نشریه مهندسی مکانیک مدرس
سال دوازدهم شماره 6 (اسفند 1391)

Internal-force-driven flows in which the force acting on channel cross sections have a perfect uniform distribution create a fully developed velocity field even the axial distribution of these forces is non-uniform. In this situation، firstly the driving force with non-uniform axial distribution can be removed temporarily and then one can use an equivalent axial uniform body forcealternatively throughout the channel. In this case، although the distribution and the driving force change but the resulting velocity profiles remain unchanged. The main advantage of thisreplacement is thatthe solution of the equations in the 3-D geometries canbe converted to a 2-D solution using Poisson equationin the channel cross section. After determining the velocity distribution in the cross section، one caninverselycalculate the actual pressure distribution easily. This will be done by resuming the real axial force. One of the applications of this simplification is that the simulation of MHD channel flows can be carried out easily. Good agreement between the results of the new solution method and the results ofthe perfect solutions shows that the present method with enough accuracy can be used for prediction of velocity and pressure fields in microfluidic networks. Consequently the heavy costs of 3-D analysis are reduced considerably.

Unique properties of boron carbide (B4C) such as high hardness، low density، and comprehensive area for Neutron attraction، have turned this material into a very suitable candidate for many industrial applications such as nuclear facilities and light armored plates. According to inappropriate sinter ability of boron carbide، phenolic resin was utilized as sintered help for this ceramic. Different free additive samples of B4C with 5wt% phenolic resin were prepared and sintered at 2200°C. Then their physical and mechanical properties were investigated. Results show that the relative density of samples including 5wt% phenolic resin is equal to %95 and for samples without additive is equal to %82 of theoretical density. Furthermore، it can be seen an improvement in mechanical properties in comparison of free additives samples; so that the flexural strength from 264 to 318MPa، the modulus of elasticity from 445 to 465GPa، Vickers hardness from 3020 to 3150GPa and fracture toughness from 2. 6 to 4. 2MPa. m1/2 will be improved.

In order to investigate the effect of various parameters on the adsorption chiller performance، the adsorbent bed should be modeled with appropriate governing equations and assumptions. In order to study the adsorption chiller numerically، the governing equations should be modeled in four domains of thermal fluid، metal tube، fins and adsorbent bed simultaneously. One of the assumptions، which greatly influenced the modeling complexity and computation cost is the uniform pressure approach for the bed. For some of modeling conditions the pressure of the bed should be calculated and the inter particle resistance cannot be neglected. In this study with comparing the governing equations of uniform pressure and non-uniform pressure approach، a dimensionless number is introduced and a limiting value is determined with the help of numerical results such as specific cooling power. This parameter can be employed to decide about the appropriate bed pressure assumption prior to start the modeling process.

This paper investigates suitable approximation for Calculating the thermal radiation flux divergence and effect of errors on performance evaluation of porous radiant burners (PRB). Thus، a single layer and a double layer of buried flame type of the porous radiant burners have been selected and numerically simulated. Due to the significant difference in the temperature of the solid matrix and the fluid passing the burner، the energy equations was considered as a non-local thermal equilibrium. Complete kinetics of methane air was used for combustion modeling. Since the effect of lateral walls should be neglected the problem was solved in 1D to present exact solution of RTE and compares the other approximations. Results show that discrete ordinate as well finite volume approximation of RTE show that eight directional spherical split is the best selection. Lower ordinates have substantial deviation and increasing the number of division enlarges computation cost without any considerable improvement on errors reductions. Furthermore، two flux method and Rosseland approximation are not valid for this kind of modeling.

The vertical wind tunnel، which has been designed for sky-diving operations، consists of an octagonal to square diffuser. The diffuser configuration indicates that the wall angles in the diffuser are not symmetrical: the smaller and larger angles being 3. 43 and 10. 66 degrees، respectively. Estimation of pressure drop and study of flow separation in the diffuser is of great importance for the design of the vertical wind tunnel. In this research work، a 7. 8 % model of the original diffuser has been constructed، and airflow، in both blower and suction modes، has been studied in the diffuser using electronic pressure gauges. Pressure drop in the test diffuser has been measured and compared with pressure drop in a circular to square diffuser، which is installed in the vicinity of the test diffuser. To estimate the pressure drop in the test diffuser، the larger diffuser angle from the above، namely 10. 66 degrees should be used in the semi-empirical equations، and use of the equivalent diffuser angle is not recommended. In addition، study of the pressure recovery coefficient showed no significant flow separation in the tested diffuser.

Passive walkers are robots، which perform a walking like، stable limit cycle on small slopes without any external control. This concept was published on 90’s by McGeer and there are lots of related researches going on in the past few years. Keeping in mind the novelty of the concept، investigating the effects of structural parameters on walking performance and finding their optimums، simulating the biped and establishing a trend to its optimal design and build، and finally doing experimental researches، would be of a great concern. In this research، a deployed model of biped that can be built has been considered، and then its walking performance sensitivity such as efficiency، stability and robustness on uneven trains due to variation of structural parameters and their optimum limits have been investigated. It was shown that the foot arc radius and center of mass height have the most important effect on walking performance. After comparing the results with previous researches and doing simulations in MSC. ADAMS software، an optimum design trend has been suggested. At the end based on experimental results، it was shown beside optimization of structural parameters، considering the impact condition as well would be very important to achieve optimal walking.

In this study، dynamic stability of a system consisting of three torsionally elastic shafts with different rotation axises is analyzed. The system stability have been investigated by means of a three degree-of-freedom model in a spatial coordinate (three dimensional). Each shaft carrying a rigid disk at one end and have been linked through two Hooke''s joints. Equations of motion for the system were derived. These equations are linearised. After linearization of the differential equations are shown to consist of a set of Mathieu–Hill equations. Their stability are analyzed by means of a monodromy matrix method. Finally dynamic stability regions have been shown on different system parameters such as rotational velocity، misalignment angle’s of shaft axis، stiffness and rigidity of shafts. The stability charts constructed on various parameters. It was observed that with increasing inertia disk ratio and decreasing Hooke''s joint angle، the stable region increases.

In this paper، an approximate solution using layer-wise theory for the vibration analysis of rotating laminated cylindrical shells with ring and stringer stiffeners under axial load and pressure is presented. The cylindrical shells are stiffened with uniform interval and it is assumed that the stiffeners have the same material and geometric properties and cylindrical shell reinforced by outer stiffeners while stiffeners are treated as discrete elements. The equations of motion are derived by the Hamilton’s principle. In deriving the governing equations three-dimensional elasticity theory are used and the study includes the effects of the Coriolis and centrifugal accelerations and the initial hoop tension. The layer-wise theory is used to discretize the equations of motion and the related boundary conditions through the thickness of the shells. The edges of the shell are restrained by simply supported boundary conditions. The presented results are compared with those available in the literature and also with the FE results and excellent agreement is observed. Finally، the results obtained include the relationship between frequency characteristics of stiffened cylindrical shell and different geometry of stiffeners، stiffener type، rotating velocities، amplitude of pressure and amplitude of axial load.

Friction Stir Welding (FSW) is a solid state welding method. Now، for joining the variety of materials، especially dissimilar metals، has many applications. This method has no restrictions on the fusion welding. In addition to its many advantages، including the joining of metals with different melting points. In this paper، a successfully joint between Al5050Aluminum alloys to AISI304 stainless steel was reported. Friction Stir Welding (FSW) process was used for joining these dissimilar materials. In friction stir welding many parameters such as tool rotational speed، feedrate، offset and pin profile were effective on microstructure and mechanical properties of weld nugget by. This paper is focused on the effect of tool rotational speed، feedrate and offset on tensile strength and micro-hardness. In addition، effect of annealing operations was investigated on microstructure and mechanical properties of the weld nugget. The elongation and tensile strength of the weld nugget were increased 100 and 9 percent respectively by the annealing process.

Plasma actuator is one of the newest devices in flow control techniques which can delay separation by inducing external momentum to the boundary layer of the flow. The purpose of this paper is to simulate a NLF0414 airfoil both experimentally and numerically in presence of the body force vector induced by a specific plasma actuator. For this reason، the simulation is done both numerically and experimentally for a NLF0414 airfoil with the compressible 25 m/s velocity airflow in two different cases: with no plasma actuator located on the airfoil and with body force produced by a plasma actuator located on the top of the airfoil in order to investigate the effect of plasma on the flow passing over it. The results showed that presence of a plasma actuator on the top surface of the airfoil، close to the separation point، transferred the separation point from x=16 mm to x=41 mm at the angle of attack of 18 degrees. This separation delay caused a 35% increase in the ratio of lift to drag coefficient or the efficiency of the airfoil in the same angle of attack.

One directional and elliptical vibration cutting of IN738 at ultrasonic frequency has been investigated both experimentally and by FEM in the present study. The influence of each process on the cutting force was studied. The FEM modeling was carried out by using MSC-MARC. The results were compared with the experimental findings of the conventional cutting. The ultrasonic vibration was applied to a rigid cutting tool along the cutting velocity in one directional vibration cutting. In elliptical vibration cutting the vibration was applied both along the cutting velocity and in the chip flow direction. The experiments were carried out on an ultra precision CNC lathe with single crystal diamond tools. The same effects were confirmed in the machining practice and by FEM. It was quite feasible when machining IN738 to obtain the advantages of elliptical vibration cutting already reported for some other materials such as copper، aluminum، tungsten and super alloys.

In this study، the effect of number of stages on Tesla microvalve performance has been studied. To do this، different layouts including one to four-stage microvalves are investigated numerically. The main criterion is used for evaluation of valves performance is diodicity. Two-dimensional and steady state computations of the fluid flow have been utilized that reveal a strong dependence of diodicity on Reynolds number and the pressure drop. The results showed that for the same flow condition، the diodicity average of the two-stage microvalve is approximately 1. 32 times of that of one-stage. Additional stages increase the complexity and they do not change the diodicity considerably. It is concluded that two-stage layout of Tesla type valve is the best option between the studied layouts. A two-stage layout of this valve in valveless micropump besides being compact، has the adaptability of the various functions. Also، the two-stage valve performance in three different sizes is compared with nozzle - diffuser type valve. Comparisons which are performed based on calculation of diodicity for applicable range of Reynolds numbers show that the diodicity is function of Reynolds number and is independent of the valve size. Also، the superiority of the Tesla type valve for higher Reynolds number and its weakness at lower Reynolds number are shown.

Flow analysis in the microchannels has recently accelerated dramatically. In this paper، numerical investigation of Joule heating effects on the electroosmotic flow through a microchannel with the trapezoidal cross-section and constant wall temperature have been presented. The energy equation for the temperature distribution، Navier–Stokes equation for the velocity distribution and a Poisson equation for the electric potential distribution have been solved by using the finite-volume method in a system curvilinear coordinates. Thermophysical properties such as the dynamic viscosity and electric conductivity vary with temperature. Results show that by increasing the Joule number، the temperature، velocity and mass flow rate increase with constant EDL number. Without considering the Joule heating effects، the increments of EDL number causes in the mass flow rate to increase، but with considering the joule heating effects، the increasing of mass flow rate continues until EDL number 15 and after that the flow rate decreases. On the other hand، when the cross-section is reduced by the increasing aspect ratio، the joule number remains constant while the mean temperature decreases.

In this paper، a novel four-variable refined theory of plate، called RPT، has been proposed for free vibration of composite laminated plates، using a hyperbolic sine function، for calculating out-of-plane shear strains. It is one of the properties of this theory that the boundary condition of zero shear stress is satisfied over upper layer and under lower layer of plate، with no reference to Timoshenko shape factor. In contrast to other higher-order shear deformation theories، in RPT theory، equations of motion are coupled dynamically only in inertial terms، while elastic energy terms are not coupled for the variables used. From this viewpoint، RPT theory is similar to classical plate theory (CLPT). Some of the objectives of this paper are the investigation of effect of influential parameters on fundamental frequency، such as modulus ratio، angle of plies، and plate length-to-thickness ratio. The results of this proposed version of RPT are compared and validated with those of first-order shear deformation theory (FSDT)، higher-order shear deformation theory (HSDT)، and the original version of RPT.