M. Fakoor; P. Mohammadzade; E. Jafari
Volume 11, Issue 2 , October 2017, , Pages 43-51
Abstract
Composite stiffened cylindrical shells are widely used as primary elements in aerospace structures. In the recent years, there has been a growing research interest in optimum design of composite stiffened cylindrical shell structures for stability under buckling load. This paper focuses upon the ...
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Composite stiffened cylindrical shells are widely used as primary elements in aerospace structures. In the recent years, there has been a growing research interest in optimum design of composite stiffened cylindrical shell structures for stability under buckling load. This paper focuses upon the development of an efficient optimization of ring-stringer stiffened cylindrical shell. The optimization problem used in this study involves weight minimization of ring-stringer stiffened composite cylindrical shell with buckling load and stress, which are considered as design constraints. The proposed methodology is based on Particle Swarm Optimization (PSO) algorithm. The material of shell is composite, but the material of stiffeners is considered to be isotropic. The approach adopted in modeling utilizes the Rayleigh-Ritz energy method and the stiffeners are treated as discrete members. In addition, a 3-D Finite Element (FEM) model of the ring-stringer stiffened cylindrical shell is developed that takes into consideration the exact geometric configuration. The results obtained using the Rayleigh-Ritz energy method are compared with those using 3-D FE model. The proposed methodology is implemented on the ring-stringer stiffened cylindrical shell using the PSO algorithm. The obtained results show a 13% reduction in the weight of the ring-stringer stiffened cylindrical shell whilst all the design constraints are satisfied. In addition, the results show that the proposed methodology provides an effective way of solving composite stiffened cylindrical shell design problems.
M.A. Azim; A.K.M. Sadrul Islam
Volume 2, Issue 3 , September 2005, , Pages 45-50
H. Ahmadikia; Ebrahim Dr. Shirani
Volume 2, Issue 4 , December 2005, , Pages 45-53
Abstract
In this paper, two-dimensional and axisymmetric, time dependent transonic and supersonic flows over a projectile overtaking a moving shock wave are considered. The flow is simulated numerically by solving full time averaged Navier-Stokes equations. The equations are linearized by Newton approach. The ...
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In this paper, two-dimensional and axisymmetric, time dependent transonic and supersonic flows over a projectile overtaking a moving shock wave are considered. The flow is simulated numerically by solving full time averaged Navier-Stokes equations. The equations are linearized by Newton approach. The roe’s flux splitting method, second order central difference scheme for the diffusion terms, and the second order approximation for time derivatives are used. For the turbulence terms, the Baldwin-Lomax and mixing length turbulence models are used. The present algorithm captures all the complicated features of flow including moving shock waves, expansion waves, boundary layers and wakes and their interactions. The results show that as the projectile passes through the moving shock wave, it changes the flow field features and pressure distribution dramatically. The drag force decreases and even becomes negative while the projectile takes over the shock wave. The flow features and the aerodynamic forces in transonic flow changes much more than those in the supersonic flow, as the projectile passes through the shock wave. The results show that when the shock wave passes though the projectile, the flow field structures and the aerodynamic forces change abruptly. The drag force reduces and the shock wave passes through the projectile. Such variations are more pronounced for transonic flows compared to the supersonic flows. For transonic flows, the drag force changes sign and accelerate of the projectile. Such behavior is important when the stability and control of the projectile is studied. Also such changes in pressure around the projectile and in the wake region change the projectile trajectory.
M. Sohankar; M. Farzin
Volume 4, Issue 3 , September 2007, , Pages 45-51
Abstract
Thin-walled tube bending is still to be considered a new and advanced technique. The process has been adopted into several industries such as aero and automotive. This process may produce a wrinkling, bulking and tearing phenomenon if the process parameters are inappropriate, especially for tubes with ...
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Thin-walled tube bending is still to be considered a new and advanced technique. The process has been adopted into several industries such as aero and automotive. This process may produce a wrinkling, bulking and tearing phenomenon if the process parameters are inappropriate, especially for tubes with large diameter and thin wall thickness. Push bending process is one of the methods used for bending tubular parts. It is a suitable technique to make considerably small bending radii. The method is performed using a rigid die to guide and form the tube into the required shape while the tube is pushed by a punch. A pressure media is used inside the tube to prevent its wrinkling and buckling. Hyper-elastic (rubber) materials are commonly used as the pressure media. This paper presents the pressure distribution within the tube, before and during the push bending process. Theoretical result of pressure distributions is compared with the finite element simulations. Effects of rubber properties on the tube quality are also studied. Finally optimum working conditions of process is predicted by the finite element method and is compared with previously published experimental observations.
M. Kadkhodayan
Volume 5, Issue 1 , March 2008, , Pages 45-50
Abstract
In this paper the effect of different hardening models in simulating the U-bending process for AA5754-O and DP-Steel, taking a benchmark of NUMISHEET 93 2-D draw bending, has been discussed. The hardening models considered in simulations are: isotropic hardening, pure (linear) kinematic hardening and ...
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In this paper the effect of different hardening models in simulating the U-bending process for AA5754-O and DP-Steel, taking a benchmark of NUMISHEET 93 2-D draw bending, has been discussed. The hardening models considered in simulations are: isotropic hardening, pure (linear) kinematic hardening and combined (nonlinear kinematic) hardening. The influence of hardening models on predicting springback and final state variables such as equivalent plastic strain, sheet thickness and punch force has been studied. The combined hardening model predicted the springback parameters well and the isotropic hardening over predicted the springback. The results of springback prediction have been compared with the results reported in the literature. A relation between the level of the final equivalent plastic strain and the amount of springback has been found. The obtained results show that attaining higher amount of equivalent plastic strain in the sheet leads to the less springback after unloading. Comparison between the two materials demonstrates that the aluminum alloy requires lower punch force which means superior formability and exhibits smaller springback.
Mehrdad bazazzadeh; Hamed Badihi; Ali Shahriari
Volume 6, Issue 1 , March 2009, , Pages 45-53
Abstract
Aircraft engines constitute a complex system, requiring adequate mon-itoring to ensure flight safety and timely maintenance. The best way to achieve this, is modeling the engine. Therefore, in this paper, a suitable mathematical model from engine controller design point of view, for a specific aero ...
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Aircraft engines constitute a complex system, requiring adequate mon-itoring to ensure flight safety and timely maintenance. The best way to achieve this, is modeling the engine. Therefore, in this paper, a suitable mathematical model from engine controller design point of view, for a specific aero turbine engine is proposed by the aid of MATLAB/Simulink software. The model is capable of reducing costs of actual engine tests and predicting some of important controlled variables, which can usually not be measured directly (e.g. compressor surge margin, the turbine inlet temperature or the engine net thrust). The model has maximum accuracy for maximal variance of the fuel flow input command consistent with the engine control system specifications. So the model is strongly adaptable to the engine control systems and real time applications. Simulation results which proved logical and well founded are obtained from applying an acquired fuel flow function to the engine model.
Aerospace Science and Technology
Sina Jahandari; Ahmad Kalhor; Babak Nadjar Araabi
Abstract
This paper presents a novel approach to modeling of jet transport aircraft. Initially, basic mathematical models of jet transport are derived. Afterwards by focusing on the bank angle system of the jet transport, considering the aileron as the input, adverse methods of identification are utilized to ...
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This paper presents a novel approach to modeling of jet transport aircraft. Initially, basic mathematical models of jet transport are derived. Afterwards by focusing on the bank angle system of the jet transport, considering the aileron as the input, adverse methods of identification are utilized to estimate parameters of the system in an online manner. Then, effects of different types of noise on identification process are analyzed. Eventually, effects of time varying parameters are discussed. Recursive least squares method and its extended version, covariance resetting and forgetting factor methods were the fundamental tools in the system identification process of jet transport. Comprehensive simulations are presented and cast some light on effectiveness and disadvantages of different approaches.
Aerospace Science and Technology
Amir Moghtadaei Rad
Abstract
Inertial navigation amplifies the noise of the input sensors over time due to the presence of an integrator in the output path to determine the position and attitude of the object. This system has high bandwidth and good short-term accuracy. On the other hand, GPS navigation has low bandwidth, low noise ...
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Inertial navigation amplifies the noise of the input sensors over time due to the presence of an integrator in the output path to determine the position and attitude of the object. This system has high bandwidth and good short-term accuracy. On the other hand, GPS navigation has low bandwidth, low noise processing power, and long-term accuracy. However, it can only determine the position and does not give us information about the object's attitude. Most papers have presented integrated algorithms related to GPS/INS tightly coupled navigation and have provided relatively acceptable results. Nevertheless, the main problem in this integration model is when there is an intentional or stochastical signal interference for GPS, which is not far from the mind in military applications. Therefore, navigation faces a problem. This article provides a solution with a tightly coupled integrated algorithm for high accuracy in integrated navigation.
Aerospace Science and Technology
Mohammad Hossein Moghimi EsfandAbadi; Adnan Mohammadi; Mohammad Hassan Djavareshkian
Abstract
This research delves into the intricate realm of supersonic inlet design for ramjet engines, honing in on the critical aerodynamic considerations and optimization of performance factors. At Mach 2.5, the study meticulously scrutinizes pivotal design parameters, including the placement and number of inclined ...
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This research delves into the intricate realm of supersonic inlet design for ramjet engines, honing in on the critical aerodynamic considerations and optimization of performance factors. At Mach 2.5, the study meticulously scrutinizes pivotal design parameters, including the placement and number of inclined shocks, cowl-lip positioning, throat area, spike location, and diffuser length. Computational fluid dynamics simulations are harnessed to unravel the intricate flow dynamics and assess the proposed inlet geometry's performance.The findings reveal a nuanced relationship between back pressure and shock wave positioning, where increasing back pressure initiates a shift in the shock wave, impacting the flow state. The paper delineates this transition, emphasizing the pivotal back pressure range of 300,000 to 350,000 pascals, where optimal shock wave alignment corresponds with design parameters, achieving a supercritical state.However, elevating back pressure beyond this range triggers a sub-critical state and mass flow overflow as the shock exits the throat.the study explores various performance metrics, encompassing drag coefficient, distortion coefficient, mass flow ratio and total pressure recovery under varying back pressure conditions. The outcomes underscore the merits of higher back pressures, which mitigate drag coefficient and distortion while amplifying TPR.In the sub-critical state, MFR diminishes due to shock wave displacement beyond the intake opening.This research illuminates the intricate dance of aerodynamics within ramjet engine inlets and underscores the paramount significance of optimizing inlet geometry to unlock heightened performance. It effectively encapsulates the essence of the full article, enticing readers to embark on a deeper exploration of this crucial area of aerospace engineering.
Aerospace Science and Technology
Sajjad hajirezayi; Ali nouri; Enayatollah Hosseinian
Volume 12, Issue 2 , October 2019, , Pages 49-60
Abstract
Till now, various models have been proposed in literature to simulate the behavior of riveted structures. In order to find the most accurate analytical method in modeling the dynamic behavior of riveted structures, a comparison study is performed on several of these models, in this research. For this ...
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Till now, various models have been proposed in literature to simulate the behavior of riveted structures. In order to find the most accurate analytical method in modeling the dynamic behavior of riveted structures, a comparison study is performed on several of these models, in this research. For this purpose, experimental modal analysis tests are conducted on a riveted plate to verify the efficacy of the analytical models. Moreover, finite element model updating is used to reduce the difference between analytical and experimental results. First, the material properties of plates are optimized using the experimental results obtained from modal tests of a simple plate. Next, the optimization is performed for the physical properties of rivet. At the end, it is concluded that the fastener model proposed by Rutman can bring about the most accurate results when it is used in combination with solid plates and gap elements in the contact region.
Aerospace Science and Technology
Sajad Ghasemlooy; mahsa dehnamaki; Hamid Parhizkar
Abstract
The calculation of aerodynamic heating is one of the most important steps in designing high speed flying bodies, especially reentry bodies. Because ignoring that, it can damage the thermal protection system and cut off the radar connections to the reentry capsule. Due to the high speed of the capsule ...
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The calculation of aerodynamic heating is one of the most important steps in designing high speed flying bodies, especially reentry bodies. Because ignoring that, it can damage the thermal protection system and cut off the radar connections to the reentry capsule. Due to the high speed of the capsule and the lack of a material medium, the radiation heat transfer rate is important in comparison to the convection heat transfer rate of the displacement in determining the total thermal flux, and ignoring it in the calculations caused many errors in the determination of the total heat flux . In this paper, various parameters affecting the heat transfer rate of the nose of the reentry capsule have been investigated. To calculate the capsule nose radiation, a theoretical method is presented which is compared with the reference simulation results to confirm its correctness. In this simulation, the heat transfer rate of the Apollo4 capsule has been investigated. Due to the low optical thickness of the model, the DO radiation model is used to simulate CFD. This simulation was carried out using Fluent software version 16 and solved with a laminar flow of gray gas and non-gray gas. The results show that the radiation heat transfer rate in non-gray gas mode is lower error than the gray gas state, and it is also observed that at high altitudes, the radiation transfer rate is 80% of the total heat transfer rate.
Aerospace Science and Technology
Reza Jamilnia
Abstract
In this paper, in order to simultaneously optimize the staging and trajectory of launch vehicles, changes are made in the structure of the trajectory optimization problem. In this approach, the flight times of all stages are considered freely and as optimization variables. During the solution and in ...
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In this paper, in order to simultaneously optimize the staging and trajectory of launch vehicles, changes are made in the structure of the trajectory optimization problem. In this approach, the flight times of all stages are considered freely and as optimization variables. During the solution and in each iteration, by using the values of the flight times in that iteration and the fuel consumption rate of each stage, the masses of the fuel and structure of the stages and the initial and instantaneous masses of the vehicle are calculated. By minimizing the initial mass as the objective function of the integrated optimization problem, the optimal flight trajectory is obtained in the form of the optimal state and control values, and the optimal budget of the fuel and structure masses between different stages is calculated. In this paper, to implement the dynamic equations, the direct collocation method is used, and to approximate the variables, the B-spline curves are used. By using the B-spline curves, despite the discreteness of the relevant parameters as optimization variables, a continuous concept for the optimal solution can be created. The presented approach in this paper for integrated optimization of staging and trajectory and the use of B-spline curves in the approximation of the multiphase problem with free final times can lead to reduce the initial weight of the Europa 2 launch vehicle by 30% to perform a specific mission.
Aerospace Science and Technology
Ahmad Sharafi
Volume 14, Issue 2 , October 2021, , Pages 52-65
Abstract
In the present study, the aerodynamic performance of the ducted fan is investigated using the surface vorticity method and the lifting line theory. In previous research, to consider the effects of the duct, most of the parameters derived from empirical tests or computational fluid dynamics. Our goal ...
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In the present study, the aerodynamic performance of the ducted fan is investigated using the surface vorticity method and the lifting line theory. In previous research, to consider the effects of the duct, most of the parameters derived from empirical tests or computational fluid dynamics. Our goal is to present a new method for considering the effects of the duct on the fan enclosed in a duct. In this method, the lift and drag coefficients are only input parameters. The present method requires considerably less computational time than CFD methods. Also, the aerodynamic optimization of fan blades geometry has been carried out using particle swarm optimization method (PSO) to achieve the optimum blade geometry and the maximum output power. The results of this method are in excellent agreement with experimental data in references. By optimizing the geometry of the blade, the output power of ducted fan increased 10 percentage in comparison to ducted fan with old blade geometry.
Aerospace Science and Technology
Enayatollah Hosseinian; Ali Nouri; Majid Zia
Volume 12, Issue 1 , March 2019, , Pages 53-63
Abstract
In this paper, nonlinear vibration analysis of functionally graded piezoelectric (FGP) beam with porosities material is investigated based on the Timoshenko beam theory. Material properties of FG porous beam are described according to the rule of mixture which modified to approximate material properties ...
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In this paper, nonlinear vibration analysis of functionally graded piezoelectric (FGP) beam with porosities material is investigated based on the Timoshenko beam theory. Material properties of FG porous beam are described according to the rule of mixture which modified to approximate material properties with porosity phases. The Ritz method is used to obtain the governing equation which is then solved by a direct iterative method to determine the nonlinear vibration frequencies of FGP porous beam subjected to different boundary conditions. The effects of external electric voltage, material distribution profile, porosity volume fraction, slenderness ratios and boundary conditions on the nonlinear vibration characteristics of the FGP porous beam are discussed in detail. The results indicate that piezoelectric layers have significant effect on the nonlinear frequencies. Also it is found that the porosity has a considerable influence on the nonlinear frequency and these effects increased especially when the electric voltage is applied.
Aerospace Science and Technology
Pooya Yousefi Khiabani; Ali Nouri; Enayatullah Hosseinian
Abstract
In this paper, a semi-analytical solution for three-dimensional transient analysis of an annular plate with piezoelectric layers is investigated. The core is a functionally graded material with an exponential distribution. This method, which is a combination of the state space method, Laplace transform ...
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In this paper, a semi-analytical solution for three-dimensional transient analysis of an annular plate with piezoelectric layers is investigated. The core is a functionally graded material with an exponential distribution. This method, which is a combination of the state space method, Laplace transform and its inversion, and the one-dimensional differential quadrature method, is used to obtain the response of three-dimensional motion equations plate and the stress-displacement relations of the state space equations obtaining an analytical solution in the direction of the thickness and by applying the differential quadrature method to the equations of state space, a semi-analytical solution of the plate is obtained. To obtain a solution in the time domain, the Laplace transform and its numerical inversion are used. Analyzing the convergence of the present method, the obtained numerical results have been compared with the results of articles and with results obtained from using finite element analysis. Various parameters were studied including boundary conditions, piezoelectric properties, voltage applied to the actuator, the ratio of core thickness to layers, the ratio of outer to inner radius, and the functionally graded material variations index.
Aerospace Science and Technology
Mana Ghanifar; Milad Kamzan; Morteza Tayefi
Abstract
This paper investigates different intelligent methods of tuning feedback-linearization control coefficients. Feedback-linearization technique is an effective method of controlling nonlinear systems. The most critical part of designing this controller is tuning the gains, especially if the plant has complex ...
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This paper investigates different intelligent methods of tuning feedback-linearization control coefficients. Feedback-linearization technique is an effective method of controlling nonlinear systems. The most critical part of designing this controller is tuning the gains, especially if the plant has complex nonlinear dynamics. In this research, to improve the performance of the overall closed-loop system, the feedback linearization method has been integrated with the conventional proportional-integral-derivative (PID) controller. Also, a quadratic performance index was used to compare the functionality of the controllers tuned by the proposed intelligent methods. These intelligent methods include Genetic Algorithms (GA), Particle Swarm Optimization (PSO), Fuzzy Logic, and Neural Network tuning algorithms. A quadrotor aircraft is used as the plant under study in order to evaluate the performance of the controllers tunned in this research. Finally, MATLAB simulation tests demonstrate the effectiveness of the presented algorithms. According to the results, it is demonstrated that the class of online algorithms performs better, even with the specified perturbation.
Soghra Rezazadeh; Esmaeil Esmaeilzadeh
Volume 6, Issue 2 , June 2009, , Pages 87-97
Abstract
In this paper, effects of EHD actuators on hydrodynamic behavior and heat transfer of air flow over a circular cylinder were considered. Pressure and temperature distributions around the cylinder were measured in presence of wire-plate EHD actuators. The Reynolds number based on cylinder diameter (d) ...
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In this paper, effects of EHD actuators on hydrodynamic behavior and heat transfer of air flow over a circular cylinder were considered. Pressure and temperature distributions around the cylinder were measured in presence of wire-plate EHD actuators. The Reynolds number based on cylinder diameter (d) were 3500, 7000. Experiments were performed for various configurations. Based on obtained results, the flow field around the cylinder was affected significantly by EHD actuation. The flow visualization confirmed the formation of separation bubble because of EHD actuation and affected pressure distribution dramatically. Measured temperatures showed that the excess flow, resulted from corona wind, enhanced local and average heat transfer over the cylinder.
R. A. Alashti
Volume 5, Issue 2 , June 2008, , Pages 91-98
Abstract
In this paper, the plastic limit load of cylindrical shells with opening subject to combined bending moment and axial force are found by finite element method using ANSYS suite of program. The effect of various parameters such as the geometrical ratios of shell and the shape, size, axial and angular ...
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In this paper, the plastic limit load of cylindrical shells with opening subject to combined bending moment and axial force are found by finite element method using ANSYS suite of program. The effect of various parameters such as the geometrical ratios of shell and the shape, size, axial and angular position of the opening on the limit load of cylindrical shells under various loading conditions are studied. The plastic limit loads of cylindrical shells with circular openings of different opening ratios and combinations of dimensionless bending moment and axial force are calculated by finite element elastic-plastic method and their interaction curves are presented.
Farzad Bazdidi-Tehrani
Volume 3, Issue 2 , June 2006, , Pages 97-103
A. M. Akhlaghi; H. Naseh; Mehran Dr. MirShams; Saeid Irani
Volume 8, Issue 2 , September 2011, , Pages 107-117
Abstract
This paper presents an extension of Bayesian networks (BN) applied to reliability analysis of an open gas generator cycle Liquid propellant engine (OGLE) of launch vehicles. There are several methods for system reliability analysis such as RBD, FTA, FMEA, Markov Chains, and etc. But for complex systems ...
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This paper presents an extension of Bayesian networks (BN) applied to reliability analysis of an open gas generator cycle Liquid propellant engine (OGLE) of launch vehicles. There are several methods for system reliability analysis such as RBD, FTA, FMEA, Markov Chains, and etc. But for complex systems such as LV, they are not all efficiently applicable due to failure dependencies between components, computational complexity and state space explosion problems. So to overcome these problems the BN modeling is preferred for OGLE reliability analysis. In this algorithm first the functional models of OGLE is constructed based on expert knowledge and experiments involving system and subsystems interactions. Then failure modes are derived through performing FMEA. Furthermore by using modeling properties of Bayesian networks, a constructional model for failure propagation is obtained based on the acquired functional model and FMEA. Finally, by allocating quantitative properties to the Bayesian model and inference of it, the reliability of OGLE is obtained. The results are verified to the Monte Carlo simulation results. Comparing the values obtained of two applied methods shows the high accuracy and efficiency of introduced algorithm to reliability analysis of launch vehicle OGLE and other complex systems with dependant failure modes.
edris yousefirad; Mohammad Pasandidefard Fard
Volume 7, Issue 2 , September 2010, , Pages 123-129
Abstract
In many applications the flow that past bluff bodies has frequency nature (oscillated) and it is not uniform. This kind of flow has effects on the formation of vortex shedding behind bluff bodies. In this paper the flow around a circular cylinder was numerically simulated. The effects of frequency variation ...
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In many applications the flow that past bluff bodies has frequency nature (oscillated) and it is not uniform. This kind of flow has effects on the formation of vortex shedding behind bluff bodies. In this paper the flow around a circular cylinder was numerically simulated. The effects of frequency variation at inlet flow on the vortex shedding frequency were investigated. The transient Two-Dimensional Navier-Stokes equations were employed to compute the unsteady laminar free stream flow over a circular cylinder. A time series of analysis for the formations of vortex shedding behind the circular cylinder was performed under uniform (Frequency=0) and oscillated flows (Frequency=0.1, 1, 10, 60 and 100Hz) at Re=300. Then the value of amplitude for one of the oscillated flows was changed. The global quantities such as drag coefficients and Strouhal number variables were compared for the above range of conditions for both uniform and oscillated flows. Results show that by increasing the inlet flow frequency, Strouhal number increases slightly and the oscillation amplitude of drag coefficient increases considerably. However the mean sizes of drag coefficients do not vary in all computations. Also the effect of variation in velocity amplitude on vortex shedding frequency and drag coefficient value were studied.
reza ansari; M. Darvizeh; Milad Hemmatnezhad
Volume 5, Issue 3 , July 2008, , Pages 129-138
Abstract
In this paper, a unified analytical approach is proposed to investigate vibrational behavior of functionally graded shells. Theoretical formulation is established based on Sanders’ thin shell theory. The modal forms are assumed to have the axial dependency in the form of Fourier series whose derivatives ...
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In this paper, a unified analytical approach is proposed to investigate vibrational behavior of functionally graded shells. Theoretical formulation is established based on Sanders’ thin shell theory. The modal forms are assumed to have the axial dependency in the form of Fourier series whose derivatives are legitimized using Stokes transformation. Material properties are assumed to be graded in the thickness direction according to different volume fraction functions such as power-law, sigmoid, double-layered and exponential distributions. A FGM cylinderical shell made up of a mixture of ceramic and metal is considered. The Influence of some commonly used boundary conditions, the effect of changes in shell geometrical parameters and variations of volume fraction functions on the vibration characteristics are studied by comparing the results from the present theory with those from the First order Shear Deformation Theory (FSDT). Furthermore, the results obtained for a number of particular cases show good agreement with those available in the open literature. The simplicity and the capability of the present method are also discussed.
Ahmad Dr. Sedaghat
Volume 3, Issue 3 , September 2006, , Pages 153-158
Abstract
This paper describes a preliminary investigation into the use of normal form theory for modelling large non-linear dynamical systems. Limit cycle oscillations are determined for simple two-degree-of-freedom double pendulum systems. The double pendulum system is reduced into its centre manifold before ...
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This paper describes a preliminary investigation into the use of normal form theory for modelling large non-linear dynamical systems. Limit cycle oscillations are determined for simple two-degree-of-freedom double pendulum systems. The double pendulum system is reduced into its centre manifold before computing normal forms. Normal forms are obtained using a period averaging method which is applicable to non-autonomous systems, more advantageous than the classical methods. Good agreement is observed between the predicted results from the normal form theory and numerical simulations of the original system.
S. H. Hashemi
Volume 5, Issue 4 , December 2008, , Pages 183-190
Abstract
Recent failure information from research teams in NASA Langley and others has shown that CTOA based fracture models calibrated on large C(T) and M(T) specimens can be transferred successfully to cracked aircraft fuselage structures for the assessment of their residual strength. A major difficulty that ...
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Recent failure information from research teams in NASA Langley and others has shown that CTOA based fracture models calibrated on large C(T) and M(T) specimens can be transferred successfully to cracked aircraft fuselage structures for the assessment of their residual strength. A major difficulty that could limit the more extensive use of this failure parameter is its experimental measurement either in the real structure or in a laboratory scale test. This paper describes the use of a DCB-like specimen for direct measurement of critical CTOA data for thin sheet 2024-T3 aluminium alloy used in aeronautical applications. It outlines the features of ductile crack growth in thin walled structures, the specimen design geometry for reproducing these features in a laboratory scale experiment, the experimental set up and loading configuration for tear test specimens, and the CTOA measurement scheme. Using the test technique developed, the CTOA resistance curves for test samples of 2.3mm ligament thickness were generated for fractures propagating in the rolling and transverse direction of the original rolled plate. The results of this research showed that the technique was capable of producing large amounts of highly consistent CTOA data even from one single specimen. This is promising as it provides a precise and relatively easy experimental method for direct evaluation of CTOA toughness levels of test materials using small-scale laboratory specimens. A comparison of the test results from the current work with similar data from the literature concludes the paper.
K. Bashirnezhad
Volume 3, Issue 4 , December 2006, , Pages 205-212
Abstract
Soot concentration distribution is investigated both numerically and experimentally in methane-air diffusion flame. The experimental work is conducted with a cylindrical swirl stabilized combustor. Filter paper technique is used to measure soot volume fraction inside the combustor. The numerical simulation ...
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Soot concentration distribution is investigated both numerically and experimentally in methane-air diffusion flame. The experimental work is conducted with a cylindrical swirl stabilized combustor. Filter paper technique is used to measure soot volume fraction inside the combustor. The numerical simulation is based on the solution of the fully-coupled conservation equations for swirling turbulent flow, chemical species kinetic modeling, fuel combustion and soot formation and oxidation. The soot particle number density and the mass density based on the acetylene concentrations are used to model the soot emission in confined swirling turbulent diffusion flame. The comparison between predictions and measurement results over a range of different swirl numbers shows good agreement. The results reveal the significant influence of swirl intensity on combustion characteristics and soot formation in diffusion flames. An increase in swirl number enhances the mixing rate, peak temperature, and soot volume fraction inside the flame zone. The locations to give the maximum temperature and soot concentration shift to backward (close to combustor inlet) with increase in swirl number.
