Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Matched Pole-Zero State-Space Model and Continuous-Time Properties
1193
1201
EN
Amir H.
D. Markazi
School of Mechanical Engineering, Iran University of Science and Technology, Tehran, Iran
markazi@iust.ac.ir
The matched pole-zero (MPZ) model is a widely used technique for discrete-
time approximation of continuous-time controllers. In this article a new state-
space representation for the (MPZ) model is presented. The new formulation
can be used for direct discretization of state-space controllers, and can be eas-
ily automated on a digital computer. The most important advantage of the
proposed representation is that it preserves the dynamic structure of the orig-
inal continuous-time realization, i.e., the physical meaning of the states and
the direction of eigenvectors remain unchanged. In fact, the new method pro-
vides, exactly, the same dynamic state equations as the step-invariant model,
together with some modifications on the static output state equation.
Up to now, due to the lack of such an eigenstructure-preserving state-
space representations, most of the time domain studies on the effects of dis-
crete approximation of analog controllers were mostly performed using the
step-invariant model, although that method is seldom used for actual dis-
cretization of controllers. The new formulation, paves the way for extending
those studies to the case of the more widely used MPZ method.
Matched pole/zero,Plant-input mapping (PIM) method,Dis- cretization,Digital redesign,Sampled-data
http://scientiairanica.sharif.edu/article_3392.html
http://scientiairanica.sharif.edu/article_3392_6215f1d41ff66a0bcf64a78d2f419ad6.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Exergy Analysis of the Triple Effect Parallel Flow Water-Lithium Bromide Absorption Chiller with Three Condensers
1202
1212
EN
Saeed
Sedigh
LNG Research Laboratory, School of Mechanical Engineering, Iran University of Science and Technology (IUST), Narmak, 16846, Tehran, Iran
saeed_sedigh@yahoo.com
H.
Saffari
LNG Research Laboratory, School of Mechanical Engineering, Iran University of Science and Technology (IUST), Narmak, 16846, Tehran, Iran
saffari@iust.ac.ir
This paper is devoted to the thermodynamic analysis and investigation of the triple effect parallel flow water/lithium bromide chiller with three condensers. For this purpose, the conservation equations governing the cycle are written and the cycle is investigated in the aspect of the first and second laws of thermodynamics. Next the thermodynamic state of various points of the cycle, cycle efficiency, work and heat transferred, and also the exergy loss in various components of the cycle are evaluated. Finally the exergy analysis is carried out and the effect of effective parameters on cycle better performance has been studied.It was concluded that when temperature of the high-temperature generator increases, the cycle COP increases and total exergy losses decrease. Though, the increase of COP and decrease of exergy losses becomes negligible for temperatures higher than 210°C. Therefore, more temperature increase does not profit considerably to increase of COP or reduction of exergy losses, and this temperature is the most optimal temperature in which both the COP and exergy losses have acceptable values.
Absorption Chiller,triple effect,water/lithium bromide,Coefficient of Performance,Exergy,thermodynamic analysis
http://scientiairanica.sharif.edu/article_3393.html
http://scientiairanica.sharif.edu/article_3393_905cad29861095c0925ea3f17202b785.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Finite element simulation of warm deep drawing process in forming circular cup from magnesium alloy sheet
1213
1220
EN
S.A.
Ossia
Department of Mechanical Engineering, Faculty of Engineering, University of Kashan, P.O. Box 87317-51167, Kashan, Iran
s.a.ossia@gmail.com
B.
Soltani
Department of Mechanical Engineering, Faculty of Engineering, University of Kashan, P.O. Box 87317-51167, Kashan, Iran
bsoltani@kashanu.ac.ir
In recent years, the use of magnesium alloys in sheet metal forming has been taken notice by many researchers due to low density and high strength. Because of low formability of magnesium alloy in room temperature, warm forming has been developed in order to produce parts of magnesium alloy sheet. In this paper, warm deep drawing process of circular cup from magnesium alloy AZ31B sheet was simulated using the finite element software ABAQUS/Standard. Simulation results obtained from this study agreed well with experimental results and show that the limit drawing ratio increases with increase in the forming temperature. Maximum LDR was obtained 2.5 at the forming temperature of 200°C. Simulation results indicate that heat transfer plays an important role in increase formability. By comparing isothermal warm deep drawing with non-isothermal one, heat transfer effect and location of sheet failure have been investigated. The effects of friction coefficient and punch speed on the limit drawing ratio were also studied. As a result, increase in the friction coefficient leads to increase in the punch force and decrease in the limit drawing ratio. Results also show that the maximum friction coefficient to produce acceptable parts increase with increase in the forming temperature. In addition, results indicate that increase in the punch speed leads to decrease in the limit drawing ratio.
Sheet metal forming,Warm deep drawing,Magnesium alloy sheet,Finite element simulation
http://scientiairanica.sharif.edu/article_3394.html
http://scientiairanica.sharif.edu/article_3394_4368100fb34cae284a650fdfe4b29963.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Sensitivity Analysis of Vibration modes of Rectangular Cantilever Beams Immersed in Fluid to Surface Stiffness Variations
1221
1227
EN
Amir
Farokh Payam
School of Electrical & Computer Engineering, University of Tehran, Tehran, Iran
a.farrokhpayam@ece.ut.ac.ir
In this paper, the sensitivity of flexural and torsional vibration modes of a rectangular cantilever immersed in a fluid to surface stiffness variations has been analyzed and a closed-form expression is derived. To represent this sensitivity, we use the analytical formulas for the vibrational resonant frequencies of a rectangular cantilever beam immersed in an inviscid fluid. The effect of the surface contact stiffness on both flexural and torsional sensitivities in a fluid is investigated and compared with the case that cantilever operates in the air. The results show that in the low surface stiffness, the first mode is the most sensitive mode. As the sample surface stiffness is increased, higher resonant frequencies show a larger shift as compared with lower resonant frequencies. In addition, comparison between modal sensitivities in air and fluid shows that the resonance frequency shifts in the air are greater than the resonant frequency shifts in the fluid.
atomic force microscope,Cantilever,Sensitivity,Surface Stiffness,Inviscid Fluid
http://scientiairanica.sharif.edu/article_3395.html
http://scientiairanica.sharif.edu/article_3395_6f29d40552c0c98d215211a4bd50488b.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Detailed Investigation of Hydrodynamics and Thermal Behaviors of Rarefied Shear Driven Flow Using DSMC
1228
1240
EN
Omid
Ejtehadi
Mechanical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, P.O.Box: 91775-1111, Mashhad, Iran
Ehsan
Roohi
Mechanical Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, P.O.Box: 91775-1111, Mashhad, Iran
e.roohi@ferdowsi.um.ac.ir
Javad
Abolfazli Esfahani
Center of Excellence on Modeling and Control Systems, Ferdowsi university of Mashhad, Mashhad, Iran
abolfazl@um.ac.ir
In the present work we simulate rarefied gas flow between two moving parallel plates maintained at the same uniform temperature using direct simulation Monte Carlo (DSMC) method.We perform simulations for monatomic argon and diatomic nitrogen gas and compare mon/diatomic gas behaviors. For both gases, we study heat transfer and shear stress and investigate the effects of compressibility and rarefaction in the entire Knudsen regime and for a wide range of wall Mach number.Slip velocity, temperature jump, wall heat flux and wall shear stress are directly sampled from the particles striking the surfaces and reported for monatomic and diatomic cases for different rarefaction regimes. We also study deviation from theequilibrium using the probability density function for argon and nitrogen gas molecules.
DSMC,mon/diatomic gases,Couette flow,Knudsen layer,compressibility effects,rarefaction effects
http://scientiairanica.sharif.edu/article_3396.html
http://scientiairanica.sharif.edu/article_3396_e7d6bcfbe40536abf2cb64c7258e36b0.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Natural convection in a nanofluid filled concentric annulus between an outer square cylinder and an inner elliptic cylinder
1241
1253
EN
M.
Sheikholeslami
Department of Mechanical Engineering, Babol University of Technology, Babol, Iran
mohsen.sheikholeslami@yahoo.com
M.
Gorji-Bandpy
0000-0002-3375-1973
Department of Mechanical Engineering, Babol University of Technology, Babol, IRAN
gorji@nit.ac.ir
D.
D.Ganji
Department of Mechanical Engineering, Babol University of Technology, Babol, Iran
ddg_davood@yahoo.com
Lattice Boltzmann Method is applied to investigate natural convection flow of a nanofluid in a concentric annulus between a cold outer square cylinder and a heated inner elliptic cylinder. In order to simulate the effective thermal conductivity and viscosity of nanofluid, Maxwell–Garnetts (MG) and Brinkman models are used, respectively. This investigation compared with other numerical methods and found to be in excellent agreement. Numerical results for the flow and heat transfer characteristics are obtained for various values of the nanoparticle volume fraction, Rayleigh numbers and eccentricity. The results show that the minimum value of enhancement of heat transfer occurs atforbut for other values of Rayleigh number it obtained at.
Elliptic cylinder,Natural convection,Lattice Boltzmann method,Nanofluid,Concentric annulus
http://scientiairanica.sharif.edu/article_3397.html
http://scientiairanica.sharif.edu/article_3397_df23ef8d024b6596662bdbcafc322bf8.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Design of mixed refrigerant cycle for low temperature processes using thermodynamic approach
1254
1268
EN
Mostafa
Mafi
Department of Mechanical Engineering, Imam Khomeini International University, Qazvin, Postal Code: 34149-16818, Iran
mostafa.mafi@ikiu.ac.ir
Bahram
Ghorbani
Faculty of Mechanical Engineering, K.N.Toosi University of Technology, Tehran, P.O. Box: 19395-1999, Iran
bahram330ghorbani@gmail.com
Majid
Amidpour
Faculty of Mechanical Engineering, K.N.Toosi University of Technology, Tehran, P.O. Box: 19395-1999, Iran
amidpour@kntu.ac.ir
Seyed Mojtaba
Mousavi Naynian
Faculty of Mechanical Engineering, K.N.Toosi University of Technology, Tehran, P.O. Box: 19395-1999, Iran
mousavi@kntu.ac.ir
Minimizing the work consumed of refrigeration system is the most effective measure to reduce the cost of products in sub-ambient chemical processes such as olefin plants. A recent advancement has been the introduction of mixed working fluids in refrigeration system in place of pure working fluids. Due to the lack of systematic design method for Mixed Refrigerant Cycle (MRC), conventional approaches are largely trial-and-error and therefore operations can be far away from optimal conditions. In this paper, a novel method for systematic design of MRCs is presented which combines the benefits of thermodynamics approach and mathematical optimization. Based on the success of the proposed systematic method for the optimal selection of refrigerant composition and operating pressures, the method is extended to give optimal arrangement of the cycle components. The procedure is demonstrated using a case study of design of MRC for a typical olefin plant.
Mixed refrigerant cycle,low temperature processes,systematic design,optimal operating conditions,optimal arrangement
http://scientiairanica.sharif.edu/article_3398.html
http://scientiairanica.sharif.edu/article_3398_7a2aeebb0ea66fce24a2aaf58bf4f7d6.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Analysis and modeling of building thermal response to investigate the effect of boundary conditions
1269
1277
EN
Zahra
Poolaei Moziraji
Department of Mechanical and Aerospace Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran
zpoolaei@gmail.com
Siamak
Kazemzadeh Hannani
School of Mechanical Engineering, Sharif University of Technology, Tehran, Iran
hannani@sharif.edu
Thermal load simulation and sensitivity analysis are performed for a building in Tehran by numerical means. Heat conduction equation of the walls together with appropriate convection and radiation boundary conditions is simulated numerically to compute temperature distributions in the walls. This research proposes a heat balance method coupled with a bulk model to calculate the building thermal load. In the first step, the results of the building thermal load for weather data of Tehran are compared and validated with those of Carrier HAP software, and a good agreement is found between them. The building thermal load depends on boundary conditions of the building. The influence of the boundary conditions such as emissivity of interior and exterior surfaces of the walls and convective coefficients on the building thermal load is investigated through the application of a sensitivity analysis. Then sensitivity coefficients, which demonstrate the significant and impact of internal and external boundary conditions of the walls on the building thermal load, are calculated numerically using a finite-difference method. The results showthat internal radiation is an important component of the building energy balance.
Building load simulation,Boundary conditions,Sensitivity coefficient,Emissivity,Convective coefficients,Numerical Modeling
http://scientiairanica.sharif.edu/article_3399.html
http://scientiairanica.sharif.edu/article_3399_eead02a6c8823a63d9a7f7a74c605dd2.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Numerical solution of general boundary layer problems by the method of differential quadrature
1278
1301
EN
S. A.
Eftekhari
Department of Mechanical Engineering, K. N. Toosi University, P. O. Box 19395-1999, Tehran, Iran
aboozar.eftekhari@gmail.com
A. A.
Jafari
Department of Mechanical Engineering, K. N. Toosi University, P. O. Box 19395-1999, Tehran, Iran
Accurate numerical solutions to some boundary layer equations are presented for boundary layer flows of incompressible Newtonian fluid over a semi-infinite plate. The differential quadrature method (DQM) is first used to reduce the governing nonlinear differential equations to a set of nonlinear algebraic equations. The Newton-Raphson method is then employed to solve the resulting system of nonlinear algebraic equations. The proposed formulation is applied here to solve some boundary layer problems including Blasius,Sakiadis, Falkner-Skan, magnetohydrodynamic (MHD) Falkner-Skan, Jeffery-Hamel, unsteady two-dimensional and three-dimensional MHD flows. A simple scheme is also presented for solving Blasius boundary layer equation. In this techniques, Blasius boundary value problem is first converted to a pair of nonlinear initial-value problems and then solved by a step-by-step DQM. The accuracy and efficiency of the proposed formulations are demonstrated by comparing the calculated results with those of other numerical and semi-analytical methods. Accurate numerical solutions are achieved using both formulations via a small number of grid points for all the cases considered
Differential quadrature method (DQM),Blasius flow,Sakiadis flow,Falkner-Skan flow,MHD Falkner-Skan flow,Jeffery-Hamel flow,Unsteady two-dimensional flow,Unsteady three-dimensional MHD flow
http://scientiairanica.sharif.edu/article_3400.html
http://scientiairanica.sharif.edu/article_3400_05b566331cf622c390e0c1e3df87dcf8.pdf
Sharif University of Technology
Scientia Iranica
1026-3098
2345-3605
20
4
2013
08
01
Three-axis Attitude Control Design For A Spacecraft Based On Lyapunov Stability Criteria
1302
1309
EN
Mostafa
Bagheri
Department of Mechanical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
m.bagheri@aut.ac.ir
Mansour
Kabganian
Department of Mechanical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
kabgan@aut.ac.ir
Reza
Nadafi
Space Science and Technology Institute, Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran
rezanadafi@aut.ac.ir
The three-axis attitude control design based on Lyapunov stability criteria to stabilizing the spacecraft and orients it to desired attitude is presented in this paper. This attitude control system is assumed to have four reaction wheels with optimal arrangement. The reaction wheels are located in square pyramidal configuration. Control system inputs are attitude parameter in the quaternion form and the angular velocity of spacecraft and reaction wheels. The controller output is the torque required to eliminate error. In this study, actuators (reaction wheels) are modeled and required torque for attitude maneuver is converted to voltage of actuators. Armature voltage and armature current is limited to 12 volts and 3 amps respectively. Also, each wheel has an angular velocity limit to 370 rad/sec. Numerical simulations indicate that the spacecraft reaches desired attitude after 34 seconds and show the reliability of mentioned configuration with respect to actuator failure. The results show that in case of failure of one reaction wheel, the spacecraft can reach desired attitude but needs more time. Moreover, results demonstrated the controller robustness against parameter variation and disturbances. It is robust against with up to 350% change in spacecraft moment of inertia and robust against disturbance up to 0.0094 N.m that is equal 38% in comparison with the allowable reaction wheel capacity.
Spacecraft,Attitude Control,DC Motor,Square Pyramidal Configuration,Lyapunov Stability
http://scientiairanica.sharif.edu/article_3401.html
http://scientiairanica.sharif.edu/article_3401_f289cce9fae3ea8ce08cdd5dca7e3e8c.pdf