Cover
text
article
2015
per
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2015
http://mme.modares.ac.ir/article_12593_91aff856a2b638dfd075ea38e29a3415.pdf
IFC
text
article
2015
per
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2015
0
0
http://mme.modares.ac.ir/article_11960_3cd5c19a702842c7be35425f5443238c.pdf
Index
text
article
2015
per
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2015
http://mme.modares.ac.ir/article_12594_ccb89b693896bccbef997f7dac810b42.pdf
Control of pneumatic servo system based on simultaneous pressure observing on both sides of the cylinder by high-Gain observer
sepehr
ramezani
پژوهشکده فناوری های نو
author
Seyed Mehdi
Rezaei
عضو هیت علمی دانشگاه صنعتی امیرکبیر
author
Mohammad
Zareinejad
عضو هیت علمی پژوهشی پژوهشکده فناوری های نو دانشگاه صنعتی امیرکبیر
author
Kevani
Baghestan
پژوهشکده فناورهای نو
author
text
article
2014
per
Nonlinear factors such as air compressibility, leakage and friction make the control of pneumatic systems complex. Model-based robust control strategies are appropriate candidates for pneumatic systems, however in such controllers the measurement of state variables of the system are needed. In a pneumatic system the state variables are position and velocity of the actuator, and pressure in both sides of the cylinder. Pressure measurement is usually obtained by means of costly and low response sensors. A better way to deal with the measurement problem is to use observers to reconstruct the missing velocity and pressure signals. However the problem in a pneumatic system is that the system is not observable and pressure signals could not be observed by means of position signals only. To deal with this problem, in this paper, the pneumatic actuator is modeled as two separate chambers and the resulting subsystems are observable independently. High gain observers are designed for mentioned subsystems and for each chamber the pressure of the other chamber is considered as a disturbance. The input signal for each observer is the actuator position signal only. Finally a sliding-mode control strategy is designed for position tracking and experimental results verify that both controller and observer objectives are satisfied.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
1
10
http://mme.modares.ac.ir/article_11808_33f0977e92847fe8cca8e10c963698e9.pdf
Studying the performance of multi-layered ceramic-epoxy armor under high velocity ballistic impact with finite element method
Mojtaba
Yazdani
Assistant Professor of Applied Mechanics (Solid)
author
Ali
Rashed
دانشگاه صنعتی سهند
author
text
article
2014
per
In this paper, the behavior of multi-layered Alumina ceramic armor against high velocity projectile has been considered. Due to the conoid failure mechanism of ceramics under impact loadings, high compressive strength and erosive behavior of them against projectiles, specially ogive nosed ones, various incorporation of this material in modern armor applications is possible. Consideration of this behavior by means of different constitutive relations in detail, such as given by Johnson & Holmquist, one can design an armor with higher ballistic performance. In this study, "conoid fracture" behavior of "ceramic - elastomer" armor has been simulated in LS - Dyna with Johnson - Holmquist constitutive relation. As a result, the conoid failure of top layer ceramic causes the impact pressure to decrease in back layer, and consequently the epoxy interface transfers the pressure distribution between ceramic layers, causing growth and propagation of conoid failure thus distributing the pressure in larger area, finally resisting against projectile penetration in the armor. Application of this armor system for light-weight and heavy back layer-free purposes, would be of the highest priority against high velocity projectiles.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
11
20
http://mme.modares.ac.ir/article_11809_2329edc3058199c61db9d1dd4b6df67a.pdf
Geometrical and physical parameters affecting free vibration and impact force of sandwich plate with Smart Flexible cores
Gholamhasan
Payghaneh
Head Of faculty Of Mechanical Engineering
author
karamat
malekzadeh fard
Head Of Air and Space
author
fahimeh
rashed saghavaz
Professor
author
text
article
2014
per
In this article, the physical and Geometrical effective parameters on free vibration and Force impact a three-layer sandwich plate in the middle layer with magneto rheological (core) Under cross-shot with low speed is investigated. The first natural frequency and loss factor of comparable modal for the first four vibrative modes for Core thickness, magnetic fields and different sheet’s geometrical parameters, is founded. The MR material shows variations in the rheological properties when subjected to varying magnetic fields. These materials with fast response time (in milliseconds) Through a detailed with variation in Magnetic field can be controlled. The governing equations of motion were obtained using Hamilton̕s principle. The results were obtained by the systematic analytical solution. Using the two degrees of freedom mass-spring model, the contact force function can be obtained analytically. The obtained natural frequency from eigen value problem, was used for calculating of equivalent mass of the plate in spring mass model. The results show that with systematic variation of magnetic field and with increasing the ratio of core thickness to the layer thickness and also with increasing the ratio of length to the whole of sheet thickness, we can in order, the stiffness, structural loss factor coefficient and maximum contact force can be changed and controlled.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
21
30
http://mme.modares.ac.ir/article_11811_7aa2d7a6159b28dec4e74189dc782bea.pdf
Experimental Study and finite element simulation of Cylindrical-conical-cylindrical horns with Circular cross-section:Application on ultrasonic vibrations Sliding friction process
davoud
shahgholian ghahfarokhi
دانشجو
author
Mahmoud
Salimi
استاد دانشگاه
author
mahmoud
farzin
استاد دانشگاه
author
text
article
2014
per
When two bodies slide on each other, friction is created. By superposing ultrasonic oscillation to one of the bodies, the friction force is reduced .This phenomenon is widely used in metal forming and metal cutting. For the production and transmission of ultrasonic vibrations to a target it is required to use an ultrasonic system the components of which are a generators, a transducers and a horn. Horn constitutes an important part of the Ultrasonic systems. The main task of the horn is to transmit the ultrasonic vibrations and amplify the ultrasonic vibration amplitude at the output. In this study, an Aluminum horn was designed in cylindrical-conical-cylindrical shape geometry and was analyzed by the finite-element method(FEM) using the Abaqus software was manufactured. The resonance frequency obtained in Abaqus was equal to 19976 Hz. The resonance frequency obtained from the generator was equal to 19920 Hz. Hence there is a very good agreement between the experimental result and the FEM simulation. The difference between the finite element simulation results and the experimental ones is less than one percent. Moreover, a horn –workpiece assembly for applying the ultrasonic sliding friction was designed and manufactured. Then the fixture and the tool holder clamp were designed for the vibrating tool so that it can be installed on a milling machine and the friction force measurement is possible while the ultrasonic vibrations are applied.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
31
38
http://mme.modares.ac.ir/article_11812_2a3b905e369592eff4436e6a0fde2d45.pdf
Study of inhibition of Aβ42 aggregation and oligomer forming with Docking and MD simulation
vahid
khandan
graduted student/sharif university of technology
author
bahar
firoozabadi
faculty/Department of Mechanical Engineering, Sharif University of Technology
author
Mohammad Said
Saidi
Faculty / Department of Mechanical Engineering, Sharif University of Technology
author
text
article
2014
per
Alzheimer’s disease (AD) is the most common type of dementia in the elderly. The neuropathology and treatment of AD is not precisely determined yet, but according to the pathological studies, AD is associated with presence of toxic soluble oligomers and insoluble senile plaques formed by amyloidosis of Amyloid Beta (Aβ) in neocortical region of brain. The V10HHQKLVFFAE22 is a critical region of Aβ42 which facilitates aggregation process. An attractive therapeutic approach to treat AD is to identify small ligands that are capable of binding to critical residues in order to inhibit or reverse Aβ amyloidosis process as source of neurotoxicity. In this area, therapeutic efforts designed various organic agents and most of them focused on the N-terminal sequence of Aβ. Here, a peptide inhibitor derived from the C-terminal of Aβ (G33LMVG37) is utilized as inhibitor and combined Docking and Molecular dynamics simulation used to find the binding sights in the critical region (V10HHQKLVFFAE22). The simulation identified tree stable binding sites for Aβ42 inhibition by penta peptide. This result indicate that this penta-peptide is capable to inhibit aggregation process and can be consider as an drug for AD preclinical studies.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
39
48
http://mme.modares.ac.ir/article_11813_d31db915e4c9087af5b9d9a1c717cc7a.pdf
Flutter analysis of an aircraft wing carrying, elastically, an external store
hamid
pourshamsi
Yazd University
author
Abbas
Mazidi
Yazd University
author
seyed ahmad
fazelzadeh haghighi
shiraz university
author
text
article
2014
per
In this paper, the Flutter analysis of an aircraft wing carrying, elastically, an external store is studied. The wing is considered as a uniform cantilever beam and the external mass is connected to the wing by one spring and damper. The aeroelastic partial governing equations are determined via Hamilton’s variational principle. Also, modified Peter's finite-state aerodynamic model is employed. The resulting partial differential equations are transformed into a set of ordinary differential equations through the assume mode method. Effects of different situations like the wing without external mass, the wing with a rigidly attached external mass, and the wing with an elastically attached external mass on the flutter speed and frequency are investigated. The numerical results for a wing are compared with published results and good agreement is observed. Then, Simulation results for the wing with an elastically attached external mass are presented to show the effects of the wing sweep angle, store mass and its location and the spring rigidity constant on the wing flutter. Results show that sliding the external mass toward the wing tip in spanwise direction and also toward the trailing edge in chordwise direction decreases the flutter speed. Furthermore, increasing the store mass and spring constant decreases the wing flutter speed. Results show that increasing the wing sweep angle, increases the flutter speed, in all situations.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
49
58
http://mme.modares.ac.ir/article_11814_474fb79183dec68a608e99178b0d44c8.pdf
Design and Modeling of an Active Hydraulic Engine Mount Using FXLMS Algorithm for Automotive Applications
Reza
Tikani
هیات علمی/دانشگاه صنعتی اصفهان
author
Saeed
Ziaei Rad
هیات علمی/دانشگاه صنعتی اصفهان
author
Nader
Vahdati
هیات علمی/دانشگاه پترولیم امارات متحده عربی
author
Somayeh
Heidari
دانشگاه صنعتی نانیانگ، سنگاپور
author
text
article
2014
per
Hydraulic engine mounts are widely used in aerospace and automotive applications for vibration isolation. Here in this paper an active engine mounting system is proposed which not only acts like an isolator in a wide range of frequencies, but also performs as a damper when shock inputs and engine resonances are present. The proposed new design consists of a conventional passive fluid mount, an electromagnetic actuator (voice coil) and a capacitive circuit. The voice coil is placed in the lower chamber of the passive fluid mount and it can change the volumetric stiffness of the bottom chamber actively such that the mount has low dynamic stiffness in a wide range of frequencies. The capacitive circuit is paralleled with the voice coil and in situations when large shock inputs are present; it adds capacitance to the electromagnetic circuit and changes the characteristics of the mount from an isolator to a damper. Here in this paper the physical and mathematical models of the new mounting system are presented, the simulation results are shown and the performances of the proposed design in all active, passive and damper conditions are demonstrated.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
59
66
http://mme.modares.ac.ir/article_11815_366eeac322024344081ba95930490a99.pdf
Influence of composite materials nonlinearity on load distribution in single-column multi-bolt composite joints
Fathollah
Taheri-Behrooz
استادیار دانشکده مهندسی مکانیک
دانشگاه علم و صنعت ایران
author
Rouzbeh
Nouhi
دانشگاه علم و صنعت ایران
author
Ali Reza
Shamaei Kashani
دانشگاه علم و صنعت ایران
author
text
article
2014
per
In this paper an analytical approach for determining load distribution in single-column multi-bolt composite joints by considering elastic nonlinear behavior of the composite materials is presented. Load distribution was calculated by writing the governing equations of the motion. This closed form solution is an integration of spring-based models with nonlinear behavior of composite plate materials. Developed method is capable to calculate taken load by each bolt and its displacement by simultaneous solving of governing equilibrium equations of the system. This manuscript specifically focused on the influence of composite material nonlinearity on the load distribution of single bolted composite joints. For this purpose, load changes versus displacement are plotted by taking into account both the linear and nonlinear material behavior. The achieved results via suggested solution revealed that displacements were increased upto 2.5-5 percent in comparison with the results of linear method available in the literature. In addition, due to the manufacturing tolerances, bolt–hole clearances can vary within allowable limits and fits. Therefore the effect of bolt hole-clearance on the composite joints with linear and nonlinear material properties was also investigated.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
67
74
http://mme.modares.ac.ir/article_11816_c5a931096a34eac2419efbd0afdfb779.pdf
Investigation of energy absorption and deformation of thin walled tubes with circle and square section geometries under transverse impact loading
Mahdi
Choubini
دانشگاه تربیت مدرس
author
Gholam Hossein
Liaghat
دانشگاه تربیت مدرس
author
MohammadHossein
Pol
دانشگاه تفرش
author
text
article
2014
per
In this study, the numerical and experimental study of energy absorption and deformation of thinwalled tubes with square geometry and circular cross under impact loading is studied. The purpose of this study was to investigate the effect of geometry on the energy absorption of aluminum tubes and the effect of foam filled tubes to absorb more energy under transverse impact. In the experimental part, the tubes of aluminum in form of hollow and filled with solid polyurethane foam prepared and then the quasi-static tests with static and dynamic loading rates by drop hammer have been performed on samples with different energy and the acceleration-time diagrams in each test is obtained. In the last part of this study simulation of the phenomenon of transverse impact on thin sections was carried out with the ABAQUS software. The discussion and conclusions of this study, the results of experimental tests carried out by the author of the thesis has been compared with the results of numerical analysis show a good agreement(difference below twenty percent). Finally, it was concluded that with regard to material of structure, at high energies square tubes have 50 percent specific energy absorbed higher than circular tubes and filled tubes have 20 percent specific energy absorbed higher than hollowone's. And transverse displacement of the hollow tube and circular tube is always higher than the filled tube and square tube.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
75
83
http://mme.modares.ac.ir/article_11810_debd3cafe4d8d331f646a6758027e024.pdf
Experimental study of surface roughness and burr width in micromilling of titanium alloy
hamed
hassanpour
PhD student, Department of Mechanical Engineering, Tarbiat Modares University
author
hamed
Rezaei
PhD student, Department of Mechanical Engineering, Tarbiat Modares University, Tehran
author
Mohammad Hossein
Sadeghi
استاد دانشگاه تربیت مدرس
author
amir
rasti
PhD student, Department of Mechanical Engineering, Tarbiat Modares University
author
mohamad mahdi
hadavi
associated professor, Department of Metallurgy Engineering, Malek Ashtar University of technology, Tehran
author
text
article
2014
per
Micromilling is a machining process in manufacturing of the miniature parts. Because of high oxidation and corrosion resistance, high fatigue strength and application of Ti6Al4V in hi-tech industries, in this paper surface roughness and burr formation in micromilling of this alloy have been investigated. Cutting parameters including spindle speed, feed rate and axial depth of cuthave been considered as input parameters of tests. Experiments have been performed for two cases: a) in presence of the minimum quantity lubrication and b) wet conditions. Carbide micro-end mill tool of diameter 0.5 mm and TiAlN coating was used. The Taguchi experimental design method has been used to design and analysis of results. Results showed that the spindle speed and feed rate were the most effective parameters on the surface roughness and burr width of titanium alloy, respectively. Also, by increasing both of these parameters, surface roughness and burr width were decreased. In addition, application of minimum quantity lubrication technique significantly improved the surface quality, and it was more effective in upper levels of spindle speed and axial depth of cut. Finally, the best surface quality was attained in spindle speed of 30000 rpm, feed rate of 0.8 μm/tooth and cutting depth of 60 μm.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
84
90
http://mme.modares.ac.ir/article_11817_28da20b80ad8bb566c0932071256a845.pdf
A model for the effective thermal conductivity of nanofluids – effect of interfacial layer and non-uniform size distribution of nanoparticles
Mohammad
Moghiman
استاد عضو هیئت علمی دانشگاه فردوسی مشهد
author
Zohreh
Shams
دانشجوی دکتری مهندسی مکانیک دانشگاه فردوسی مشهد
author
text
article
2014
per
This work presents a model for calculating the effective thermal conductivity of nanofluids. In this method, the effect of non-uniform sizes of nanoparticles and interfacial layer is investigated simultaneously. The developed model for the thermal conductivity of nanofluids takes into account the effects of the thermal conductivity of base fluids, the thermal conductivity, the volume fraction and the size of nanoparticles, the interfacial layer, non-uniform sizes of nanoparticles, Brownian motion and temperature. Hence, this model has the capability of offering both analytical and numerical Predictions. The accuracy of proposed model for the effective thermal conductivity of water-〖 Al〗_2 O_3, ethylene glycol-〖 Al〗_2 O_3, water- CuO, ethylene glycol-CuO, ethylene glycol-Al, water- TiO_2 is investigated. The effect of temperature, size of nanopartcles and volume fraction of nanopartcles is determined. Results show that the interfacial layer at the nanoparticle-liquid interface and non-uniform sizes of nonparticles are the important parameters for calculating the thermal conductivity of nanofluids. The Comparison between the result and available experimental data of several types of nanofluids indicates that the proposed model provides accurate results and the maximum error is 5%.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
91
98
http://mme.modares.ac.ir/article_11819_0f29dc02685502c2aefe30264e8763be.pdf
Effects of regular and random distribution of silica nanoparticles on the thermo-elastic and viscoelastic properties of polymer nanocomposites- Micromechanics-based analysis
Reza
Ansari
استادیار دانشگاه گیلان
author
Mohammad Kazem
Hassanzadeh
دانشجوی دکترا
author
text
article
2014
per
In this study, a three-dimensional micromechanics-based analytical model is developed to study the effects of regular and random distribution of silica nanoparticles on the thermo-elastic and viscoelastic properties of polymer nanocomposites. The Representative Volume Element (RVE) of nanocomposites consists of three phases including silica nanoparticles, polyimide matrix and interphase. Since the polymer in the vicinity of the nanoparticles shows distinct properties from those of the bulk matrix, because of nanoparticle–polymer matrix interactions, this region as interphase is considered in micromechanical modeling with specific thickness and properties. In order to simulate random distribution of silica nanoparticles into polyimide matrix, the RVE is extended to c×r×h cubic nano-cells in three dimensions. Perfect bonding conditions are applied between the constituents of RVE. It is assumed that all three phases of the RVE to be homogeneous and isotropic to obtain the thermo-elastic response of nanocomposite. The extracted thermo-elastic properties by the micromechanical model with random distribution of silica nanoparticles are closer to the experimental data. To predict the effective viscoelastic properties of the nanocomposites, silica nanoparticles are modeled as a linear elastic material, while polyimide matrix and interphase are assumed to be as a linear viscoelastic material. The model is also used to examine the influence of varying interphase properties and silica nanoparticle size on the effective nanocomposite behavior. The overall creep behavior of the nanocomposite for several stress levels is also presented.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
99
107
http://mme.modares.ac.ir/article_11818_42f563da8980b57ac842ad73939fb7ba.pdf
Analytical and experimental modeling of cutting force in plasma assisted turning of hardened steel
Masoud
Farahnakian
Ph.D student, Department of Mechanical Engineering, Amirkabir University of Technology
author
Mohammad Reza
Razfar
دانشیار، دانشکده مهندسی مکانیک، دانشگاه صنعتی امیرکبیر
author
Farid Reza
Biglari
دانشیار، دانشکده مهندسی مکانیک، دانشگاه صنعتی امیرکبیر
author
text
article
2014
per
Plasma assisted machining (PAM) is a method to improve machinability of hard turning. The process of plasma assisted machining for turning applications utilizes a high-temperature plasma arc to provide a controlled source of localized heat, which softens only that small portion of the work material removed by the cutting tool. The goal of this study is to present a methodology for determination cutting force during plasma enhanced turning of hardened steel AISI 4140. In this regard, a finite differential model was made to estimate the uncut chip temperature under different plasma currents, cutting speeds and feeds during PAM. A mechanistic model developed to estimate cutting force under different PAM conditions by considering shear stresses in the primary, secondary shear zones and force on the tool edge. The proposed model was calibrated with experimental hard turning data, and further validated over practical PAM conditions. Mean errors of predicted values and experimental data is lower than 10 percent. It is shown that PAM can decrease main cutting force in comparison to convectional to 40 percent in turning of hardened steel at high levels of uncut chip temperature due to softening the material.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
108
116
http://mme.modares.ac.ir/article_11854_b029d98fcbcd2ba8c31f543ab0215df9.pdf
Effect of Secondary Flow Injection Area on Thrust Vectoring Angle in Double Throat Nozzles
Hadi
Hamedi
دانشجوی کارشناسی ارشد
author
Mehdi
Jahromi
استادیار دانشگاه صنعتی مالک اشتر- مجتمع هوافضا
author
Mostafa
Mahmoodi
استادیار
author
Jamasb
Pirkandi
استادیار دانشگاه صنعتی مالک اشتر
author
text
article
2014
per
The present paper discussed about the technique that can be used to vector the exhaust flow in the pitch directions with using Double Throat nozzle (DTN). Compressible and supersonic gas flow inside a Double Throat nozzle and its exhaust plume at specific nozzle pressure ratios have been numerically studied with several turbulence models. The numerical results reveal that, the SST k–ω model gave the best results compared with other models in time and accuracy. In the present research, effects of changes in injection area of secondary flow and percent of secondary mass flow rate, on performance of Double Throat nozzle and thrust vectoring system have been investigated. The predicted results show that by decreasing the value of secondary flow injection area in a case with 7% secondary injection, the thrust vector angle increase 18º to 21º and thrust vectoring efficiency will increase. But by decreasing the value of secondary flow injection area, the thrust and discharge coefficient will decrease. Also when secondary mass flow rate increases, the discharge coefficient will decrease. So that in the design of fluidic thrust vectoring with double throat nozzle, the value of secondary mass flow rate should be low.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
117
125
http://mme.modares.ac.ir/article_11855_1724eb0df1b08af069ee6b09f89e7224.pdf
The effect of pass numbers over microstructure and mechanical properties of magnesium alloy of AZ31C in the tubular channel angular pressing (TCAP) at temperature of 300 °C
Abdulrahman
Soufi Mohammadi
Student/University of Tehran
author
Mahmoud
Mosavi Mashhadi
استاد/دانشگاه تهران
author
Ghader
Faraji
ssistant Professor/ University of Tehran
author
text
article
2014
per
This research studies pass numbers effect on microstructure and mechanical properties of magnesium alloy of AZ31C in the tubular channel angular pressing (TCAP) at the temperature of 300 °C. Pressing process has been carried out through four pass over AZ31C Magnesium tubes and in each pass the sample is exposed to Tensile and microhardness test and Metallography. The microstructure and mechanical properties of processed tube through one to four passes of TCAP process were investigated. Microhardness of the processed tube was increased to 62Hv after one pass from an initial value of 55 Hv. An increase in the number of passes from 1 to higher number of passes has not more effect on the microhardness. Yield and ultimate strengths were increased 1.97 and 1.49 times compared to as cast condition. Notably increase in the strength was achieved after two pass TCAP while higher number of passes has not more effect. Microstructural investigation shows notably decrease in the grain size to around 6 µm from the primary value of ~200 µm. microscope images show that the grain size is getting smaller by the first pass buy bigger in the next passes.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
126
130
http://mme.modares.ac.ir/article_11856_798ba2653ef0eca9139ed4c94edef853.pdf
Investigation of energy harvesting from vibrating nonlinear Timoshenko beam under base oscillating with electromagnetic energy harvester
Behzad
Janizadeh
دانشجو
author
Morteza
Dardel
عضو هیات علمی
author
Mohammad Hadi
Pashaei
استاد دانشگاه
author
Reza
Akbari Alashti
استاد دانشگاه
author
text
article
2014
per
Abstract In this work, nonlinear electromagnetic vibration energy harvesting from cantilever beam under base harmonic oscillation is investigated and the effects of electromagnetic parameters on behavior of system is considered. For modeling assumed mode method is used, and beam is modeled in according to Timoshenko theory, which includes shear deformation and rotary inertia. In energy harvesting, the frequency response of the system is very important because it shows the best areas for energy harvesting and it is a good criterion for designing energy harvesters, hence a semi analytical method is used to find simply the amplitude of oscillation in terms of excitation frequency. In this method, at first equations of motion are solved with complex averaging method and obtained equations are solved with continuation method. For validation, comparison between results obtained from numerical and semi analytical method is given. Also comparison between linear and nonlinear system, and stability of periodic response and their bifurcations are given. In addition, in order to the effect of number of mode shapes and convergence of solution, frequency response of one, two, three modes and four modes cases are compared with together.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
131
140
http://mme.modares.ac.ir/article_11857_8df3f3b192e04b168089181f5987c0c3.pdf
The Effect of Advanced BBC2003, Yld2004 and BBC2008 Yield Criteria on FLDs Based on M-K, Swift's and Hill's Models
Mohammad
Hossein Pour
استادیار عضو هیئت علمی دانشگاه تربیت دبیر شهید رجایی
author
S.M. Hossein
Seyedkashi
استادیار/دانشگاه بیرجند
author
Sepideh
Shahi
دانشگاه تربیت دبیر شهید رجایی
author
text
article
2014
per
In this paper, effect of advanced yield criteria including BBC2003, Yld2004 and BBC2008 are studied using Swift hardening law on determination of limit strains based on Marciniak–Kuczynski (M-K), Swift’s diffuse necking and Hill’s localized necking models. These models are used to assess the formability prediction of orthotropic AA2090-T3 and AA6111-T4 sheets. The Portevin-Le Chatelier effect on formability of AA5182-O sheet is also studied. The sensitivity of Marciniak–Kuczynski model to the initial imperfection factor is investigated and the effect of this parameter on prediction of forming limit diagram has been shown. Study of the effect of yield criteria exponent on formability shows the higher exponents which are recommended for FCC metals have higher forming limits in comparison with lower exponents adopted to BCC metals. The effects of strain hardening coefficient and anisotropy are studied too. The results show that the increase of strain hardening exponent improves the formability, while the PLC effect and increase of anisotropy coefficient reduce it. Experimental results show better conformity with M-K model at the right side of FLD and with Hill’s localized necking model at its left side.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
141
150
http://mme.modares.ac.ir/article_11858_06670519bd70ed20ba9142787b564cbe.pdf
Slip compensation of grasped objects in robot gripper
Habib
Ahmadi
دانشگاه شارود
author
Hanieh
Esmaeeli
دانشگاه شاهرود
author
Mohammad Mehdi
Fateh
دانشگاه شاهرود
author
text
article
2014
per
Grasping in robot gripper is an operation that is inevitably performed by prosthetic hands or industrial robots. Meanwhile, slipping of the grasped object is considered as an undesirable phenomenon in any kind of grasping. Here, the computed torque control is used in order to slip control and also guarantee the desired behavior of the closed loop system. Nevertheless, any acceleration changes of the robot’s joints before completing the response time of the slip controller, influence directly on the object position relative to the robot and causes slip phenomenon. However, the applied computed torque controller is proper for tracking trajectory but this desired trajectory will be altered according to slip occurrence. This paper introduces a method to modify the desired trajectory during grasping an object. The modification is done according to the measured slip. These methods not only control the slip of the grasped object, but also compensate it. So the object could be handled and placed in its proper position in the task space. This approach guarantees the safe grasping and moving objects according to object position relative to the gripper.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
151
162
http://mme.modares.ac.ir/article_11859_51ca7cd816dcb107dfe6612a5daf2f2f.pdf
Numerical investigation of magnus wind turbine new blade section performance
Seyed Ali
Kazemi
دانشگاه صنعتی اصفهان
author
Mahdi
Nili Ahmadabadi
Isfahan Univ. of Tech.
author
Mohsen
Saghafian
دانشگاه صنعتی اصفهان
author
Ahmad
Sedaghat
دانشگاه صنعتی اصفهان
author
text
article
2014
per
The ratio of lift to drag coefficient in wind turbine blades is within the most important parameters affecting the power coefficient of wind turbines. Due to the performance of Magnus wind turbines in low speed air flow; such turbines are attractive for research centers. In the present work, a new geometry for the blades of Magnus wind turbines is defined. The defined geometry is based on the geometry of a Treadmill with a difference that the diameter of its leading circle is greater than that of its trailing one. In the present work, the body is supposed to a low speed air flow while a tangential velocity is applied to the airfoil surfaces and then, its effect on the lift and drag coefficient is studied by numerical method. The effect of generated tangential velocity on the surfaces is investigated for different air flow speed and attack angles and then, its results are compared with that for stationary surfaces. The results show that generating tangential velocity along the surfaces caucuses the lift and drag coefficients and, their ratio to be varied, greatly. By the tangential movement of the surfaces, the maximum ratio of lift to drag coefficient occurs in zero attack angle which is equal to 109. Moreover, maximum magnitude of lift to drag coefficient for attack angles 5, 10, and 15 degrees are 81, 64, and 57; respectively.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
163
172
http://mme.modares.ac.ir/article_11860_9bc30a72d1325982107cb7e4c27eb847.pdf
Analysis of the passengers effect on chaotic vibrations of a nonlinear full vehicle model
Javad
Fakhraee
دانشجو
author
Heshmatallah
Mohammad Khanlo
استادیار دانشگاه شهید ستاری
author
Mostafa
Ghayour
Professor
author
text
article
2014
per
In this paper, the effect of passengers on the chaotic vibrations of the full vehicle model is investigated. The vehicle system is modeled as a full nonlinear seven-degrees of freedom with an aditional one -degree of freedom for each passenger. Four passengers are added sequentially to the vehicle that produces eight, nine, ten and eleven degrees of freedom models, respectively. The effect of passengers on the chaotic vibrations of vehicle is studied for the above mentioned cases. The nonlinearities of the system is due to the nonlinear springs and dampers that are used in the suspension and tires. Roughness of the road surface is considered as sinusoidal waveforms with time delay for tires. The governing differential equations are extracted by Newton-Euler laws and are solved numerically via forth-order Runge-Kutta method. The analysis is conducted first by detecting the unstable regions of the system and then followed by a specific excitation frequency, where there is possibility of chaos. The dynamic behavior of the system is investigated by special nonlinear techniques such as bifurcation diagram, power spectrum, pioncare section and maximum lyapunov exponents. The obtained results represents different types of nonlinear dynamic absorbers in the vehicle with and without passengers. Consideration the passengers and increasing the mass of the system can resultes in a significant changes in the dynamic behavior where improves the chaotic vibration of the vehicle.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
173
184
http://mme.modares.ac.ir/article_11862_3e077a7b1e404f852e2c846349630193.pdf
Experimental investigation of quasi-static perforation on laminated glass epoxy composites by indenters with different geometries
Erfan
Mehrabani Yeganeh
دانشجوی کارشناسی ارشد / دانشگاه تربیت مدرس
author
Gholam Hossein
Liaghat
دانشگاه تربیت مدرس
author
Mohammad Hossein
Pol
استادیار / دانشگاه تفرش
author
text
article
2014
per
This paper, experimentally evaluates the effects of indenter geometry on quasi-static perforation process of laminated woven glass epoxy composites. Low loading rate tests were performed, using six indenters with blunt, hemispherical, conical (cone angle of 37˚ and 90˚) and ogival (caliber radius head of 1.5 and 2.5) nose shapes. Composite behaviors like energy absorption, contact force, failure mechanisms and friction force were investigated for different indenter shapes. Hand lay-up method has been used to manufacture composite targets with 18 layers of 2D woven glass fibers of 45% fiber volume fraction. The epoxy system is made of epon 828 resin with jeffamine D400 as the curing agent. The results show that the load displacement curve is divided to five areas. Some of these areas may have higher or lower magnitude, depending on indenter nose shape. The highest contact force is exhibited by unsharpened indenter. The lowest contact force and so the best performance is seen in ogival (CRH=2.5) indenter. Comparing absorbed energies shows that for an identical dent depth, the amount of absorbed energy is major for unsharpened indenters. The 37˚ conical indenter needs the highest energy for perforation, which is 2.6 times more than blunt indenter’s.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
185
193
http://mme.modares.ac.ir/article_11861_de318551581f0eedf2847e5c31a64e32.pdf
Evaluating the effects of layer arrangement of a porous clothing on heat and mass transfer from the body under cold conditions
Hassan
Hassanzadeh
استادیار گروه مهندسی مکانیک دانشگاه بیرجند
author
Seyed Alireza
Zolfaghari
عضو هیات علمی گروه مهندسی مکانیک دانشگاه بیرجند
author
Hossein
Shariati
دانشجوی کارشناسی ارشد مهندسی مکانیک دانشگاه بیرجند
author
text
article
2014
per
Although,the physical activity in the cold condition causes the body temperature to rise,it can be a significant factor in the occurrence of thermal discomfort due to increase in the perspiration rate and water gathering in the fabric.Moreover,the accumulated water at the inner side of the clothing can cause a difficulty in the skin respiration. So, the amount of accumulated water and interior surface wetness are important indices for evaluating the suitability of clothing for winter activity. The aim of this study is to determine the amount of accumulated water in various arrangements of multi-layer clothing assemblies containing of three bathing layers of Polyester and Viscose in a very cold environment (with -20C temperature).For this reason,the clothing has been modeled as a porous media with multi-phases and multi-species flow by considering the sorption and condensation phenomena.Also,the implicit finite volume numerical method has been used for discretizing and solving the governing equations.The results show that locating the non-absorbing polyester fabric at the layer adjacent to the skin causes the wetness to decrease at this region. Also, locating the polyester at the outer layer can help to maintain the clothing temperature at the proper conditions.Also,the results indicate that using the viscose fabric as the middle layer leads to decrease in the water content value at the center of clothing. Therefore, the “polyester-viscose-polyester” arrangement can properly remove the perspiratory moisture from the skin to environment, with the minimum of inner water content index (0.02) and maximum inner surface temperature(33C) and average clothing temperature(16.1C).
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
194
202
http://mme.modares.ac.ir/article_11863_954c540a1a83bb0dc744f915b23aff16.pdf
Experimental investigation of force convection heat transfer in porous channel with internal heat generation
meysam
nazari
MSC student of mechanical engineering of sahand university of technology
author
Davood
JalaliVahid
Associate Professor, Department of Mechanical Engineering, Sahand University of Technology
author
samira
khanjani
MSC student of chemical engineering of sahand university of technology
author
text
article
2014
per
In this study forced convection heat transfer in a pebble bed cylindrical channel with internal heat generation was investigated experimentally. Dry air has been used as working fluid in heated spheres cooling process. Internal heating was generated uniformly, by electromagnetic induction heating method in a metallic spheres which have been used in test section. Spheres are made of stainless steel and their diameter is in the range of 5.5-7.5 mm. Present study was performed at steady state and turbulence flow regime, with Re number in the range of 4500-9500. Different parameters resulted by variation of spheres diameter, flow velocity and generated heat on forced convection heat transfer was studied. According to thermal and hydrodynamics studies, it can be said as Re number increases, heat transfer coefficient will increase. Also heat transfer coefficient has been increased by spheres diameter decrement. The generated heat has a little influence on heat transfer coefficient. The effect of pressure variations on forced convection heat transfer can be neglected. Porous channel has greater friction factor in comparison with an empty channel. The friction factor in empty channel is always less than 1 but for porous channel this parameter is in the range of 10-25.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
203
210
http://mme.modares.ac.ir/article_11864_7b9ef9edb79ccddef0fb8226d038f72b.pdf
Thermo Elasto-Plastic analysis of functionally graded thick-walled cylindrical shells based on Prandtl-Reuss flow rule
Fatemeh
Farhatnia
Academic member of Islamic Azad University-Branch of Khomeinishahr
author
Mahsa
Ghanbari
دانشگاه آزاد اسلامی واحد خمینی شهر
author
text
article
2014
per
This study deals with thermo-elasto-plastic behaviour of functionally graded thick-walled cylinder that is exposed to internal pressure and temperature gradient. For this purpose, Von-Mises yield criterion and Prandtl-Reuss flow-rule under state of plane strain are utilized. The modulus of elasticity, the thermal conductivity and thermal expansion coefficients are assumed to obey the power function in the radial position according to Erdogan’s model. In this work, the presented approach leads to the definition of new formulation to determine the elastic limit pressure and predict the onset radius of yielding, spread and growth of plastic zone. The governing equilibrium equation of cylindrical shell in axi-symmetrical status is solved in order to determine the distribution of radial, circumferential stresses and radial displacement. Various examples are handled to investigate the effect of FG-power law parameters on the yield pattern and distribution of plastic zone. The distribution of radial displacement, radial and circumferential stresses are expressed as the functions of radial position. The numerical results show that by the appropriate choice of the FG parameters and the specified thermal gradient, the plastic zone can commence simultaneously from inside and outside or intermediate radius.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
211
218
http://mme.modares.ac.ir/article_11870_2196bcb1afd046d5f4e81ced2eb089d2.pdf
Experimental and numerical investigation of the maximum deflection of circular aluminum plate subjected to free air explosion
Jamal
Zamani
دانشگاه صنعتی خواجه نصیرالدین طوسی
author
Mohsen
Goudarzi
دانشگاه خواجه نصیرالدین طوسی، تهران
author
text
article
2014
per
In this study, the numerical and experimental investigation of the maximum deflection of circular plates under shock wave from the air blast is discussed. Shock wave will generated by explosion of a spherical charge at different distances from center of the plate. Two series of tests were designed, in the first series of non-uniform shock wave reached the structure, and the second series of shock waves, is uniformly. The purpose of design and implementation of experiments to investigate the effect of waves on the deformation behavior and extract semi-empirical model to predict the maximum deflection of the center of the circular plate subjected normal shock wave uniform and non-uniform. These two models as a function of the Nurick damage number, that effects of all parameters of explosive material and structures are summary and dimensionless, are presented. Then for the experimental verification using finite element software LS-DYNA, the numerical simulation of the behavior of plate under free air explosion was performed and the results were compared with experimental results. Results obtained with acceptable margin of error is close to the experimental results. Finally the results obtained were compared with semi-empirical models of other scholars who have researched this area
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
219
226
http://mme.modares.ac.ir/article_11872_5aa43fa5887dcb7e082d1c71295bf474.pdf
Point clouds curvature estimation using umbrella curvature
khalili
khalili
دانشیار هیات علمی- دانشگاه بیرجند
author
Abolfazel
Foorginejad
عضو هیات علمی دانشگاه صنعتی بیرجند
author
text
article
2014
per
With the rapid development of 3D laser scanners, point-based discrete shape modeling is being widely used in many engineering applications, e.g. quality control, reverse engineering, computer graphics and machine vision. Point cloud discrete curvature estimation is considered basic operation in point cloud operations and is used in many applications related to cloud points. This paper presents a novel method for point clouds surface curvature estimation. One of the key components of point clouds surface curvature calculation is neighbor coordinates of query point. For selecting neighbors homogeneous neighborhood method is used. This method of choosing neighbors, in addition to the distance takes into consideration the directional balance by improving the k nearest neighbors. Surface normal vector is estimated by neighbors coordinates. In this paper surface curvature is calculated based on normal vector and homogeneous neighbors coordinates. Surface curvature calculated using the novel method is called umbrella curvature. To evaluate how this method performs, umbrella curvature values are calculated for a number of cloud points and the results are used in some different applications. The results show that the proposed method performs well in point clouds curvature estimation.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
227
235
http://mme.modares.ac.ir/article_11871_96afc36e0ef1d9d8da2911af2839de60.pdf
Optimal Parameter Estimation of Ultrasonic Signals by Using a Combination of Particle Swarm Optimization and Gauss-Newton Algorithms
Ali
Gholami
دانشکده مکانیک دانشگاه صنعتی خواجه نصیرالدین طوسی
author
Farhang
Honarvar
دانشکده مکانیک دانشگاه صنعتی خواجه نصیرالدین طوسی
author
Hamid
Abrishami Moghadam
دانشکده برق دانشگاه صنعتی خواجه نصیرالدین طوسی
author
text
article
2014
per
The echoes obtained from ultrasonic testing of materials contain valuable information about the geometry and grain structure of the test specimen. These echoes can be modeled by Gaussian pulses in a model-based estimation process. For precise modeling of an echo, the parameters of the Gaussian pulse should be estimated as accurately as possible. There are a number of algorithms that can be used for this purpose. In this study, three different algorithms are used: Gauss-Newton (GN), particle swarm optimization (PSO), and genetic algorithm (GA). The pros and cons of each of these three algorithms are reviewed and by combining them, the benefits of each algorithm are used while its shortcomings are avoided. For signals containing multiple echoes, the minimum description length (MDL) principle is used to estimate the numbers of required Gaussian echoes followed by space alternating generalized expectation maximization (SAGE) technique to translate it to separate echoes and to estimate the parameters of each echo. The performance of the proposed algorithms for simulated and experimental signals with overlapping and non-overlapping echoes is evaluated and shows to be quite effective.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
236
244
http://mme.modares.ac.ir/article_11873_751a760acec3c1508a6040d64c0f2175.pdf
Optimization of Surface roughness in stereolithography
Seyed Javad
Aaraj Khodaii
دانشگاه صنعتی امیرکبیر، دانشکده مهندسی مکانیک، گروه ساخت و تولید
author
Abdolreza
Rahimi
دانشگاه صنعتی امیرکبیر، دانشکده مهندسی مکانیک، گروه ساخت و تولید
author
text
article
2014
per
Stereolithography has the most portions through rapid prototyping techniques in injection molding and vacuum casting manufacturing because of simultaneously possessing of dimensional accuracy and strength. However low surface finish and appearance of stair-step phenomenon, restrict extension of use of this process. In this research, the influence of process parameters and part orientation on surface finish was studied. For this purpose parts were built in various conditions of surface angle, hatch space and post curing time. Surface roughness measured by contact and non-contact method. Surface angle was in 0-180 degree range by 2 degree step. Considered post curing time was 20, 50 and 80 minute and hatch spacing was 50, 75, 100 and 125 micrometer. In non-contact method using digital microscope, surface profile obtained and then surface roughness calculated using MATLAB software. Finally using Analysis of Variance (ANOVA) a mathematical model has been developed among parameters and responses. Results showed that surface finish has reverse relation with hatch space and are almost Independent of post curing time. Surface roughness in up-facing increases with surface angle enhancement swiftly and then it drops. In down-facing Surface roughness increases with surface angle enhancement slowly and then it falls. Comparison of real dates with estimated values showed that estimation average error is less than 14 percent.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
245
256
http://mme.modares.ac.ir/article_11890_250cb7b2856364d8a4ffd19263581317.pdf
Reliability Assessment of Multi-State System Based-On Fault Tree Analysis
mohammad ali
farsi
عضو هیئت علمی
author
Mohammad
Najafi
پژوهشگاه فضایی
author
text
article
2014
per
In this paper, the reliability assessment based-on fault tree analysis in coherent multi-state emergency detection system in a sounding rocket is explained. A system with more complexity and contains much intersection terms was built, that, the analysis of such system whether by means of minimal cut-set nor Binary Decision Diagram (BDD) approaches need complex analysis and will be time consuming owing to intersection terms. Also, the determined results have less accuracy and contain uncertainty. To overcoming this problems, a combinatorial method for solving static fault tree are used. This method combines minimal cut-set with BDD methods, then, it computes the occurrence probability of top-event in fault tree. To describe and show capability of this method, Emergency Detection system (EDS) of a space craft is studied as a case-study. We assume that, overall system consists of two sub-systems: sending signal sub-system and cabin indicator instrumental sub-system. Two different fault trees are constructed; ones for each are constructed and explained. At the end, the results of the multi-state systems consist of multi-state elements are presented. Also, the reliability value of this system is determined.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
257
266
http://mme.modares.ac.ir/article_11891_377d89ab62fde84c3c46fea538cbcb62.pdf
Investigation of stress field around circular tunnels with concrete lining using complex potential functions
Alireza
Kargar
دانشجو در دانشگاه شهید باهنر کرمان
author
Reza
Rahmannejad
استاد دانشگاه شهید باهنر کرمان
author
Mohammad Ali
Haja Abasi
استاد دانشگاه شهید باهنر کرمان
author
text
article
2014
per
Using Muskhelishvili and Kolosov complex potential functions, an elastic solution is presented in this study in order to investigate stress components around circular tunnels reinforced by concrete lining with constant thickness. It was assumed that rock mass and concrete behave as isotropic linearly elastic materials. The rock mass undergoes an in situ stress field. It was also supposed that rock and concrete interface is in no-slip condition so that they have common displacement. Due to complexity of the problem for concrete reinforced layer, conformal mapping functions were utilized in order to find a solution. Supposing plane strain condition, the problem was solved, and a closed-form solution was obtained. The solution was compared to Kirsch solution, in which the lining thickness was reduced to zero, and also ABAQUS finite element software results, which showed a good agreement, except for ABAQUS software predictions around crown of tunnel lining periphery where some discrepancies were found; also it was demonstrated that this solution predicts stress components at inner lining periphery much more accurately than ABAQUS software. Finally, a sensitivity analysis based on rigidity and thickness of liner was conducted and some propositions were made on design of concrete liner. The advantage of this solution lays in the fact that it has quicker and more accurate calculation process compared to numerical methods.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
267
276
http://mme.modares.ac.ir/article_11892_e02f57fbb6d46ec3e7c0cb030418281a.pdf
Investigating the domain length for simulation of gaseous detonation
Saeed
Parvar
دانشگاه تربیت مدرس
author
Kiumars
Mazaheri
Tarbiat Modares University
author
text
article
2014
per
Numerical simulation of gaseous detonation is one of the most challenging problems in computational fluid dynamics (i.e., CFD). The presence of sonic locus at the end of the reaction zone isolates the reaction zone and the leading shock from the far-field flow perturbations, so computational domain may be truncated by artificial boundary conditions. However, some artificial boundary conditions generate spurious waves that introduce some errors into the results. The computational domain is usually considered very large for protecting the domain from spurious waves. A systematic study of boundary conditions’ role in simulation of self-sustained detonation has not been performed yet. In the present study, it is aimed to investigate the influence of the width and length of the computational domain on numerical simulation and the effect of activation energy on the length and width of the domain. Instead of considering a very large domain, the so-called non-reflecting boundary condition is implemented in the present investigation. Characteristics method was employed to define the non-reflecting boundary conditions. Finite length of domain was computed for 1D and 2D simulations. Suitable length of the domain was determined for different activation energies. The results indicate that the suitable length and width of the domain for high activation energy mixtures are larger with respect to the corresponding length and width for low activation energy mixtures. Results also show that, using non-reflecting boundary condition, the computational time decreases considerably for both one and two-dimensional simulations.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
277
284
http://mme.modares.ac.ir/article_11893_888af36b1e48130ff0162ad3811b33a9.pdf
A nonlinear study on transient vibration and limit cycle oscillation of composite sandwich plates with electrorheological fluid layer
Jalil
Rezaeepazhand
عضوهیات علمی گروه مهندسی مکانیک دانشکده مهندسی دانشگاه فردوسی مشهد
author
Jafar
Rahiminasab
PhD Student of Ferdowsi University of Mashhad
author
text
article
2014
per
This paper deals with the nonlinear transient vibration of composite sandwich plates with an electrorheological (ER) fluid core. The initial excitation is a distributed transverse load or the flutter instability due to supersonic airflow. The Bingham plastic model is adopted to accurately model the post-yield behavior of the ER material. . The first order piston theory is used for evaluating the aerodynamic forces. The von Karman strain-displacement relations are employed to account for moderately large deflection. The Hamilton’s principle is applied in conjunction with the finite element method to derive the equations of motion. The solution is then obtained through use the Newmark time integration scheme. Numerical investigations are conducted to study the effect of ER core layer on the vibration characteristics of the sandwich plate. The influence of the electric filed strength, ER core thickness, initial excitation and the boundary conditions on the settling time of transient vibration are also examined. The results show that the damping of transient vibration is significantly improved as the electric field applied to the ER layer, but the amplitude of post-flutter oscillations remains unchanged.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
285
293
http://mme.modares.ac.ir/article_11894_9e06d79cd9f4be008e468225fed3274f.pdf
Dynamic Modeling and simulation of nano particle motion in different environments using AFM nano –robot
Amin
Habibnejad Korayem
School of Mechanical Engineering, Iran University and Science Technology, Tehran, Iran
author
Moein
Taheri
School of Mechanical Engineering, Iran University and Science Technology, Tehran, Iran
author
Moharam
Habibnejad Korayem
Prof. of Mechanical Eng. of Iran University of Science and Technology
author
text
article
2014
per
Nowadays one of the arguments that have been raised in the world of nanotechnologies is moving or manipulation nanoparticles. This discussion is important because the displacement of nanoparticles can make structurally different than what is currently available. So to achieve this goal, the atomic force microscope probe is used as manipulator. In this way, the use of nanoparticles by pulling or pushing on the surface, are displaced and brought to the desired point. If you apply too much force is needed, Nanoparticle Continued movement (sliding or rolling) after standing atomic force microscopy probes and away from the desired final. On the other hand, if the force is low, so that it can’t overcome the static friction force, Nanoparticles will be no movement. So finding the optimal force is important in nanomanipulation. In this paper, with using nanoparticle dynamic simulation, the governing equations on nanoparticle are derived and simulated during manipulation happen that they can be used to obtain the critical force and time for gold, yeast and platelets nanoparticles, in gaseous, water, alcohol, and plasma environments. By comparing the results obtained in this paper, it is concluded that the movement of particles in different biological environments starts later and by a force of higher magnitude relative to the gaseous medium.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
294
300
http://mme.modares.ac.ir/article_11895_a622d4deb2894c5dbf3c2c12201af042.pdf
A methodology for positioning of holes in extrusion with multi-hole flat-faced dies to minimize exit profiles distortion
Sina
Nazari Onlaghi
M.Sc./Sharif University of Technology
author
Ahmad
Assempour
استاد دانشکده مهندسی مکانیک دانشگاه صنعتی شریف
author
text
article
2014
per
In the extrusion of sections with a multi-hole flat-faced die, the proper positioning of the die holes is of critical importance in avoiding the appearance of geometrical defects. In this paper, a methodology has been presented for radial positioning of the die holes in multi-hole extrusion process. A die with two non-symmetric T-shaped holes has been chosen as the computational example. A kinematically admissible velocity field at deformation zone has been obtained. The effects of dead metal zone formation have been considered in prediction of the velocity field. To measure the exit profile curvature a deviation function has been suggested. Using the proposed function, the velocity field has been used for prediction of the exit profile curvature and accordingly positioning of the die holes. It was found that a balanced metal flow at the exit of extrusion die could be achieved if the position of holes is near the centroid of the die area. In order to validate the results, finite element simulation has been used. The proposed methodology can be extended to dies with greater number of holes and more complex shapes. This methodology helps the die designer to have a better quality extrusion process.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
301
309
http://mme.modares.ac.ir/article_11899_f7307d54fc256b0ea6e80d88009f3c59.pdf
Effect of different geometrical parameters of atomic force microscope cantilevers in critical force and time based on manipulation with applying EFAST sensitivity analyses
Moharam
Habibnejad Korayem
Prof. of Mechanical Eng. of Iran University of Science and Technology
author
Sepehr
Dehghani Ghahnaviyeh
School of Mechanical Engineering, Iran University and Science Technology, Tehran, Iran
author
Mehdi
Ghasemi
School of Mechanical Engineering, Iran University and Science Technology, Tehran, Iran
author
Moein
Taheri
School of Mechanical Engineering, Iran University and Science Technology, Tehran, Iran
author
text
article
2014
per
In nanoparticles manipulation with atomic force microscope for modeling exact manipulation dynamics and prevent of damaging nanoparticles, it is necessary to compute critical force. For modeling dynamics and computing critical force that apply to nanoparticles it is necessary to modeling cantilever stiffness and determine sensitive geometrical parameters which are effect cantilever stiffness and critical force. In this paper at first it is investigated on two common different kinds of cantilevers which are V-shaped and dagger cantilevers. For modeling V-shaped cantilever stiffness this cantilever is divided into two parts a triangular head section and two slanted rectangular beams. After that the stiffness of each part is modeled separately and the total stiffness is computed. For modeling dagger cantilever stiffness it is used the same method and cantilever is divided into two parts a triangular head section and a rectangular beam and then the total stiffness is computed. Cantilevers stiffness and critical force in manipulation of biological particles and non-biological particles are very important because of that it is used EFAST sensitivity analyses for selecting suitable cantilever and its parameters. In this paper it has been shown that the dagger-shaped cantilever is more suitable for the manipulation of biological particles.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
310
316
http://mme.modares.ac.ir/article_11901_29c08a06b824014fcfd03c337563cc05.pdf
Lubrication enhancement in deep drawing process by utilizing nanoparticle additives
Behrooz
Zareh-Dsari
داشجوی دکتری تخصصی مهندسی مکانیک ساخت و تولید دانشگاه تبریز
author
Mohammad
Abaszadeh
دانشگاه صنعتی ارومیه
author
Behnam
Davoodi
دانشگاه تبریز
author
text
article
2014
per
Lubrication is an essential factor in sheet metal forming processes such as deep drawing in order to reduce friction at contact surfaces, forming load, tool wear rate and increasing of sheet formability. Various metal oxide nanoparticles can be used as additives to create desirable tribological properties in base lubricants because of their unique properties such as specific surface area. In the present study, the conventional lubricant enhanced by alumina nanoparticles (Al2O3) is utilized in deep drawing process in order to improve frictional conditions. The forming load, surface roughness (Ra) and thickness distribution values of the formed cups were assessed to evaluate the performance of the enhanced conventional lubricant with alumina nanoparticles (Al2O3) in comparison to the conventional lubricant and dry forming condition. The obtained results from experimental tests revealed that adding 0.5 wt.% Al2O3 nanoparticles to the conventional lubricant improves lubrication property significantly and reduces forming load by 16.39% and surface roughness by 19.33% compared to the conventional lubricant. Furthermore, it is observed that using lubricant containing nanoparticle additives results in 23.94% improvement in maximum thickness reduction in critical zone.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
317
322
http://mme.modares.ac.ir/article_11900_e05557c4483409e9d26825d664238ac9.pdf
Experimental Investigation of dissimilarity between heat transfer and momentum transfer in a distributed boundary layer using design of experiments method
Ghodrat
Ghassabi
دانشجو
author
Mohsen
Kahrom
Professor of Mechanical Engineering/
Thermodynamics and Fluid Mechanics
author
text
article
2014
per
In previous studies, there is no comprehensive experimental study that has evaluated dissimilarity between heat transfer and momentum transfer for all the interactions between effective variables. On the other hand, when a rectangular cylinder is located near a flat plate, skin friction coefficient and heat transfer coefficient are effected some variables that change in an extensive range. Therefore, testing all possible combinations of effective variables will not be reasonable. In this paper, maximum and minimum of skin friction coefficients and heat transfer coefficients were determined using robust Taguchi design. Design of experiments method was applied for decreasing the number of experiments without losing the required information in the first step. Then, experiments were performed in a wind tunnel, the maximum speed of which was 13 m/s. Finally, skin friction coefficient and heat transfer coefficient were optimized using Taguchi method and Minitab software. Results showed that dissimilarity between heat transfer and momentum transfer has occurred for all the possible combinations of the effective variables. Additionally, the gap height between the rectangular cylinder and flat plate was the most effective variable on generating the dissimilarity.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
323
331
http://mme.modares.ac.ir/article_11902_3dfa0e62b8bfd6f798bf8201aa204cdc.pdf
Proper locus for mounting strain gage around cold worked hole for residual stress determination
Ehsan
Pooladi Borj
Mechanical engineering department, engineering faculty, Ferdowsi university of Mashhad
author
Khalil
Farhangdoost
دانشیار دانشگاه فردوسی مشهد
author
text
article
2014
per
Cold working a hole decreases tendency of fatigue crack near the hole to initiate or growth. It is due to creation some compressive tangential residual stresses around the hole. Determination of mentioned residual stresses with a non-destructive, simple and non-expensive method is the key step in design process of holed components. In this article, residual stresses have been determined by mounting some strain gages around the hole and in fact surface strains during cold working process have been introduced as a feature for residual stress field. Delineation the numbers of needed strain gages and also proper place for mounting them around the cold worked hole is the main object of this research. Results have a good agreement with test result of cold working on specimens made of Al2024. According to the results, mounting two strain gages at same radius in opposite side of hole edge; which one in radial and another in tangentially direction; is needed for determining the residual stress field. Also, strain gages should be mounted in elastic zone. Mounting the gages in plastic zone led to have errors and unreliable results.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
332
338
http://mme.modares.ac.ir/article_11903_d40b13ab387fc760b17518bce8ba2bad.pdf
Configuration design and layout optimization in GEO satellite
mahdi
fakoor
University of Tehran
author
Marziye
Taghinnezhad
دانشگاه تهران
author
Amir Reza
Kosari
دانشگاه تهران
author
text
article
2014
per
This research deals with configuration design and layout optimization of communication satellite. First, an approach is proposed to design the configuration of GEO satellite. Since propulsion subsystem in GEO satellite is the massive item, it has a significant impact on satellite configuration. Consequently, it is necessary to consider the propulsion subsystem influence on satellite configuration. Then layout design process of the satellite components which is one of the complex problems in engineering is performed. In this paper, in order to optimize the layout design of satellite components, the algorithm which consists of two stages, primary and detail layout, is proposed. In order to express geometric constraints mathematically, the Finite Circle Method (FCM) is used. For The mathematical expression of performance constraints, the distance constrains related to distance relationships between components have been developed. The hybrid optimization method is proposed to optimize layout design which is a combination of Simulated Annealing optimization and Quasi Newton methods. The optimization method validation is applied on simple test problem. Finally, the proposed algorithm for configuration and optimal layout design is implemented on communication satellite. The results show that product of inertial (objective function) are minimized and considered constrains of communication satellite are satisfied.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
339
351
http://mme.modares.ac.ir/article_11915_1fab4f107eb1a36cfba4c0fd81b96fb1.pdf
A new inverse method for determination of unidirectional ply mechanical properties of a laminated composite
Mahmood
Mehrdad Shokrieh
استاد دانشگاه علم و صنعت ایران دانشکده مهندسی مکانیک،
author
Reza
Madoliat
دانشگاه علم و صنعت ایران
author
Bahador
Bostani
دانشگاه علم و صنعت ایران
author
Ahmad
Ghasemi
دانشگاه سمنان
author
Vahid
Mohmoodian
دانشگاه علم و صنعت ایران
author
text
article
2014
per
In the present paper, a new combined technique consist of experimental results and numerical solution for determination of elastic constants of thin and thick orthotropic plates with various stacking sequences; and also isotropic plates under different boundary conditions is proposed. This new proposed technique makes use of vibrational test data, corresponding numerical solution and optimization methods. The vibration test data consists of a set of eigen frequencies that are obtained from transverse vibration test of the plate. The numerical solution is based on a finite element method using a commercial program. Material constants of the plate are determined by using of the inverse method and a particle swarm optimization algorithm in MATLAB software. The error function is based on the sum of square difference between experimental data and numerical data of eigen frequencies solution. The validation, performance and ability of the proposed technique in this paper are discussed using experimental and numerical data available in the literature. The higher accuracy of results that obtained by the present method in comparison with other methods proved the validity and capability f the new proposed method.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
352
360
http://mme.modares.ac.ir/article_11917_4b76a97be8401f4bc98675f062e4b177.pdf
Determination of a general close form relation for crack density and induced delamination evolution in cross ply laminates under uniaxial loading condition based on an extended shear lag model
amin
farrokhabadi
استادیار گروه هوافضای دانشگاه سمنان
author
Maryam
Aghaebrahimi
فارغ التحصیل کارشناسی ارشد
author
text
article
2014
per
In the present study, using a precise shear lag parameter in an extended shear lag model, by considering the effects of out of plane shear stresses, the stress fields distribution as well as strain fields and displacement distributions will be obtained for a typical [0m/90n]s cross ply composite laminate containing a specified matrix cracking density. Then, the stiffness degradation due to existence of matrix cracking in these cross-ply composite laminates will be evaluated and specific damage parameters, which affect the stiffness matrix of composite ply, will be defined. Furthermore, using the concept of fracture mechanics by applying two different criteria including the maximum stress and strain energy release rate, the matrix cracking initiation and evolution as well as induced delamination propagation will be studied. Finally, a closed form relation will be presented which predicts the evolution of matrix cracking under uniaxial loading conditions in cross-ply composite laminates. At last, the obtained results by present study will be compared with available semi-analytical and experimental results. The obtained results reveal that the proposed closed form relations by the authors have a less difference with experimental results in comparison with the previous semi analytic results.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
361
370
http://mme.modares.ac.ir/article_11916_7425a830e9546b402d8d6b9dcbe3db7e.pdf
Experimental Investigation of Expanded Metal Tube Absorbers under Axial Impact Loading
mohammad
damghani noori
Associated of Mechanical Engineering of Semnan University
author
Hossein
Hatami
دانشجوی دکتری دانشکده مکانیک دانشگاه سمنان
author
Ali
Ghodsbin Jahromi
دانشجوی ارشد دانشکده مکانیک دانشگاه سمنان
author
text
article
2014
per
Today, in order to reduce the damage caused by the collision, energy absorbers are used. Thin-walled structures are most popular as energy absorbent that are used in various forms. In this research, the cylindrical absorber made of expanded metal sheets (expanded metal tube) under impact loading has been examined. Expanded metal sheets due to their low weight, effective collapse mechanism has a high energy absorption capacity. Two types of absorbers with different cells angle were examined. First, the absorber with cell angle α =0 and then the absorber with angle cell α =90. Tests are done by drop hammer device. The output of device is acceleration - time Diagram which is shown by Accelerometer that is located on the picky mass. In this study the type of collapse, force - displacement diagram and effective parameters has been investigated. From the obtained results it was observed that the absorber with cell angle α =0, have symmetric collapse and had high energy absorption capacity but the absorber with cell angle α =90, had global buckling and the energy absorption value was not suitable.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
371
378
http://mme.modares.ac.ir/article_11920_1f652e95406f05f8b31a97890d81c523.pdf
Numerical study of magnetic fields effects on the electrical conducting non-Newtonian ferrofluid flow through a vertical channel
Mousa
Mohammadpourfard
عضو هیئت علمی/دانشگاه شهید مدنی آذربایجان
author
text
article
2014
per
This Study presents a numerical investigation of the hydro-thermal behavior of a Non-Newtonian ferrofluid (non-Newtonian base fluid and 4% Vol. Fe3O4) in a rectangular vertical duct in the presence of different magnetic fields, using two-phase mixture model, power-law model, and control volume technique. Considering the electrical conductivity of the base fluid, in addition to the ferrohydrodynamics principles, the magnetohydrodynamics principles have also been taken into account. To study the effects of non-Newtonian base fluid using power-law model, assuming the same flow consistency index with viscosity of Newtonian fluid, two different power law indexes (i.e., n=0.8 and 0.6), have been investigated and the results have been compared with that of Newtonian ones (i.e., n=1). Three cases for magnetic field have been considered to study mixed convection of the ferrofluid: non-uniform axial field, uniform transverse field and another case when both fields are applied simultaneously. The results indicate that the overall influence of magnetic fields on Nusselt number and friction factor is similar to the Newtonian case, although, by decreasing the power law index, the effect of axial field on velocity profile, Nusselt number and friction factor become more significant. Moreover, the results indicate that electrical conductivity has a significant effect on the behavior of ferrofluid and cannot be neglected and also negative gradient axial field and uniform transverse field act similarly and enhance both the Nusselt number and the friction factor, while positive gradient axial field decreases them.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
379
389
http://mme.modares.ac.ir/article_11921_f306009755a1d6d3b72d6cf755482383.pdf
Comparison of the Effect of Different Yield loci to Prediction of Ti64 Titanium Alloy Forming Limit Diagram
Ali Akbar
Allahdadian
دانشجوی کارشناسی ارشد / دانشکده فنی مهندسی، گروه مهندسی مکانیک، دانشگاه اصفهان
author
Kourosh
Hasanpour
استادیار دانشگاه اصفهان
author
text
article
2014
per
Mathematical modeling is an important step in the design and optimization of process parameters for metal forming. Researchers have been concerned the metal forming limit diagram as an efficient tool to optimize the production of components using forming methods. Due to the low ductility of titanium alloys and wide applications of these alloys in advanced industries such as aerospace, researchers have focused on studying the forming behavior of these alloys. Due to the high cost of experimental methods, especially at high temperatures, numerical methods, has attracted the attention of many researchers. The accuracy of the numerical methods is affected by model of elastic-plastic material behavior. Unusual mechanical behavior of Ti-64 titanium alloys such as high in-plane anisotropy/asymmetry of yield stress and hardening response has been observed. In this paper, the Marciniak model with Cazacu and Hill yield criterions has been used for forming limit prediction. It is observed that the prediction of forming limit using the Cazacu criterion is closer to the experimental results. This is due to the better prediction of the behavior of the titanium alloy, specially Lankford and stress anisotropy coefficients by Cazacu criterion. Cazacu and Hill criterions prediction of Lankford coefficients and yield stresses have been compared.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
390
396
http://mme.modares.ac.ir/article_11918_281989fd6a744173064a980eb3bc9b23.pdf
Investigation of annealing treatment on the mechanical and metallurgical properties of explosive-welded Al/St/Al multilayer
Mahdi
Pourjafari Kasmaee
دانشجو
author
mohammad
honarpishe
دانشگاه کاشان
author
text
article
2014
per
Explosive welding is used for excellent bonding of similar and dissimilar materials with the wide variety of thicknesses,area dimensions and different thermal and mechanical properties. In this study, an Al/St/Al multilayer sheet was fabricated by explosive welding process and the effects of annealing temperature on the interfacial properties of explosively bonded Al/Cu bimetal have been investigated. For this purpose, hardness changes along the thickness of the samples have been measured, and the thickness and type of intermetallic compounds formed at the joining interface have been explored by means of optical microscopy (OM), scanning electron microscopy (SEM) and also energy dispersive spectroscopy (EDS). By heat treatment of the samples at 300, 350 and 400°C, it was observed that intermetallic layer was formed at the interfaces. The obtained results indicate that, with the increase of the annealing temperature, the thickness of intermetallic compounds has increased and the amount of hardness along the thickness of the joining interface has diminished. In the annealed sample at 300 °C for 60 min, it was observed that intermetallic layers have formed at the interface of Al/St bimetals. These layers consist of the intermetallic compound Al2Fe and its thickness gets to about 35 μm at some points.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
397
402
http://mme.modares.ac.ir/article_11922_467c75fc4012a20e079c0a03af4742e5.pdf
Numerical Investigation of Water/〖Al〗_2 O_3 Nanofluid Forced Convective Boiling flow in a Vertical Channel
ataollah
rabiee
shiraz university
author
Alireza
Atf
دانشگاه شیراز
author
text
article
2014
per
Nowadays boiling phenomenon has been an important issue in various fields such as petroleum industries and nuclear power plants due to enhancement of the total heat transfer coefficient. One method to increase the level of heat transfer coefficient is to add certain nanoparticles such as 〖Al〗_2 O_3 to the base fluid. The present paper concerns the effect of nanoparticles on forced convective boiling within the general- purpose computational fluid dynamics (CFD) solver FLUENT. The governing equations solved are generalized phase continuity, momentum and energy equations. Wall boiling phenomena are modeled using the baseline mechanistic nucleate boiling model developed in Rensselaer Polytechnic Institute (RPI). To simulate the critical heat flux phenomenon, the RPI model is extended to the departure from nucleate boiling by partitioning wall heat flux to both liquid and vapor phases considering the existence of thin liquid wall film. In addition to validating the subcooled boiling phenomenon, the effect of aluminum oxide 〖Al〗_2 O_3 nanoparticles on heat transfer coefficients has been analyzed. It is concluded that by increasing the volume fraction of 〖Al〗_2 O_3 nanoparticles in the base fluid, wall temperature has been dropped and the heat transfer coefficients have been increased significantly.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
403
411
http://mme.modares.ac.ir/article_11923_5891032c35d5c6c39f0f84b1f8307f11.pdf
Calibration of Weibull Parameters of Local Approach to Fracture by Using Experimental Cleavage Fracture data of Three Point Bending Specimens
Ahmad
Mohammadi Najafabadi
دانشگاه صنعتی امیرکبیر
author
Iraj
Sattarifar
دانشگاه صنعتی امیرکبیر
author
Mohsen
Rezaeian
دانشگاه صنعتی امیرکبیر
author
text
article
2014
per
Cracks due to manufacturing processes or in-service applications can propagate and cause failure in structures. Therefore, it is of interest to find a suitable fracture assessment method for predicting crack initiation. Main approaches for fracture assessment of structures are global approach and local approach. In the global approach, it is assumed resistance against fracture can be measured by a critical values of a far from crack tip parameter like K or J. In this study, Beremin model of local approach is used for predicting brittle fracture which studies stress and strain fields at the crack tip. The model introduces unknown parameters which have to be calibrated using experimental fracture data. The purpose of this study is evaluating of conventional calibration methods of local approach parameters using the experimental brittle fracture data of three point bending specimens, determining limitations, and finally presenting a new calibration method to produce suitable parameters for predicting brittle fracture of the specimens by using local approach to fracture. This study shows that conventional calibration method using experimental fracture data of three point bending specimens has limitation in some cases. Also, by introducing location parameter of Weibull distribution as a stress triaxiality criteria in Beremin model, a new rational method for predicting brittle fracture of the three point bending specimens with different constraints is presented.
Modares Mechanical Engineering
Tarbiat Modares University
1027-5940
15
v.
1
no.
2014
412
418
http://mme.modares.ac.ir/article_11926_dfdac8388cc899c770427cca82e8a33d.pdf