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Continuous model for flexural vibration analysis of Timoshenko beams with a vertical edge crack
, Article Archive of Applied Mechanics ; 2014 ; ISSN: 09391533 ; Ebrahimi, A ; Behzad, M ; Sharif University of Technology
Abstract
In this paper, a continuous model for flexural vibration of beams with a vertical edge crack including the effects of shear deformation and rotary inertia is presented. The crack is assumed to be an open-edge crack perpendicular to the neutral plane. A quasi-linear displacement filed is suggested for the beam, and the strain and stress fields are calculated. The governing equation of motion for the beam has been obtained using Hamilton principle. The equation of motion is solved with a modified weighted residual method, and the natural frequencies and mode shapes are obtained. The results are compared to the results of similar model with Euler–Bernoulli assumptions and finite element model...
Continuous model for flexural vibration analysis of a Timoshenko cracked beam
, Article Archives of Mechanics ; Volume 65, Issue 4 , 2013 , Pages 265-288 ; 03732029 (ISSN) ; Ebrahimi, A ; Behzad, M ; Sharif University of Technology
2013
Abstract
In this paper, a continuous model for vibration analysis of a beam with an open edge crack including the effects of shear deformation and rotary inertia is presented. A displacement field is suggested for the beam and the strain, and stress fields are calculated. The governing equation of motion for the beam has been obtained using Hamilton's principle. The equation of motion is solved with a modified Galerkin method and the natural frequencies and mode shapes are obtained. A good agreement has been observed between the results of this research and the results of previous work done in this fiels. The results are also compared to results of a similar model with Euler-Bernoulli assumptions to...
Experimental investigation on aerodynamic performance of a flapping wing vehicle in forward flight
, Article Journal of Fluids and Structures ; Volume 27, Issue 4 , 2011 , Pages 586-595 ; 08899746 (ISSN) ; Ebrahimi, A ; Sharif University of Technology
2011
Abstract
The aerodynamic performance of a flexible membrane flapping wing has been investigated here. For this purpose, a flapping-wing system and an experimental set-up were designed to measure the unsteady aerodynamic forces of the flapping wing motion. A one-component force balance was set up to record the temporal variations of aerodynamic forces. The flapping wing was studied in a large low-speed wind tunnel. The lift and thrust of this mechanism were measured for different flapping frequencies, angles of attack and for various wind tunnel velocities. Results indicate that the thrust increases with the flapping frequency. An increase in the wind tunnel speed and flow angle of attack leads to...
Experimental study on interaction of aerodynamics with flexible wings of flapping vehicles in hovering and cruise flight
, Article Archive of Applied Mechanics ; Volume 80, Issue 11 , 2010 , Pages 1255-1269 ; 09391533 (ISSN) ; Ebrahimi, A ; Sharif University of Technology
2010
Abstract
Flapping wings are promising lift and thrust generators, especially for very low Reynolds numbers. To investigate aeroelastic effects of flexible wings (specifically, wing's twisting stiffness) on hovering and cruising aerodynamic performance, a flapping-wing system and an experimental setup were designed and built. This system measures the unsteady aerodynamic and inertial forces, power usage, and angular speed of the flapping wing motion for different flapping frequencies and for various wings with different chordwise flexibility. Aerodynamic performance of the vehicle for both no wind (hovering) and cruise condition was investigated. Results show how elastic deformations caused by...
A continuous vibration theory for beams with a vertical edge crack
, Article Scientia Iranica ; Volume 17, Issue 3 B , 2010 , Pages 194-204 ; 10263098 (ISSN) ; Ebrahimi, A ; Meghdari, A ; Sharif University of Technology
2010
Abstract
In this paper, a continuous model for flexural vibration of beams with an edge crack perpendicular to the neutral plane has been developed. The model assumes that the displacement field is a superposition of the classical Euler-Bernoulli beam's displacement and of a displacement due to the crack. The additional displacement is assumed to be a product between a function of time and an exponential function of space. The unknown functions and parameters are determined based on the zero stress conditions at the crack faces and the concept of J-integral from fracture mechanics. The governing equation of motion for the beam has been obtained using the Hamilton principle and solved using a modified...
Experimental investigation of the effect of chordwise flexibility on the aerodynamics of flapping wings in hovering flight
, Article Journal of Fluids and Structures ; Volume 26, Issue 4 , May , 2010 , Pages 544-558 ; 08899746 (ISSN) ; Ebrahimi, A ; Sharif University of Technology
2010
Abstract
Ornithopters or mechanical birds produce aerodynamic lift and thrust through the flapping motion of their wings. Here, we use an experimental apparatus to investigate the effects of a wing's twisting stiffness on the generated thrust force and the power required at different flapping frequencies. A flapping wing system and an experimental set-up were designed to measure the unsteady aerodynamic and inertial forces, power usage and angular speed of the flapping wing motion. A data acquisition system was set-up to record important data with the appropriate sampling frequency. The aerodynamic performance of the vehicle under hovering (i.e., no wind) conditions was investigated. The lift and...
Catalytic wet peroxide oxidation of phenol in a new two-impinging-jets reactor
, Article Industrial and Engineering Chemistry Research ; Volume 48, Issue 23 , 2009 , Pages 10619-10626 ; 08885885 (ISSN) ; Ebrahimi, A. A ; Sharif University of Technology
Abstract
The catalytic wet peroxide oxidation (CWPO) of phenol with activated carbon (AC) as the catalyst has been successfully tested in a novel type of two-impinging-jets reactor (TIJR). The TIJR is characterized by a high-intensity reaction chamber, which is separated by a perforated plate from other parts of the reactor. The perforated plate was used as a filter to keep the catalyst particles within the reaction chamber. The influences of various operating and design parameters such as jet Reynolds number, feed flow rate, internozzle distance, and the jet diameter on the performance capability of the TIJR were investigated. As a result of the impinging process, turbulence, complex trajectory of...
Heat transfer aspects of regenerative-cooling in methane-based propulsion systems
, Article Aerospace Science and Technology ; Volume 82-83 , 2018 , Pages 412-424 ; 12709638 (ISSN) ; Ebrahimi, A ; Sharif University of Technology
Abstract
In the present article, thermal behavior and heat transfer deterioration (HTD) of transcritical methane as well as the fluid state change in regenerative cooling with straight/curved rectangular channels are studied numerically. Simulations are conducted with a finite-volume based CFD solver utilizing reliable turbulence models and thermo-fluidic relations in transcritical conditions. The experimental and numerical results of hydrogen inside a heated tube in the literature are used for validation. The effects of mass flow rate, outlet pressure, wall temperature, surface roughness, and the channel geometry on the thermal behavior of the coolant fluid are studied in detail. According to the...
Improvement of heat-transfer correlations for supercritical methane coolant in rectangular channel
, Article Applied Thermal Engineering ; Volume 147 , 2019 , Pages 216-230 ; 13594311 (ISSN) ; Ebrahimi, A ; Sharif University of Technology
Elsevier Ltd
2019
Abstract
In this article, the conjugate heat transfer of the methane coolant inside a rectangular channel is studied and the related Nusselt correlations are improved. The compressible methane flow enters the cooling channel at supercritical pressure and subcritical temperature. The coolant flow absorbs heat from the heated walls and exits the channel with a supercritical temperature. An in-house solver is developed employing Semi-Implicit Method for Pressure-Linked Equations-Consistent (SIMPLEC) algorithm accompanied by the appropriate thermodynamic and transport property relations for the supercritical conditions of the methane coolant. The solver is validated with the experimental data found in...
A new continuous model for flexural vibration analysis of a cracked beam
, Article Polish Maritime Research ; Volume 15, Issue 2 , July , 2008 , Pages 32-39 ; 12332585 (ISSN) ; Ebrahimi, A ; Meghdari, A ; Sharif University of Technology
2008
Abstract
In this paper a new continuous model for vibration analysis of a beam with an open edge crack is presented. A quasi-linear displacement filed is suggested for the beam and the strain and stress fields are calculated. The equation of motion of the beam is calculated using the Hamilton principle. The calculated equation of motion is solved with a modified weighted residual method and the natural frequencies and mode shapes are obtained. The results are compared with those obtained by finite element method and an excellent agreement has been observed. The presented model is a simple and accurate method for analysis of the cracked beam behavior near or far from the crack tip
A linear theory for bending stress-strain analysis of a beam with an edge crack
, Article Engineering Fracture Mechanics ; Volume 75, Issue 16 , 2008 , Pages 4695-4705 ; 00137944 (ISSN) ; Meghdari, A ; Ebrahimi, A ; Sharif University of Technology
2008
Abstract
In this paper, a new linear theory for bending stress-strain analysis of a cracked beam has been developed. A displacement field has been suggested for the beam strain and stress calculations. The bending differential equation for the beam has been written using equilibrium equations. The required constant for this model is also obtained from fracture mechanics. The bending equation has been solved for a simply supported beam with rectangular cross-section and the results are compared with finite element and empirical results. There is an excellent agreement between theoretical results and those obtained by numerical and empirical methods. The model developed in this research is a simple and...
A continuous model for forced vibration analysis of a cracked beam
, Article 2005 ASME International Mechanical Engineering Congress and Exposition, IMECE 2005, Orlando, FL, 5 November 2005 through 11 November 2005 ; Volume 74 DSC, Issue 2 PART B , 2005 , Pages 1849-1855 ; 0791842169 (ISBN); 9780791842164 (ISBN) ; Meghdari, A ; Ebrahimi, A ; Sharif University of Technology
2005
Abstract
In this paper the equation of motion and corresponding boundary conditions has been developed for forced bending vibration analysis of a beam with an open edge crack. A uniform Euler-Bernoulli beam and the Hamilton principle have been used in this research. The natural frequencies and the forced response of this beam have been obtained using the new developed model in conjunction with the Galerkin projection method. The crack has been modeled as a continuous disturbance function in displacement filed which could be obtained from fracture mechanics. The results show that the first natural frequency will reduce when the crack depth ratio increases. Also the rate of this reduction depends on...
A new approach for vibration analysis of a cracked beam
, Article International Journal of Engineering, Transactions B: Applications ; Volume 18, Issue 4 , 2005 , Pages 319-330 ; 1728-144X (ISSN) ; Meghdari, A ; Ebrahimi, A ; Sharif University of Technology
Materials and Energy Research Center
2005
Abstract
In this paper the equations of motion and corresponding boundary conditions for bending vibration of a beam with an open edge crack has been developed by implementing the Hamilton principle. A uniform Euler-Bernoulli beam has been used in this research. The natural frequencies of this beam have been calculated using the new developed model in conjunction with the Galerkin projection method. The crack has been modeled as a continuous disturbance function in displacement field which could be obtained from fracture mechanics. The results show that the natural frequencies of a cracked beam reduce by increasing crack depth. There is an excellent agreement between the theoretically calculated...
Plasma actuator effects on the flow physics of dynamic stall for a vertical axis wind turbine
, Article Physics of Fluids ; Volume 34, Issue 7 , 2022 ; 10706631 (ISSN) ; Ebrahimi, A ; Sharif University of Technology
American Institute of Physics Inc
2022
Abstract
Darrieus type vertical axis wind turbines have several advantages over other wind turbines for local electricity generation in urban environments. However, the main aerodynamic challenge is the negative impacts of the dynamic stall phenomenon on the turbine performance. This study numerically scrutinizes the effects of plasma actuators on the dynamic stall control and performance improvement of a Darrieus turbine. For this sake, unsteady Reynolds-averaged Navier-Stokes equations are solved using a pressure-based finite volume method. The Suzen-Hoang plasma actuator model is employed to calculate the body forces attributed to the plasma actuator. First, the dynamic stall characteristics of...
Active flow control of a wing section in stall flutter by dielectric barrier discharge plasma actuators
, Article Physics of Fluids ; Volume 34, Issue 7 , 2022 ; 10706631 (ISSN) ; Ebrahimi, A ; Amandolese, X ; Sharif University of Technology
American Institute of Physics Inc
2022
Abstract
This paper investigates the potential of using an active flow control technique to modify stall flutter oscillations of a NACA (National Advisory Committee for Aeronautics) 0015 wing section. Wind tunnel experiments have been performed with a test-rig that provides the elastic degree of freedom in pitch. Measurements of the clean airfoil are taken at preset angles of θ 0 = 6 ° - 12 °, and for Reynolds numbers of R e c = 6.2 × 10 4 - 1.25 × 10 5, which reveal the dependency of the stall flutter oscillations to Rec and θ0. Then, flow control experiments are carried out at θ 0 = 10 ° and R e c = 1.04 × 10 5. Two dielectric barrier discharge plasma actuators have been employed simultaneously to...
Exploring self-organized criticality conditions in Iran bulk power system with disturbance times series
, Article Scientia Iranica ; Vol. 21, issue. 6 , 2014 , p. 2264-2272 ; 10263098 ; Ebrahimi, A ; Fotuhi-Firuzabad, M ; Sharif University of Technology
Abstract
Ubiquitous power-law as a fingerprint of Self-Organized Criticality (SOC) is used for describing catastrophic events in different fields. In this paper, by investigating the prerequisites of SOC, we show that SOC-like dynamics drive a correlation among disturbances in Iranian bulk power systems. The existence of power-law regions in probability distribution is discussed for empirical data using maximum likelihood estimation. To verify the results, long time correlation is evaluated in terms of Hurst exponents, by means of statistical analysis of time series, including Rescaled Range (R/S) and Scaled Windowed Variance (SWV) analysis. Also, sensitivity analysis showed that for correct...
Application of genetic algorithm for optimization of separator pressures in multistage production units
, Article Chemical Engineering Communications ; Vol. 201, issue. 7 , 2014 , p. 926-938 ; Ebrahimi, A. N ; Pishvaie, M. R ; Sharif University of Technology
Abstract
In this article, a genetic algorithm is used to optimize the separator pressures in a multistage crude oil and multistage gas condensate production unit with four and three separators respectively. This leads to the generation of more accurate results for the quality and quantity of oil remaining in the stock tank for both crude oil and gas condensate production units. Genetic-based optimized pressures for crude oil separators resulted in 1.8% and 2% enhancement in oil remaining in the stock tank for summer and winter respectively. API gravity of the stock tank oil was improved 2.4% in summer and 2.2% in winter. For the gas condensate production unit, optimized pressures can enhance by 8.6%...
Preparation and evaluation of bioactive and compatible starch based superabsorbent for oral drug delivery systems
, Article Journal of Drug Delivery Science and Technology ; Volume 23, Issue 5 , 2013 , Pages 511-517 ; 17732247 (ISSN) ; Ebrahimi, A. A ; Barzegar, S ; Sharif University of Technology
2013
Abstract
Novel types of highly swelling hydrogels (superabsorbent) were prepared by grafting crosslinked poly acrylic acid-co-2-hydroxyethylmetacrylate (PAA-co-HEMA) chains onto starch through a free radical polymerization method. The effect of grafting variables (i.e., concentration of methylenebisacrylamide (MBA), acrylic acid/2-hydroxy methymetacrylate (AA/HEMA) weight ratio, ammonium persulfate (APS), starch, neutralization percent, were systematically optimized to achieve a hydrogel with a maximum swelling capacity. The superabsorbent (SAP) formation was confirmed by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). The controlled-release behavior of...
Constructing the Bayesian network for components reliability importance ranking in composite power systems
, Article International Journal of Electrical Power and Energy Systems ; Volume 43, Issue 1 , 2012 , Pages 474-480 ; 01420615 (ISSN) ; Ebrahimi, A ; Fotuhi Firuzabad, M ; Sharif University of Technology
Abstract
In this paper, Bayesian Network (BN) is used for reliability assessment of composite power systems with emphasis on the importance of system components. A simple approach is presented to construct the BN associated with a given power system. The approach is based on the capability of the BN to learn from data which makes it possible to be applied to large power systems. The required training data is provided by state sampling using the Monte Carlo simulation. The constructed BN is then used to perform different probabilistic assessments such as ranking the criticality and importance of system components from reliability perspective. The BN is also used to compute the frequency and...
Three dimensional finite element modeling of laser solid freeform fabrication of cobalt alloy stellite 21 with 1.5% nanoCeO2 on the low carbon steel 1015
, Article Physics Procedia, 21 September 2010 through 24 September 2010, Erlangen ; Volume 5, Issue PART 2 , 2010 , Pages 405-411 ; 18753884 (ISSN) ; Ebrahimi, A ; Zakeri, S. S ; Sharif University of Technology
2010
Abstract
Our purpose is to model the multilayer laser solid freeform fabrication (LSFF) process for the material properties of low carbon steel 1015 for workpiece and cobalt alloy stellite 21 with 1.5 wt.% nano CeO2 as the powder particles. In this paper, transient heat transfer and mass transfer equations in laser solid freeform fabrication process are solved by Finite Element Method (FEM). In this approach, the geometry of the deposited material, temperature and thermal stress fields across the process area are predicted. For each layer the clad height is computed and computations are done by the use of MATLAB and COMSOL software