Showing 19 results for Rahimi
Saatchi, A. Najafi- Zadeh, M. R. Rahimipour,
Volume 14, Issue 1 (1-1994)
Abstract
M. H. Rahimian and M. Farshchi,
Volume 16, Issue 2 (1-1998)
Abstract
The objective of this research is to develop an accurate numerical method to be used in showing the deformation of a liquid fuel droplet in a convective field. To simultaneously solve the internal liquid droplet flow field as well as the external gas phase flow field, a nonstaggered rectangular grid system without any coordinate transformation is used. Transition from the gas field to the liquid field is performed through consistent balancing of kinematic and dynamic conditions at the liquid-gas interface. An implicit fractional step-type method is used to capture pressure and velocity field with proper coupling at low Mach number limit. To show the accuracy of the method, the solution of the driven cavity flow and flow over a solid cylinder is presented. Next, two phase flow field solution of moving and deforming droplet in a gaseous surrounding, with appropriate surface tracking, is presented. While gas Reynolds number and Weber number are shown to play an important role in droplet deformation, liquid Reynolds number and density ratio have no significant effect.
M. H. Rahimian and M. Farshchi,
Volume 21, Issue 1 (7-2002)
Abstract
The internal flow circulation dynamics of a liquid drop moving in a co- or counter-flowing gas stream has been numerically studied. The present work is concerned with the time accurate numerical solution of the two phase flow field at the low Mach number limit with an appropriate volume tracking method to capture motion and deformation of a liquid drop. It is shown that relative velocity between gas and liquid and the parameters controlling the deformation of the drop have the strongest influence on its internal circulation, too. The effects of the liquid Weber number, ranging from 8 to 32, and of gas stream Reynolds number, ranging from 1 to 20 are studied. It was revealed that the largest and the most lasting internal circulation are observed in drops with small deformation in high Reynolds number gas streams. In the case of counter-flowing gas stream, there is a strong internal circulation inside the liquid drop. The locations of the gas separation points on the drop are strongly influenced by the internal circulation of the drop, resulting in a complex wake dynamics.
Keywords: Numerical solution, Two phase flow, Moving droplet, Droplet internal circulation
B. Saghafian, S. Rahimi Bandarabadi, H. Taheri Shahraeeni and J. Ghayoomian,
Volume 24, Issue 1 (7-2005)
Abstract
Rainfall is one of the most important climatic variables in the hydrology cycle. In flood estimation as well as environmental pollution studies in medium to large watersheds not only mus temporal pattern of rainfall t be known, but also the knowledge of its spatial distribution is required. Estimation of daily rainfall distribution without comparison and selection of
suitable methods may lead to errors in input parameters of rainfall – runoff models. Interpolation methods are among the techniques for estimating spatial distribution of rainfall. In this study, Thin Plate Smoothing Splines (TPSS), Weighted Moving Average (WMA) and Kriging are applied to estimate spatial daily rainfall in the southwest of Iran. Cross validation technique is used for comparison and evaluation of the methods. The results of analysis with two different station density showed that the TPSS method with power of 2 is the most accurate method in estimating daily rainfall. Zoning of the region also increased the interpolation accuracy. Generally speaking, division of the region based on cluster analysis improves accuracy compared with division by inter basin boundaries
M. Ebrahimi and A. Ghaderi,
Volume 24, Issue 2 (1-2006)
Abstract
Stator flux oriented vector control of induction motor (IM) drives for speed sensorless control has several advantages. But the application of a pure integrator for the flux estimation is difficult due to the presence of measurement noise and dc offset. To overcome these problems, some have used a programmable cascaded low pass filter (PCLPF). In this paper, it is shown that some problems still exist and some new problems arise from this approach. In order to solve these
problems, a novel compensation method is proposed. In this scheme, the dc offset is detected and subtracted from the estimated flux along d and q axes. The simulation results show that it works well in the low speed region as well as in the transient state. The oscillation of the torque and the estimated flux are also reduced notably when the torque reference changes rapidly.
A. Ebrahimi, S. A.a. Mousavian, and M. Mirshams,
Volume 24, Issue 2 (1-2006)
Abstract
The rapid growth of space utilization requires extensive construction, and maintenance of space structures and satellites in orbit.
This will, in turn, substantiate application of robotic systems in space. In this paper, a near-minimum-time optimal control law is developed for a rigid space platform with flexible links during an orientating maneuver with large angle of rotation. The time optimal control solution for the rigid-body mode is obtained as a bang-bang function and applied to the flexible system after smoothening the control inputs to avoid stimulation of the flexible modes. This will also reflect practical limitations in exerting bang-bang actuator forces/torques, due to delays and non-zero time constants of existing actuation elements. The smoothness of the input command is obtained by reshaping its profile based on consideration of additional first-order and second-order derivative constraints. The platform is modeled as a linear undamped elastic system that yields an appropriate model for the analysis of planar rotational maneuvers. The developed control law is applied on a given satellite during a slewing maneuver. The simulation results show that the modified realistic optimal input compared to the bang-bang solution agrees well with the practical limitations and also alleviates the vibrating motion of the flexible appendage, which reveals the merits of the new control law developed here.
M. Ebrahimi, M. Moradiyan, H. Moeshginkelk, M. Danesh, and M. Bayat,
Volume 25, Issue 1 (7-2006)
Abstract
This paper presents a method based on neural networks to detect broken rotor bars and end rings in squirrel cage induction motors. In the first part, detection methods are reviewed and traditional methods of fault detection as well as dynamic
model of induction motors are introduced using the winding function method. In this method, all stator and rotor bars are considered independently in order to check the performance of the motor for any faults in the parts. Then the frequency spectrum of current signals is derived using the Fourier transform and analyzed under various conditions. In the second part of the paper, an analytical discussion of the theoretical principles is presented to arrive at a simple algorithm for fault detection based on neural networks. The neural network has been trained using the information from a 1.1 KW induction motor. Finally, the system is tested with different values of load torque and is found capable of working on-line to detect all normal and ill-performing conditions.
M. R. Rahimi Pour and M. Moayeri, ,
Volume 25, Issue 1 (7-2006)
Abstract
The effective parameters that influence in situ cast ferroTic composites were investigated. Centrifugal casting of specimens was carried out using ceramic & metallic molds. OM, SEM and XRD techniques were used to examine the existence of flows in the specimens. Results show that the control of chemical composition, processing, cooling rate and heat treatment has
a promising effect on the quality of specimens. Also remelting process leads to the hemogeneity of matrix by uniform distribution of secondary phase.
B.ebrahimian and M.vafaeian,
Volume 26, Issue 1 (7-2007)
Abstract
In spite of the fact that the effect of earthquake on earth dams has been widely studied during the past decades, the complicated behavior of such earth structures against different seismological characteristics is still unknown. Such ambiguities necessitate more accurate studies using more comprehensive computation tools to achieve new results describing the behavior of such structures subjected to earthquake loading. In the present study, the simple soil model of elastic, perfectly plastic (based on the Mohr-Coulomb criterion), and Rayleigh damping criterion have been adopted for the soil. First, the numerical model employed was verified by dynamic analysis of real cases such as “Long Valley” and “santa Felecia” earth dams. The computational results were then compared with real recorded data or with those reported by other researchers. In addition to evaluating seismic stability of earth dams, their seismic stability was verified using pseudo-static analyses. Therefore, the “Carsington” dam was analyzed to verify the results of pseudo-static analyses and to check the results of FLAC software in calculating the pseudo-static factor of safety. The values of calculated factors of safety in the present study are in good agreement with the published results in the literature. Furthermore, the failure behavior revealed in the analysis shows the ability of FLAC software in defining the failure surface. In the main part of the analyses, a parametric study was conducted for different selected conditions and specially the effect of dam height and the optimum size of crest width were investigated. The results are presented in relevant diagrams.
H. Ebrahimifar , M. Zandrahimi,
Volume 29, Issue 2 (Dec 2010)
Abstract
In order to increase the efficiency and working life of mettalic interconnects used in solid oxide fuel cells, protective coatings with high electrical conductivity are used. In this study, AISI 430 ferritic stainless steel was coated in a cobalt-base pack mixture by pack cementation. The effect of oxide thickness on the area specific resistance (ASR) was investigated by applying isothermal oxidation at 800 °C and non-isothermal oxidation at a temperature range of 400 – 900 ºC. Results showed that the formation of MnCo2O4 and CoCr2O4 Spinels during oxidation improved electrical conductivity. The increase of isothermal oxidation time and temperature increases the oxide thickness, and consequently the ASR increased.
M. Ahmadi, Sh. Ebrahimi, M. Ahmadi,
Volume 33, Issue 2 (Journal of Advanced Materials-fall 2015)
Abstract
This research aimed at producing microcellular foams (with cell size of 1-10 μm and cell density above 10 9 cell/cm3)
from PC/EPDM in order to use in medical devices. Due to the weak nucleation behavior of microcellular polycarbonate foams, it is difficult to prepare them. This research provides valuable information regarding the possibility of making microcellular foams from this polymer by using multiwalled carbon nanotubes (MWNT,s) as nucleating agents (the value of 1-3 phr). The nanocomposite samples were prepared in an internal mixer and foamed via a batch processing method using supercritical carbon dioxide as the foaming agent. The results showed that the addition of nanoparticles up to 3 phr improves the foamability of PC/EPDM blend. Furthermore, as nanoparticle content increased a decrease in cell size and hence an increase in cell density were observed. Another finding showed that cell size distribution is directly related to uniform dispersion of carbon nanotubes.
M. Haerifar, M. Zandrahimi,
Volume 34, Issue 1 (Journal of Advanced Materials-Spring 2015)
Abstract
In the present study, 304 stainless steel (SS) was electrochemically plated with nanocrystalline Mn-Cu alloy coatings from a bath containing ammonium sulfate. The effects of current density on the microstructure, crystallographic structure, and chemical composition of the deposits were studied. The results showed that at low current densities, discontinuous coatings with a large amount of Cu can be obtained. Further increase in current density resulted in amorphous, compact and heterogeneous coatings with a small amount of Cu. The presence of Cu at low contents in precipitated coatings delayed the phase transformation of as-deposited ductile g-Mn to the brittle and hard a-Mn. However, the results did not show any specific changes in the grain size of the coatings with variation of current densities.
F. Ebrahimi, F. Ashrafizade, S. R. Bakhshi,
Volume 36, Issue 3 (Journal of Advanced Materials-Fall 2017)
Abstract
In this research, ordered porous anodic templates with 30 nm diameter and 15 µm thickness were prepared by using double anodization process. Dip coating method was employed to synthesize strontium ferrite in the form of nanowires in sol dilution. Ferrite nanopowders were also synthesized using sol gel method. The characterization of the nanostructures were examined by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive Spectroscopy (EDS). Hysteresis loops of nanopowders and nanowires, parallel and perpendicular to the wires axes, were measured by Superconducting Quantom Interference Device (SQUID). The results showed that double anodization in 0.3 M oxalic acid at 4 oC with a single anodization for 12 hours could produce ordered template. Dip coating in 80 oC for two hours could form fine and uniform strontium ferrite nanowires. The produced material showed parallel anisotropy.
S. F. Shams, M. Ebrahimian-Hosseinabadi,
Volume 37, Issue 2 (Journal of Advanced Materials-Summer 2018)
Abstract
The purpose of this paper was modeling and mechanical analysis of the biodegradable biphasic calcium phosphate/silk (BCP/Silk) laminated composite bone plate for fractured tibia healing; to this aim,ABAQUS 6.13 was employed for modeling and mechanical analysis. First, the tibia bone was considered based on the anthropometric measurements of an average person as a two-layer cylinder; the inner part was the bone marrow and the outer one was the cortical bone. Then, the bone plate and screws were designed according to the defined standards and the properties of new composite in the ABAQUS software. The mesh of bone plate and other equipments were selected to be tetragonal and cubic, respectivelly. After that, the bone plate was placed on the bone while the bone was bounded along the Y axis and the force of around 400 N was loaded. The results showed that the biocompatible and biodegradable composite bone plate had the elastic modulus of about 21 GPa, which was close to the bone modulus.
M. Mahmoudi Saleh Abad, M. Zandrahimi, H. Ebrahimi Far,
Volume 37, Issue 3 (Journal of Advanced Materials-Fall 2018)
Abstract
In order to improve the oxidation and hot corrosion resistance of steels, various elements including aluminum, chromium, silicon, titanium or combination of these elements can be diffused on to the surface of steel. In this study, aluminum and titanium were simultaneously co-deposited onto the AISI 430 ferritic stainless steel substrate by the pack cementation process. Coating was examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The coating consised of two layers with the thickness of approximately 14 microns. The results obtained by XRD showed the existence of FeTi, TiO2, AlTi, Al3Ti and Al5Ti phases in the coating. Isothermal oxidation and cyclic oxidation were carried out at 1000○C. It was showed that the diffusional coating of aluminum-titanium led to the improvement of cycle and isothermal oxidation resistance.
M. Akbarzadeh, M. Zandrahimi, E. Moradpour,
Volume 37, Issue 4 (Journal of Advanced Materials-winter 2019)
Abstract
Molybdenum disulfide (MoS2) is one of the most widely used solid lubricants. In this study, MoS2-Cr composite coatings were deposited onto AISI 1045 steel substrates by direct-current magnetron sputtering. The MoS2/Cr ratio in the coatings was controlled by sputtering the composite targets. The coatings were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and nano-indentation and nano-scratch techniques. The tribological behavior of the coatings was investigated using the pin-on-disc test at room temperature. The results showed that the thickness and the hardness of the coating were 6 µmand 850-1300 HV, respectively. The degree of the crystallization of the composite coatings was enhanced with increasing the Cr contents. The incorporation of Cr into MoSx coatings resulted in the considerable improvement of coating adhesion and hardness. The optimum doping level for MoS2-Cr coatings to show the best tribological propertie was 13 atomic percent. The main wear mechanisms of the coating were delamination, tribochemical and abrasive micro cracking
F. Saeidpour, M. Zandrahimi, H. Ebrahimifar,
Volume 38, Issue 1 (Journal of Advanced Materials-Spring 2019)
Abstract
Crofer 22 APU ferritic stainless steel has been evaluated as one of the favorable materials for utilization in Solid oxide fule cell (SOFC) interconnects. However, there are difficulties in utilizing these metallic interconnects, including the quick decrease of their electrical conductivity and evaporation of Cr species. To overcome the above problems, the application of protective coatings has been proposed. In this work, Co/Y2O3 composite coatings were deposited onto Crofer 22 APU stainless steels by direct current electrodeposition method. Oxidation and electrical properties of uncoated and coated steels were evaluated. Surface and cross-section of the bare and coated steels were characterized using scanning electron microscopy and X-ray diffraction techniques. Results showed that oxidation rate of the coated specimen was reduced by about 4 times, as compared to the uncoated one after 500 h isothermal oxidation in air at 800˚C. Formation of Co3O4 and MnCo2O4 spinel compositions improved electrical conductivity of the coated sample. After 500 h of isothermal oxidation at 800˚C, ASR value of the Co/Y2O3-coated and uncoated steels was 15.8 mΩ·cm2 and 25.9 mΩ·cm2 , respectively.
M. Eshraghi, Z. Mosleh, M. Rahimi,
Volume 38, Issue 1 (Journal of Advanced Materials-Spring 2019)
Abstract
In this investigation, the structural and magnetic properties of Cr and Zn substituted Co ferrite with the general formula Co1-xZnxFe2-xCrxO4 (x= 0.1, 0.3, 0.5, 0.7) as prepared by sol- gel method were studied. The structural, morphological and magnetic properties of the samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), Scanning electron microscopy (SEM) and Vibrating sample magnetometer (VSM). XRD measurements along with the Rietveld refinement indicated that the prepared samples were single phase with the space group of Fd-3m. Results of SEM images also showed that the particles were in the nanosize range. Also, the magnetic properties of the samples indicated that the magnetization was first decreased, reaching the minimum value for x=0.1 sample; then it was increased. This behavior was related to the cation distribution at the tetrahedral and octahedral sites. Moreover, coercivity was significantly decreased with increasing the doping level due the decrease of magnetocrystalline anisotropy because of the nonmagnetic Zn ion substitution.
H. Ebrahimifar, M. Zandrahimi, F. Ekhlaspour,
Volume 38, Issue 3 (Journal of Advanced Materials-Fall 2019)
Abstract
One of the most effective ways to improve oxidation resistance of interconnects used in solid oxide fuel cells (SOFCs) is to apply a layer of conductive protective coating. In this study, Crofer 22APU ferritic steel was coated in a titanium- based powder mixture by pack cementation method. The powder composition for titanium coating was Ti 20 wt.%, NH4Cl 5 wt.% (activator) and Al2O3 75 wt.%. The optimum temperature and time to obtain the best coating quality in terms of adhesion and porosity were 800 °C and 7 hours, respectivly. The obtained titanized coating consisted of TiFe, TiFe2 and TiCr2 phases. The results of isothermal and cyclic oxidation tests carried out at 900 °C, showed that titanium-coated samples had better oxidation resistance than non-coated samples. Microstructural and phase studies of coated and oxidized samples were performed by scanning electron macroscopy (SEM) and X-ray diffraction analysis (XRD). During oxidation process, the coating layer was converted into TiFe, TiFe2, TiFe2O5, TiO2 and TiCr2O4 phases. The coated specimens had lower weight gains relative to uncoated samples showing that coating effectively protects the substrate against oxidation. Moreover, coated samples had higher electrical resistance than uncoated ones.
