جستجوی مقالات مرتبط با کلیدواژه « روش المان مرزي » در نشریات گروه « مکانیک »

In this paper, three-dimensional code has been developed to simulate the partial cavitating flow around projectiles with various heads (blunt, hemispherical, and conical) using the BEM. For this purpose, after generating the geometry using quadrilateral elements with four control points, using the integral expression of Green's theory, source and dipole have been distributed on elements, and using an iterative algorithm, simulation is done and results are compared with the available experimental data and other numerical results. Despite the low computational cost of this method, the results have a high accuracy and convergence rate. One of the main contributions of this work is to present a correlation between the properties of cavity around projectiles with different heads (in the limit of 0.075≤σ≤0.5). Also, Analysis of the results shows that the method has a suitable ability to predict the properties of cavitation flow at non-zero angles of attack (up to 8° angle of attack) in the shortest time. Of course, by increasing the angle of attack and getting away from the potential assumption, the results are associated with some errors (15% in geometrical characteristics and 12% in aerodynamic coefficients). Due to high convergence rate and acceptable accuracy, this method can be used for initial design and optimization of subsurface projectiles with cavitation.

In this article, a review of the evaluation methods of the domain integrals in the boundary element method will be presented. The emergence of domain integrals in the formulation of the boundary element method mainly originates from the inertia term in dynamic problems, body forces in static problems or the effects of material heterogeneity. There are several approaches to calculate boundary and domain integrals in boundary element methods. Choosing the type of integration method has a prominent effect on the accuracy of the numerical solution. In this research, a comprehensive review on the techniques of domain integrals computation will be presented based on two approaches, i.e. domain splitting, and converting the domain integrals to boundary ones. The review focuses primarily on the formulation of approaches without requiring domain splitting, because of their popularity. Among them, the dual reciprocity method and the radial integration method have been described as the most efficient. At the end, the details of the modified radial integration method for calculating the integrals within non-convex domains will be stated.

A Modified Single-Level Fast Multipole Method (MSLFMM) for large-scale heat conduction problems is presented. This method is obtained by embedding the far-field approximation (FFA) within the traditional single-level fast multipole method (SLFMM). The FFA is used to compute the influence coefficients of the far elements within adjacent cells, and also to determine the moments of the elements within far cells. This approximation not only reduces the difficulty of procedures and programming, but also causes a significant decrease in the CPU time. Several problems are considered to verify and evaluate the proposed method. The computational cost of the MSLFMM is demonstrated by comparing the Conventional Boundary Element Method (CBEM), the SLFMM, and the Multi-Level Fast Multipole Method (MLFMM). It is shown that the MSLFMM is much faster than the SLFMM, and comparable with the MLFMM due to its ease of use. Finally, to check the ability of the proposed method in modeling a complicated problem, steady-state heat conduction in an engine block is solved. The numerical results show a good agreement with those obtained by a Finite Volume Method (FVM), and its difference is less than 1.5%.

Calculating acoustic loads due to the flow field produced by the outlet flow of launch vehicles impinging on the launch pad is one of the main challenges in the space industry. The sound level of outlet flow from the engine and reflection of produced acoustic waves from the launch pad and their effect on payloads depends on the turbulence parameters, created vortices, nozzle geometry, and launch pad geometry. The present paper aims to calculate the sound level generated by supersonic flow at the outlet of the launch vehicle engine besides the sound reflection from the flow deflector below the engine using a hybrid computational fluid dynamics/ boundary element method. For this purpose, the sound produced by the nozzle outlet flow in the supersonic engine of a launch vehicle is studied. In order to observe the effect of the reflection of acoustic waves from the launch pad, results are compared between two cases (with a flow deflector and without it). Numerical simulation is performed for the three-dimensional viscous compressible turbulent flow, and the boundary element method is used to compute the propagation and reflection of acoustic waves. Obtained results indicate that the generated noise level impressively increases when considering acoustic wave reflection from the deflector. The noise level generated by the projectile engine in the presence of a jet flow deflector is higher by about 8-10 dB than in the absence of a deflector. Also, results show that the acoustic waves over the projectile become more uniform by using a deflector.

Underwater vehicles in industrial applications, have various control surfaces for stability, maneuverability, gudance and control. These control surfaces affect the body hydrodynamic characteristics, including the resistance forces and the form of the generated wave due to the motion of the vehicle near the free surface. In this paper the effect of a vertical control surface on the hydrodynamic characteristics of an underwater vehicle near free surface is studied using Boundary Element Method.Results, including characteristics of free surface wave, pressure coefficient and wave resistance are calculated for Froude numbers in the range of 0.1-0.5 and non dimensional submergence depths of 1.25, 2.25, 3.5 & 4.5 for the bare model and model with control surface. Comparing BEM and experimental results shows the discrepancy of less than 3% for maximum free surface wave height and 17% for wave resistance. Predicted results indicates that control surface causes an increase of about 13% in maximum free surface wave height and 16% in wave resistance. This increment is due to the interaction of the main body with control surface and also direct relation of wave resistance with free surface wave height.

In this paper the boundary element method is used to calculate the added mass matrix of an airship. Here, the governing equations are desecrated on the triangular computational cells on the surface of the airship geometry. The computational grid cells are generated on the surface of the body using the Gambit software and inserted to the BEM Fortran code. First, in order to validate the numerical code, the added mass of a sphere and ellipsoid with two different fineness ratio (a/b=2, 3.85) is obtained and the results are compared with the analytical results. The BEM results are in a very good agreement with the analytical results. Then the BEM code is used to calculate the envelope added mass matrix with NPL and GNVR body profile which are two common body profiles in airships. Finally, the simulation is conducted for the whole airship with fins and gondola and the added mass matrix is obtained for two envelope profiles.

The aim of this paper is to investigate the growth of pitting corrosion in CUSTOM 450 stainless steel and to obtain strain values in growing pits at the maximum bending region. In this regard, a two-point bending specimen was made and subjected to a potentiostatic test under the potential of 350 mVSCE in the 3.5 wt% sodium chloride solution. Then, the depth of the grown pits is calculated using Eddy Current device. By simulating a sample under the pitting corrosion in COMSOL Multiphysics software and matching its results with the results of the Eddy Current device, it was found that the simulation can largely replace the laboratory test. To calculate the tensile stress distribution in the cross section of the sample under pitting corrosion, the Laplace equation governing the sample was discretized. The same results were obtained by solving the discrete equations and comparing them with the results of COMSOL Multiphysics software. According to the results, the pit tends to grow superficially. This means that the surface growth of the pit is greater than its growth in the direction of depth. This is due to the fact that near the sample surface, tensile stress and electrical potential are high, as well as chemical reactions and corrosion in areas near to the pit surface.

In this research, the governing differential equation of heat conduction problem in a non-homogenous, functionally graded material (FGM) is solved using the boundary elements method (BEM). Except for some limited cases, there is not known Green function or fundamental singular solution for this kind of problems, which is necessary to have the boundary elements analysis. In this paper, the thermal conductivity of the functionally graded plate is considered as a quadratic function of one direction and then an auxiliary variable is introduced into the governing differential equation in order to simplify the problem to a kind with known fundamental singular solution and therefore, heat transfer is solved in a 2D functionally graded material by simple boundary elements method which is not possible by common methods. Based on the proposed approach, a computer code is developed using the MATLAB. The validity of this developed code is verified by solving and analyzing a number of heat transfer problems.

In this study, we numerically and theoretically investigated the effect of magnetic field on shape of red cells. The two phase model was used for the dynamics of red cells. We considered red cells as deformable drops flowing through a flat microfluidic channel. We employed boundary element method (BEM) to numerically solve the two-dimensional Darcy equation by applying magnetic normal stress as a boundary condition at the interface of red cells and blood plasma. Our numerical and theoretical results indicate that red cells elongate in direction of magnetic fields. The final stable shape is a result of the balance between the surface energy and the magnetic energy of the drop. Our numerical and theoretical results are in good agreements with the experimental results.

With the development of computers, the application of numerical methods in solving engineering problems has increased considerably. Methods such as Finite Element Method, Finite Volume Method and Finite Difference Method can be mentioned as some. In this research a Boundary Element Method is applied for numerical simulation. The main difference among the Boundary Element method and other numerical methods is the governing mathematics. At first In this method the governing equation is integrated. This leads to a decrease in the dimensions of the problem and then the simulation is performed. In this research, by a change of variable, the Navier Stokes equation is transformed to Navier equation in Elastostatics at first. Subsequently the methods proposed for solving the problems in Elastostatics is utilized to solve the viscous fluid flow. In fact, the applied fundamental solution is the main difference among the proposed method and other Boundary Element Methods. In the proposed method, in contrast to previously proposed methods, the fundamental solution of the Navier equation is utilized for simulation. At last, by considering the governing mathematics a computer code is developed for viscous flow simulation. The code is applied to two different geometries, a lid-driven-cavity and a backward facing step. Convergent solutions is achieved up to Reynolsds numbers equal with 600 and 100 respectively.

In this paper, added mass coefficients of an underwater vehicle were calculate using boundary element method. Fluid flow was solve using boundary element method on triangular meshes created by GAMBIT. For verification purposes, added mass coefficient of typical ellipsoid and sphere in various aspect ratios were considerd. The results for typical ellipsoid and sphere were compared with analytical results. As an applicable research, Suboff submarine was investigated. Added mass coefficients of Suboff submarine which was determine by boundary element method was compare with experimental results then. The results show good consistency in comparison with experimental one.

Since the design algorithm of partially submerged propeller (PSP) is under the influence of the various geometrical and physical parameters; so a new convenient method and numerical tools is required to flow analysis on propeller and consider the effect of all parameters simultaneously. The aim of this study is the development of boundary element method (BEM) based on potential for PSP analysis under open water condition. Using the concept of material derivative and kinematic boundary condition, a BEM algorithm has been developed to analyze the growth, development and change in thickness of ventilation-cavitation regimes in both chord and radial directions on wide range of advance velocity coefficients. Based on the obtained results, in the high advance coefficients there are very good conformity between the values obtained from the numerical simulation compared with experimental data and observations. This adaptation is reduced by reducing the value of advance coefficient. For the low advance coefficients ((J0.4), hydrodynamic coefficients obtained from numerical results and efficiency of propeller compared with experimental measurements are desirable.

This paper presents an study and analysis of acoustic wave scattered and radiated from a truncated conical shell excited by an time-harmonic constant amplitude acoustic wave arriving from infinity by specified angle of incidence. The shell immersed in unbounded air and inner face has in-vacuo condition. Donnel-mushtari theory of shell displacement field proposed to investigate the kinetic and potential energy of shell and Hamilton principal is employed to extract the shell dynamic equation. Incident sound wave is considered as plane wave which is an incoming wave solution of reduced homogenous wave equation. The Helmholtz integral equation is use to model the scattered and radiated sound by shell. Boundary element method (BEM) is employed to relate the surface nodal pressure to nodal displacement. Then by combination of BEM and Rayleigh-Ritz method, the coupled structural-acoustic problem is solved and the sound pressure in any point of medium and shell surface is obtained. The final result has been compared with Finite Element – Boundary Element (FE-BE) method and result shows that the analytical result is in good agreement with the numerical FE-BE method. Also the bahaivor of medium fluid is studied by considering air and water as two case of fluid medium

Podded drive systems are one the recent innovation in marine propulsion systems. Hydrodynamic analysis of this system is more complicated than conventional propeller-rudder systems. The different numerical methods have been used in the hydrodynamic analysis of podded drive systems. The range of these methods is from the potential method or potential/viscous approach to pure viscous methods. In this paper, we applied coupled approach in this regard. The main purpose of this research is developing a BEM/RANS coupled method for numerical simulation of podded drives. In the proposed Potential/Viscous coupled method, the flow around rotating part (propeller) is simulated by a BEM code. Then fixed parts (pod and strut) are modelled by a RANS solver. In RANS solver, the propeller can be substituted by a set of equivalent forces which called body force and added in the right hand of momentum equation. Two cycles are available for coupling the result between potential and viscous method. The coupled method is first studied and validated with a single propeller. Afterward, the propulsive performance of the podded drive systems is studied. The results include the propeller thrust coefficient, the propeller torque coefficient, and the axial force coefficient. The results obtained by coupled method are compared to and verified by the experimental data.

In this article, cavitation flow around axisymmetric projectiles with ringed and non-ringed cavitator has been investigated using control volume and boundary element methods. In the numerical method, the homogeneous equilibrium approach as well as the zwart model, for modeling the mass transfer and forming the system of equation, have been used. In the boundary element approach with dipole distribution on the body and cavity surfaces and source distribution on the cavity surface, the right conditions were set for using the Green's theorem in solving the potential flow. Moreover, some source components were imposed on the cavitator surface in order to add the hole effects. The validation procedure for both methods has been done by analytical and experimental data. In general, the results of this research are presented in two parts. In the first part, hydrodynamic properties of ringed cavitator such as cavity dimensions, intended forces, flow behavior and etc are analysed deploying the numerical methods based on Navier Stokes equations. In the second part, the boundary element method has been used for the analysis of the cavitation flow around practical geometries with ringed cavitator. The most important finding of this study is reduction of the cavity dimensions and also an increase in the force on the projectile during the use of annular cavitator. In addition, as a result of this study, two equations for maximum length and maximum diameter of the formed cavity on the cylindrical body in relation to the cavitation number and hole diameter have been provided.

One of the most important issues of applied Hydrodynamics is Analysis of Moving hydrofoil near the free surface. In this paper attention is being paid to the analysis of hydrofoil near the free surface. For this simulation, an iterative method based on Green’s theorem is employed, and the problem is divided to hydrofoil and free surface and the effects on each other is calculated, and then perturbation potential on hydrofoil and free surface are acquired. Next, the values of these potentials are modified with an iterative algorithm until the results converge to real values. Then by having these potentials, Pressure distribution on hydrofoil surface and curve wave on free surface are obtained. Having validated this method, various factors on the hydrofoil performance such as thickness, camber, angle of attack, the Froud number, distance from the free surface, and distance from depth are surveyed. It can be observed that the results of boundary element method with good approximation predict the flow performance. However, the existence of an ideal fluid is assumed.

In this paper simulation of steady super cavitation phenomenon Çhas been considered by using partial non-linear model of Boundary Element Method(BEM).The grid mesh used is fixed and the strength of dipole and source are constant on each element. With the assumption of a partial non-linear model the cavity condition is applied on the body with the assumption that cavity height is low. Thus there is not any calculation on the cavity surface, but it is restricted to only the panels on the body surface. Cavitation number is known at first and the cavity length is determined in every iteration. When the lengths obtained in two successive iterations are very close to each other it assumed to be the answer. Based on this method two Kutta conditions including Morino condition and Iterative Pressure Kutta Condition(IPKC) are studied to satisfy the wake surface condition. The application is a wing with NACA16006 section. Iterative pressure Kutta condition compared to Morino condition needs higher computational costs, but on the other hand leads to more accurate results. It has been shown that the simulation of the flow with super cavitation over wing leads to a pressure difference at the trailing edge of each strip if we use Morino’s Kutta condition. While if Iterative Pressure Kutta Condition is usedthe results are satisfactory. Comparing the results show that this method leads to very accurate predictions for the behavior of flows with cavitation, while significantly lower computational cost is required if we use the simple cavity closure condition.

The different factors such as material, height, thickness, installation status, and geometry of the wall's head can influence on the efficiency of the sound walls to decrease the noise pollution of theairports. This paper is presented to improve the effective geometry of the wall's head as well as finding the best wall's head to maximize the noise reduction in an airport. To investigate the performance of the sound wall, the boundary element method is used. Then, in order to modeling the sound walls with different dimensions and sizes, PATRAN software is utilized. In the next step, the models are meshed and finite element method is used to analyze the vibrations of the models. Consequently, the natural frequencies and the mode shapes of sound's walls are predicted and finally the insertion loss via modeling of sources of noise and sound receivers are calculated. The design method of Taguchi experiments is applied to decrease the total numbers of the different models of Y shape geometries. Lastly,the governing equations with approximately fitted over the test cases are determined by neural network. Finally, the genetic algorithm is used to obtain the ideal head's wall.

High speed submerged bodies, such as projectiles, are subjected to cavitation phenomena which often take place when velocity increases to an extent where pressure reduces to vapor pressure and, consequently, liquid changes into vapor. This phenomenon is often undesirable but sometimes it is useful because of the drag reduction, due to the lower viscosity of the vapor phase relative to that of the liquid. Thus, the formation of cavitation in submerged bodies is of interest as a drag reduction technique, and therefore, has attracted many researchers to study its characteristics. When the cavity covers the entire solid body, the phenomenon is called supercavitation. However, if the cavity length is smaller than that of the body, i.e., the cavity closes on the body, partial cavitation occurs. Partial cavitation may also occur during flight, when the maneuvering of a vehicle is necessary. In this paper, the partially cavitating flow over an axisymmetric projectile was studied in order to obtain the optimum cavitator. The procedure used for this purpose was based on the minimization of the total drag coefficient at a given cavitation number. The boundary element method (BEM), along with CFD simulations, was employed in obtaining the optimum cavitator. Using a parabolic relation with three geometric variables, a large number of cavitators for a certain projectile were created and the BEM method was used to solve the potential fluid flow. Next, the optimum cavitator was selected based on the goal function of the minimum total drag coefficient. To examine the optimization results, several cavitators with a total drag coefficient close to that of the optimum cavitator were simulated using a CFD program (Fluent V6.3). Finally, the optimum cavitator was selected, based on both BEM and CFD results. The simulations showed that for a given projectile at all cavitation numbers, the cavitator that generates a cavity covering the entire conical section of the body with a minimum drag coefficient is optimum. It was found that increasing the cavitation number causes the optimum cavitator to approach the disk cavitator.

In this paper, Improve of transmission loss (TL) for simple geometry mufflers using micro-perforated panels (MPP) is studied. The TLs given by numerical method using SYSNOISE are compared with experimental and analytical results of other researchers. Different configurations have been used so as to detect the real effect of resonator absorbers based on micro-perforated panels in the expansion chambers. It is shown that applying these absorbers can effectively enhance the TL in desired frequencies if their parameters are well chosen. In addition, the reactive effects of the geometry are of high importance in compare with dissipative ones. However, their dissipative effects are negligible when they act at a reactive effect resonance. Finally, numerical modeling of micro-perforated panel in numerical software that it was impossible, in this paper a suitable solution using the equivalent impedance method for modeling in boundary element software, SYSNOISE, is presented.