نشریه مهندسی مکانیک مدرس
سال پانزدهم شماره 4 (تیر 1394)

Active structural acoustic radiation control of a piezolaminated arbitrary thick rectangular plate with a mixed-norm H2/H∞ robust controller is developed. The structure is made of a transversely isotropic host layer with a distributed piezoelectric sensor layer as well as a matched piezoelectric actuator layer، facing high frequency uncertainties and random external disturbances. The elasto-acoustic formulation، based on the exact linear 3D piezo elasticity theory، is developed for problem of fully coupled structure and acoustic mediums. Identification of the fluid/structure interaction system with subspace algorithm is implemented on actuator/sensor data sets. A multi-objective controller with regional pole placement، formulated in LMI framework is synthesized while unmodeled dynamics are treated as multiplicative uncertainties. Numerical simulations confirm effectiveness of the implemented multi objective robust active control scheme for reduction of radiated acoustic power from a piezocomposite plate، without stimulating any instability. Also، better tracking performance and disturbance rejection of mixed-norm controller is observed in comparison to that of ℋ􀬶 and ℋ􀮶 controllers. Finally، validity of the elasto-acoustic model is proved by results obtained from a finite element software، as well as with the data available in the literature.

Emulsion consists of drops of one liquid dispersed into another immiscible liquid, and is a novel technique for producing monodisperse droplets. The aim of this research is to use the Lattice Boltzmann Method (LBM) to simulate two-phase flows in micro-channels to access the emulsification process. For this approach, the Index-Function Model proposed by He is used to simulate drop formation in emulsification process in a co-flowing micro-channel with a complex geometry and three inlets. The simulation is performed to investigate the mechanism of drop generation due to dripping and jetting modes and the mode between them. Index function model, which is a new reliable model to evaluate two-phase flows is applied to track the motion and deformation of the interface between the two immiscible fluids. Accuracy of our results is examined by two well-known basic analytical models including Relaxation of a rectangular drop and Coalescence of two static droplets. Our results indicate good agreement with the analytical data. The dimensionless numbers such as Capillary and Velocity ratio were used. The Capillary number is one of the most important dimensionless numbers in determination of fluid flow characteristics in micro-channels. The simulations reproduce dripping, widening jetting and narrowing jetting simultaneously in a co-flowing microchannel in agreement with the experimental ones. This indicates that index function LBM model has good accuracy and high stability to simulate this kind of flow.

In this paper, thermo-hydraulic behavior of a solar heater is numerically investigated and optimum geometry for the angled-rib turbulator which produces the maximum thermal enhancement factor is determined by Taguchi method. A general innovative geometry is introduced for the rib, by which changes in its geometrical parameters will lead to three convenient types of turbulators, namely triangular, rectangular and trapezoidal ribs, simultaneously. Applying Taguchi method leads to the optimum geometry parameters. A 16 (44) orthogonal Taguchi array is used. The aim of the Taguchi analysis is to maximize the thermal enhancement factor. The factors employed for the analysis are rib relative pitch, height, tip width and percentage of upstream-side perpendicularity. The analysis has been carried out for a flow of Re=10000 and a constant rib width. Due to the twofold effect of turbulators on thermo hydraulic behavior, thermal enhancement factor concept is employed, which stands for both heat transfer rate and friction factor. Results show that rib pitch and height have the most significant effect on thermal enhancement factor, respectively. The optimum rib geometry is found to be a triangular rib which has P/H=2, e/H=0.2 and 100% perpendicularity.

In recent years, rubber pad forming process has conferred many advantages, such as high flexibility, good surface quality and lower manufacturing costs. RPF has been widely used in automotive, aerospace and military industries. In the present research, numerical and experimental analysis of free bulging 304 stainless steel seamed tube, using a polyurethane elastic pad has been studied. First, 3D simulation of seamed tube bulging using the finite element ABAQUS/Explicit 6.12 software by several frictional conditions has been performed. An incompressible hyperelastic pad has been modeled by Mooney-Rivlin constitutive equation and the elastic-plastic behavior as a more progressive ductile damage criterion FLD for steel tube were assumed. In the experimental activity, compression test of rubber was carried out according to ASTM D575-91 standard and compressive stress-strain curve and the Mooney-Rivlin constants were determined. Forming of meshed tubes by using elastic pad with different lubricating systems has been conducted up to onset of bursting in the seam weld and longitudinal, hoop and thickness strains were measured. Results showed that friction, especially between rubber and tube plays the main role in controlling wrinkles, increasing the bulge depth, reducing the forming load and friction dissipation energy of the process. Also, it was observed that the intact parts without any wrinkles were formed using nylon lubricant between tube and rubber and drawing oil between tubes and die.

One of the challenging problems of tribology is cam and follower elastohydrodynamic lubrication due to the simultaneous effect of various lubrication mechanisms. These mechanisms are transient, squeeze film, elastic deformation of contacting surfaces and variation of lubricant properties with pressure. In this paper, besides studying the mentioned factors, the effect of using a non-Newtonian lubricant such as grease is numerically investigated. The lubrication governing equations and Oswald’s grease behavior equation have been discretized using finite difference technique. The system of equation has been solved via Multi-Grid method which is an advanced iterative method in solving system of partial differential equations. The results are shown for Newtonian oil compared to grease for different cam rotational speed. Also, different grease behaviors are investigated. The results are verified by comparison of the results obtained using the famous Newton-Raphson method. Findings show that the minimum lubricant thickness as well as the maximum pressure is lower when using grease compared to the case that a Newtonian lubricant is used. In the case of Newtonian lubricant, increasing the speed results in an increase in the lubricant film thickness but it is shown that, in the case of Non-Newtonian lubricant, speed does not affect the lubricant thickness.

In this paper, the behavior of sandwich structures with composite skins and the core that consists of foam and a corrugated composite laminate under compressive load is investigated experimentally. Three geometrical shapes including square, trapezoid and triangle forms are considered for corrugated composite laminate and results are compared with reference specimen with simple core that is made of foam. Skins and corrugated composite laminate are made from woven glass and epoxy resin; PVC foam is used in core. Corrugated composite laminate is built using the hand layup method and vacuum assisted resin transfer molding technique is used to join the skins to the core. In order to confirm experimental accuracy of test three of the same samples have been produced for each type of core shape and the average of the data tests has been used to compare the results. The edgewise compressive test based on ASTM is used to perform the experimental test. The results show that set corrugated laminate composite in core improves the mechanical properties (such as compressive strength, axial stiffness, toughness) and the ratio of mechanical properties to the weight of these structures to a large extent. Also, by approaching the square shape the structure achieves more favorable properties. The results have shown that compressive strength is increased from 188.56% (triangle core) to 362.37% (square core) and the ratio of compressive strength to weight is increased from 92.36% (triangle core) to 141.4% (square core).

The purpose of this investigation is to study control and monitoring of a rehabilitation robot with two degrees of freedom (2-DOF) for rehabilitation of the lower limbs of patients with loss of ability for movement due to injury, disease, stroke or surgical operations. After determining the movements which include flexion-extension movements of the knee and hip joints, the performance of the mechanism was investigated using dynamic analysis and simulation. Then, a programmable logic controller (PLC) was employed to control the robot performance. Finally, the accuracy of PLC program was guaranteed by monitoring the robot. Passive, assistive and resistive exercises were considered in programming the controller. In assistive exercises, the forces needed by the patient to perform the movements were actually set automatically by using the feedback data provided by the patient's forces. In addition, to perform the resistive exercises, rather than using actual weights, negative loads were employed. The results obtained indicate considerable accuracy to perform the movements and create safe conditions for the patient. Also, high flexibility in programming provides the possibility of performing a wide range of rehabilitation exercises.

In this study, performance of gas liquid cylindrical cyclone separators and the effect of changing geometrical parameters of cyclone separators entrance are investigated. The cyclone is simulated with computational fluid dynamic methods. After choosing a suitable mesh grid for the cyclone and checking grid independency, the effect of changing entrance geometry on gas carry under and liquid carry over is investigated. Geometrical parameters, especially inlet geometrical parameters, have considerable effect on optimizing cyclone separators performance. RSM model is used for turbulence simulation of the flow and two phase flow is simulated using Eulerian- Eulerian approach. In this simulation, inlet cross section, inlet angle and inlet height relative to the cyclone bottom part are optimized. Results show that GCU decreases with decreasing nozzle’s inlet angle. An optimum point for GCU was given with changing inlet altitude relative to the bottom of the cyclone and inlet nozzle’s width. An optimum point for LCO was obtained with changing inlet altitude and inlet nozzle width. Increasing inlet angle causes a decrease in LCO. In optimum model, gas carry under decreases significantly and liquid carry over is eliminated.

Human bones experience different modes of loading including tension, compression, bending, and torsion. The modes of loading depend on the activities performed by the body. Regarding the crack shape and loading modes, by the time only the first mode of fracture has been studied in order to analyze the fracture toughness. However, it is necessary to analyze different modes of fracture in order to achieve more reliable results. In this research, finite element analysis and calculations for geometric coefficients were done to obtain the toughness of bone. Hence,first, second, and combined modes of fracture in cortical samples having cracks were studied numerically and experimentally. To this end, bovine tibia was used to make standard tensile samples for implementation in Arcan’s device. Some optimizations were made on the Arcan’s device. These were included of bone fixation in the device and ability of performing tests in different angels. Stress intensity factor (Kc) was obtained for different fracture modes. Results showed a decrease in KIc with respect to change in loading angle while KIIc acted vice versa. Performing some extra optimizations, the device can be used for torsional fracture mode in a torsion test device.

Increasing usage of metals in engineering structures has made the metal forming process superior in the solid mechanic research. Meanwhile, the physical theories are of considerable significance due to their individual features. The crystal plasticity theory is one of these theories. This theory predicts the texture evolution and deformation of these materials by modeling the plastic deformation mechanisms of crystal material’s micro-structure (such as metals). Connecting with micro-structure enables this theory to predict the anisotropy of single crystals, and also the prediction of some phenomena in polycrystals which are aggregates of single crystals, is possible. Presenting a suitable work hardening model which contains the anisotropy behaviors of singlecrystals is very important. In this paper, first, the principles of crystal plasticity are explained, then by evaluating several experimental results and the most commonly used work hardening models, a new work hardening model will be presented. This model adapts better with experimental results, compared to the previous models. The scope of this research is specifically for crystal materials with FCC structure, nevertheless, some parts of this research are applicable to the other structures.

An extensive experimental study has been conducted to investigate the performance and stability of a supersonic axisymmetric mixed compression air intake. The intake has been designed for a free stream Mach number of 2.0. However, tests were conducted for free stream Mach numbers of 1.8, 2.0, and 2.2. This investigation is aimed at studying effects of Mach number and back pressure on the intake flow stability during the buzz phenomenon. Further, the effect of acoustic resonance on the Buzz frequency has been investigated. Buzz phenomenon is defined as the shock oscillation ahead of the intake that may occur when the intake mass flow ratio reduces. Results show that the stability margin reduces when the free stream Mach number increases. In addition, reducing the free stream Mach number and increasing the back pressure cause the oscillation frequency to increase. The main cause of instability start is flow separation on the compression ramp and two ranges of frequency of buzz oscillations are obtained, range of 100 Hz for flow instability for Mach numbers of 1.8, 2.0 and 2.2, and range of 475 Hz for flow instability in Mach number of 1.8. For both cases, the spatial domain of buzz oscillations covers the entire intake length. Further, these low and high frequency ranges have significant conformity with the zeroth-order and first order of the acoustic resonance frequency, respectively, that increase the probability of existence of acoustic resonance driving the buzz oscillation.

In this paper, pull-in instability of a cantilever beam type nanoactuator made of the functionally graded material (FGM) based on higher order modified strain gradient theory is investigated. It is assumed that the functionally graded beam, made of germanium and silicon, follows the volume fraction definition and law of mixtures, and its properties change as a power function through its thickness. By changing the germanium constituent volume fraction percent of the nano beam, five different types of the nano-beams are investigated. The influences of the volume fraction index, length scale parameter and the intermolecular forces, on the pull-in instability are examined. Principle of minimum total potential energy is used to derive the nonlinear governing differential equation and consistent boundary conditions which is then solved using the differential quadrature method (DQM). The present analysis is validated through direct comparisons with other published research methods and experimental results and, after comparison, excellent agreement is achieved between the new solution method and other experimental and numerical solution results. Besides, the results demonstrate that size effect and amount of volume fraction have a substantial impact on the pull-in instability behavior of beam-type nanoactuator.

In this study, spray behavior of bio-ethanol as a regenerative fuel that reduces emissions such as NOX and CO is investigated in a combustion chamber and compared to its different blends with gasoline. For this purpose, microscopic and macroscopic spray characteristics and also evaporated fuel mass after the injection are modeled and investigated using Fire 2013. It is concluded by increasing bio-ethanol content in the fuel, evaporated fuel mass, spray cone angle, spray area and sauter mean diameter increases, however spray tip penetration remains roughly constant. Increase of injection pressure, decreases spray cone angle and suater mean diameter and increases evaporated fuel mass, spray area and spray tip penetration. If the energy content and time of injection of bio-ethanol and gasoline be equivalent the results vary significantly compared to previous cases. In this case bio-ethanol has a longer spray tip penetration and spray area, higher fuel mass evaporated and smaller spray cone angle and sauter mean diameter compared to gasoline. The increased spray tip penetration and spray area in this case may lead to piston impingement and bore wetting resulting increased hydrocarbon emissions and decreasing engine efficiency.

In a structural analysis, dynamic response of a crack is of significant importance as well as the impacts of elastic waves on stress intensity factors (SIF). In this paper, dynamic analyses of multiple cracks on a half-plane subjected to anti-plane shear stresses are presented. Stress intensity factors are calculated and the interaction of elastic waves with the boundary of plane and the crack's tips is investigated at different locations. The distribution discontinuous displacement techniques are used, enabling us to solve the crack problems in dynamic fracture mechanics. Integral transformations (Laplace and Fourier) are applied to elastodynamics equations and by using a set of appropriate boundary conditions, the crack problem is solved through discontinuous displacement method. As a result, the stress equations with hypersingularity terms are obtained. Using Chebyshev series expansion and collocation points in Laplace domain, the crack solution is achieved. Finally, different algorithms of numerical Laplace inversion are presented and the stress intensity factors (SIF) are obtained. The presented results are compared with published data and a good agreement is observed. Moreover, it is also demonstrated that the present theoretical study is capable of modelling multiple cracks with different arrangements.

In this study, dilatometry examination was conducted on API X70 steel in a wide range of cooling rates from 0.5 to 40 °C/s. The optical microscopy observation and microhardness measurement were used to verify the observed microstructures. From the experimental results, the continuous cooling transformation curves (CCT) were constructed for API X70 pipeline steel. Different microstructures including granular bainite, pearlite, acicular ferrite and bainitic ferrite were observed depending on the cooling rate of tested samples. The obtained microstructure in 3.5 to 5 °C/s cooling rate was mainly granular bainitic ferrite and little pearlite. With increasing cooling rate, the observed dominant microstructure in 5 and 7.5 °C/s cooling rate was acicular ferrite (which is the desired microstructure in energy transportation pipeline steels) and the bainitic ferrite microstructure in 10 °C/s and upper cooling rates predominantly observed at these speeds. Thus, by carefully comparing of the observed microstructures with the studied steel microstructure in the same micrograph direction, it concluded that the production of steel in 7.5 °C/s cooling rate has the highest microstructure proportion with the desired steel. These results can be used to design the optimum thermo-mechanical control process (through the selection of proper cooling rate) in domestic manufacturing of the API X70 steel.

Steam flow at blade passages of low pressure steam turbines becomes a supercooled drysteam due to rapid expansion and crossing the saturation line. But after nucleation and condensation shock, phase change from vapor to liquid droplets occurs which is called two-phase or wet steam flow. In this paper, the aim of developing finite-volume flow of wet Jameson is considered for the first time in two-dimensional study by using the advantages of CUSP's numerical method. In this paper, equations governing the formation of liquid phase are combined with equations of survival and by using CUSP's numerical approach in Jameson's finite-volume method (the integrated method) the positive features of both of these methods can be simultaneously used in the modeling of two-phase flow. To calculate nucleation, the classical equation of nucleation with appropriate correction and also Lagrangian solution for growth of droplets are used in integrated method. Additionally, condensation shock effect on the pressure distribution and the droplet size has been calculated and compared with experimental data. Given the importance of areas of shocks on the suction surface of the blade, the focus of integrated method is shifted to this area. The results of integrated two-phase model are examined in subsonic and supersonic flow output. In the shock area on suction surface blade, using the CUSP's method (the integrated method) demonstrates better coverage in predicting attributes of flow in target area in comparison with the experimental data by a reduction of 20 percent in numerical errors.

Neurological diseases such as cancer damage blood brain barrier and consequently cause more permeability in tissues. Generally, if there is damage to the brain tissues the contrast agent used in MRI diffuses outside the capillaries and the MRI picture brightness changes. The purpose of this paper is to show the effects of different parameters on the contrast agent diffusion in the brain capillary. In this study, the lattice Boltzmann method with multi-relaxation time (MRT) is used to simulate the flow in the capillary and porous media around it. The results show that the porosity in extravascular tissues (it shows the tissue damage), the kind of contrast agent and capillaries curvature have an impact on the contrast agent diffusion in the tissues. The presented results show the effects of curvature on shear stress and thus on mass transfer in the capillary. It should be noted that the presented results have been evaluated by previous statistical and analytical results for flow in the damaged brain capillary with different permeability. It has been shown that the lattice Boltzmann method is able to simulate the complex problems, especially in porous media.

Forming media in metal forming processes is so important. Since the forming media in Ball deepdrawing process is discrete, it is quite flexible. In this paper, thickness distribution and required force for forming of conical part by ball deep-drawing and conventional deep-drawing processes using finite element simulation and experimental stages were studied. In this research, sheets used were made from St14 steel and brass having 1mm thickness. The experimental results are in good agreement with simulation results. The results showed the sample formed by conventional deep-drawing process had more uniform thickness distribution than ball deepdrawing, but the maximum thinning in the parts of ball forming process was less than conventional deep-drawing process. Also, it was observed that required force for ball deep drawing process is more than the conventional deep-drawing process. It was observed that with increasing radius of the input die, the force required to stretch the ball deep-drawing and ball processes is decreased, and with increasing radius of the input die thinning amount is reduced. It was noted that one of the advantages of ball deep drawing process over traditional deep drawing process is that a negative slope part is achieved.

Dissemination of an analyte under the laminar flows plays a major role in measuring and assessment of biological fluids such as sample preparation in the context of microfluidic systems. Due to the development of manufacturing technology in the Lab-on-a-chip devices, the production of rectangular microchannels with finite aspect ratios and micron and submicron sizes has been provided by which the effect of electrokinetic phenomena on concentration distribution will be magnified in these systems. Since the recent researches in this field have overlooked such effects, the present work will be conducted analytically to study the effect of electric double layer on cross stream diffusion of the analyte in the combined electroosmotic and pressure driven flows. Three flow scenarios, the favorable, adverse and zero pressure gradients are analyzed. The results demonstrate that the width of the diffusion region near the top and bottom walls of the microchannel becomes broader with the increase in the Debye length. Also, the results of the scaling analysis reveal the decrease in mixing intensity with increasing the Péclet number based onHelmholtz-Smoluchowski velocity and dimensionless Debye–Hückel parameter. As well, the average scaling exponent of this criterion is a descending function with respect to the thickness of the electric double layer.

Production of double-stepped metal sheet parts is considered a complex and difficult task in industries. The crankcase is such a complex double-stepped part for the automobile industry and its production with traditional methods is associated with many problems. In this paper, the formability of this stepped part has been studied experimentally and by simulation using hydrodynamic deep-drawing with radial pressure. It is shown that the crankcase can be formed successfully in one step by the hydroforming process. Moreover, the effect of fluid pressure on the thickness distribution and die filling and the effect of geometric parameters such as punch corner radius and the height of the steps on the thinning, and also optimal shape design of the original blank were investigated. The study showed that choosing the correct forming pressure can improve formability and increase the amount of die filling. It is also illustrated that by increasing the punch corner radius, the maximum thinning is reduced and the thickness distribution is improved and, by increasing the height of the steps, thinning in the wall of the first step and the punch corner radius increase; by decreasing the height of the steps, thinning position will be shifted toward the second step. Also, by optimizing the original blank, it has been concluded that the optimization of the shape of original blank has a major impact on the material flow and will delay the sheet rupture.

In the current study, for a special fabric that is used for parachute canopies the permeability of the canopy has been estimated empirically and numerically. Moreover, the coefficients of Darcy's equation resulted from experiment are used in 2D numerical simulation of a single parachute-like body. Assuming permeability for the fabric, the drag coefficient showed a 39 percent decrease rather than solid canopy. Comparison between a solid canopy and a permeable one showed significant differences in the results, especially in streamlines and pressure distribution. In order to diminish the numerical effort, the canopies were taken as 2D hemi-spherical porous cups. Socalled two dimensional numerical simulations using FLUENT® software was conducted on a group of paired permeable with two different diameters in various vertical and horizontal distances. The diameter of lower canopy was considered as half that of the upper one. Tandem canopies were used in order to reduce the inflation shock of main parachute. The lateral relative displacement of lower canopy to upper one has been considered in order to stimulate true descending conditions. The results showed the interaction between flow fields of canopies has significant effect on drag coefficient of the cluster parachutes. Therefore, determining the length of the risers as the vertical distance and relative diameter of parachutes and their interactions was found to have a tremendous effect on the design of cluster parachutes. The study showed that the most desirable longitudinal distance between two canopies was equal to the diameter of lower canopy.

Numerical and experimental models have been developed to investigate the effect of implementing in-line and staggered arrangement of dimple on heat transfer augmentation and pressure drop of the air flow through a multilouvered fin bank. Three-dimensional simulations of single row of louvers have been conducted for the aforementioned geometry. Simulations are performed for different Reynolds number. The simulation revealed that heat transfer and temperature augmentations occur due to existence of a circulation region created by dimple. Additionally, continuous temperature gradients have been observed over the louver surface with the highest temperature at the base of the louver and the lowest temperature at the middle of the louver. Besides, the difference between these two points is more obvious in higher Reynolds numbers, especially in the first louvers. Fin efficiency and fin effectiveness have been calculated to assess the louver performance. The air-side performance of heat exchanger is evaluated by calculating Colburn j factor and Fanning friction f factor. Results show that implementing dimples on the louver surface increase j factor and f factor. The present results indicated the staggered arrangement, in comparison with in-line arrangement, could effectively enhance the heat transfer performance. The results show that both fin bank performance and louver effectiveness will be increased against an acceptable increase in pressure drop.

Mixing within electrokinetic micromixers is studied numerically in this article. The micromixer studied here is simply a heterogeneous parallel plate microchannel which is imposed to the electroosmotic flow field. For the thorough modeling of such flows, the coupled equations of Navier-Stokes, Nernst-Planck, Poisson-Boltzmann and concentration equations are solved for the flow motion, electric charges transport, electric field and species concentrations, respectively. Numerical solution of these set of equations for the heterogeneous microchannels is complicated and difficult. Therefore, a simple and approximate model such as Helmholtz-Smoluchowski has been proposed which is appropriate for the case of microchannels with the homogenous properties on the walls. Validation of Helmholtz-Smoluchowski model has been well-used for the prediction of two dimensional flow fields, yet its application is rarely validated for the prediction of concentration field and mixing performance. In this article mixing due to electroosmotic flow field is investigated using Nernst-Planck equations as well as Helmholtz-Smoluchowski models and the accuracy of the Helmholtz-Smoluchowski model is evaluated. Comparison of the results indicates that in the proper condition the approximate model can predict the mixing performance accurately along the micromixer length.