testing in high inlet flow. Black powder particles with certain particle size distribution have been tested, whose average particle size has been determined 0.327 µm by DLS and SEM images processing. CFD simulation of helical one-channel dust concentrator for air-black powder separation has been done with FLUENT software. The Realizable k-ε turbulent model, as an optimal turbulence model in terms of accuracy and speed in simulation, has been used. According to evaluation of the results, the experimental results have been compared and it showed 5.2% error. To investigate the effect of operating condition, the various air flow rate and solids mass fractions were investigated and the results showed that the simulation efficiency has increased more than 4.1% by increasing 58% of the inlet volumetric flow rate. The separation efficiency had no change by increasing the solid mass fraction from 7% up to 20%.
H.r. Rashidi, M. Zandi, F. Mossaiby,
Volume 19, Issue 6 (6-2019)
Abstract
Sloshing phenomenon is one of the complex problems in free surface flow phenomena. Numerical methods as a new method can be used to solve this problem. In these methods, the lack of a mesh and complex elements the domain of problems due to the change in geometry of the solution over time provides a lot of flexibility in solving numerical problems. In the previous researches, the sloshing problem reservoirs , using the Laplace equation with respect to the velocity potential, but the solution to this problem with pressure equations has not much considered; therefore, using the pressure equations and a suitable time algorithm, generalized exponential basis function method has been developed for dynamic stimulation reservoirs. The approximation is solved, using a meshless method of generalized exponential basis functions and the entire domain of problem will discrete to a number of nodes and then with appropriate boundary conditions, the unknowns are approximated. In this study, linear and nonlinear examples have been solved under harmonic stimulation, in two-dimensional form of rectangular cube tanks, and the results of them have been compared with the analysis solving methods, other numerical methods, and experimental data. The results show that the present method in two-dimensional mode is very noticeable compared with other available methods because of accuracy in solving problem and spending time.
