JOURNAL OF SOLID AND FLUID MECHANICS
Year:2015 | Volume:5 | Issue:3|Papers Count:19

The necessity of being regular in plan and elevation of concrete tunnel form buildings in spite of accelerating the manufacturing process and high quality assurance, following several limitations in architectural design and puts limits on the application of this system in areas where there is no possibility of symmetrical construction. Loss of exact amount for R-factor in the current earthquake regulations is a major challenge in design of these structures by force. In this study, the seismic behavior of two-tunnel form structures with 5 and 10 stories with irregular floor plans is investigated and the coefficients of demand and capacity, respectively, based on seismic demand and capacity of structure have been calculated. Also, uncoupled frequency ratios and fragility curves are determined for studied models by incremental dynamic analysis (IDA). The results show high capacity of the system and flexible torsional behavior of the structures due to their irregularity. Since both structures are placed in the immediate occupancy performance level in design earthquake, it seems that criteria which necessitate tunnel form structures to be regular in plan, are strict and prudent for the studied structures.

Actuators of robot operate in the joint-space while the end-effector of robot is controlled in the task-space. Therefore, designing a control system for a robotic manipulator in the task-space requires the jacobian matrix for transforming joint-space to task-space. Using an imprecise Jacobian matrix and the presence of uncertainties degrade the control performance. Uncertainties include the parametric uncertainty, unmodelled dynamics and external disturbance. In this paper, a novel decentralized adaptive fuzzy sliding mode control approach in the task-space is presented using the voltage control strategy. The control design is simple, free from model and robust agains uncertainties. These advantages are because of using voltage control strategy instead of commonly used torque control strategy. The case study is an articulated robot manipulator driven by permanent magnet DC motors. The stability analysis, simulation results and comparison with a torque based control method are presented to verify the effectiveness of the control method. Simulation results show the effectiveness of proposed control method.

The wind turbine blades are made of composite materials and subjected to severe operational loadings. The complexity of composite materials behavior along with variable amplitude loadings dictates the need for experimental setups which can conduct real part test as close as possible to in service loadings. In this regard, wind turbine blade manufacturers are obliged to perform standard ultimate and fatigue tests on their products. In this research a cost effective Blade Testing Machine (BTM) is proposed which is capable of conducting ultimate static and fatigue tests according to wind turbine blade standards. A new control unit is designed and implemented to track fatigue block loading in the frequency range of 1 to 3Hz. The main focus is on designing a controller to perform desired block loading fatigue tests with proper performance. PI and robust feedback controllers are designed and analyzed. Due to the poor robust performance, an adaptive feed forward controller is proposed based on the gain scheduling algorithm. The proposed robust controller compensates un-modeled high frequency dynamics and rejects measurement noises while the adaptive controller compensates low frequency uncertainty and improves reference tracking. Extensive experimentations in the desired frequency range confirm proper performance of the developed controller.

In this article, the high strain rate, dynamic plastic response of materials in the standard plate impact test is numerically studied. To get the closest results to the experimental data available in the literature, different types of hardening as well as dynamic fracture models are employed. With the aid of this standard plate impact test, one can verify the new models in the ideal condition of uniaxial-strain cases.To properly model this test, to discrete the field, von-Neumann Finite-Volume method is utilized. Jonson-Cook and perfectly elastoplastic hardening models as well as the Zerilli-Armstrong model are used beside the dynamic fracture model of modified Tuler-Butcher, to predict the spall phenomenona. In this work, the impact of two plates (the flyer plate and the target plate) is analyzed. Results of the simulation is compared with the experimental data as well as the other numerical results reported in the literature. The results of the present work are in a better correspondence comparing to the experimental data. Among investigated models, employing JC constitutive model, accompanying with the modified Tuler-Butcher fracture model and Steinberg model for the elastic modulus gives the most accurate results compared to other model combinations.

Target function in vehicle crash, is about the suitable deformation of vehicle body structure in the case of do not harm occupant’s space. As a consequence the vehicle body structure should be deformed to absorb the impact’s energy and decrease the transmitted forces to occupants. In this paper at design process the vehicle’s body in white has been simulated in CATIA software firstly, and then the element has been created by the Hyper mesh pre-processor software and finally in Pam Crash software, after introducing appropriate materials, standards joints and contacts between vehicle’s parts, and then loads has been created by initial velocity for able to deform barrier toward side panel of vehicle. The vehicle body structure is designed in Vehicle Research Center, based on the style of B-class vehicle platform’s product and the analysis results of this paper is part of new product development process of national platform’s project. As a result, the requirements out puts such as displacement, velocity, acceleration, and energy absorption and section forces have been extracted. After all the results has been compared with the targets which are predicted. Then the suitable thickness for B-pillar illustrated based on the out puts which are extracted.

In this paper, one-dimensional design of centrifugal compressor with inverse design of 90-degree bend duct between radial and axial diffuser is accomplished using an iterative method. In the design procedure, all overall dimensions of the centrifugal compressor including impeller, vanned radial diffuser, 90-degree bend duct and axial diffuser are computed. To evaluate the results, after the geometry modeling and grid generation, 3-D flow field in the whole compressor are numerically analyzed using CFX software. The numerical simulation shows that there is a high loss region through the 90-degree bend between the radial and axial diffuser because of its high curvature. In order to reach a minimum loss in the 90-degree bend duct, the aerodynamic design of the 90-degree bend duct is carried out using an inverse design method. At the first step, the shape modification capability of the 90-degree bend duct is evaluated by linking up the Ball-Spine inverse design algorithm and CFX software. Then, the geometry is modified by improving the hub and shroud pressure distribution and applying it to the inverse design code. Finally, the designed compressor with the modified 90-degree bend duct is analyzed. The results show that, the total pressure ratio and overall efficiency increases about 3 and 4 percent, respectively.

In this research, for the first time, the net of control points in the isogeometric analysis has been improved by employing an error estimator based on a stress recovery method. First, an error estimation algorithm based on stress recovery is used to obtain the energy norm for each element. Then, artificial rods are defined between control points and the estimated values of errors in the control points located at the vicinity of a typical point is assigned to each rod as a thermal gradient. Now, by analyzing this hypothetical truss problem under temperature changes a new arrangement of control points and consequently the knot vectors can be obtained. Repeating this process in isogeometric analysis will lead to a better distribution of errors in the domain of the problem and results in an optimal net of control points to calculate the integrals. To evaluate the efficiency of this method, the results of modeling and analysis of two elasticity problem is presented. The obtained results show that this innovative approach has a good performance and can be employed for reducing the analysis errors.

The present paper investigates the effects of grid configuration on the buckling and vibration behavior of grid structures. Hence, four similar simply supported plates with equal weights and different grid patterns are considered. Using, bending stiffness matrix, the buckling load and free vibration frequency of the plates are computed. To investigate the effects of the grid orientation on the mechanical behavior, the orientation of the grids is changed in the plates. The Rayleigh–Ritz method is applied to obtain the axial and shear buckling load and free vibration frequencies. The results are compared with an angle-ply laminated composite plate with similar inplanedimensions and equal weight. The results show, changing the grid pattern and orientation, will affect bending stiffness and consequently, significantly change the buckling and vibration behaviors of the grid plates.

In this paper, the stress intensity factor for a circumferential crack in a thick-walled cylinder is derived analytically and numerically which is subjected to the non-Fourier (hyperbolic) thermal shock. The uncoupled thermoelasticity governing equations for an uncracked cylinder are solved analytically. The weight function method is implemented to obtain the stress intensity factor. The non-dimensional hyperbolic heat equation is solved using finite Hankel transform and separation of variables method. Results show the different behavior of the crack under hyperbolic thermal shock. For relatively short cracks, the maximum stress intensity factor of Fourier and hyperbolic models is closed. But for longer cracks, the stress intensity factor of the hyperbolic model is significantly greater than Fourier model. Moreover, the maximum stress intensity factor in hyperbolic model occurs for a crack the peak of stress wave reaches to its tip. According to the results, assumption of adequate heat conduction model for structure design under transient thermal loading is critical.

In this paper a collaborative simulation between Matlab/Simulink and Fluent softwares is done to active control of an elastically mounted circular cylinder, free to move in in-line and cross-flow directions. The control goal is reduction of the two-dimensional vortex-induced vibrations (VIV) of cylinder. The natural oscillator frequency is complemented with the vortex shedding frequency of a stationary cylinder. A parallel simulation scheme is realized by linking the PID controller employed in Matlab/Simulink to the plant model constructed in Fluent, aiming at calculation of the control force necessary for total annihilation of the transverse cylinder vibrations. The simulation results reveal the high performance and effectiveness of the adopted control algorithm in diminishing the VIV of elastic cylinder. Once the control algorithm is turned on, there is a extreme reduction in the transverse and in-line cylinder oscillation amplitudes as well as lift and drag coefficients values. In particular, it is observed that the coalesced vortices in the far wake region of the uncontrolled cylinder are seprated and displaying wake vortices of weaker strengths.

This research investigates the mechanical properties of High Performance Fiber Reinforced Cementitious Composites (HPFRCC) with two volume fractions of fiber (1% and 2%). Hooked steel fibers were incorporated into a mortar matrix with 49 MPa compressive strength. Four point bending tests were carried out according to ASTM C1018 and ASTM C1609 Standards. Parameters such as: load carrying capacity (equivalent bending strength), energy absorption capacity (toughness), deflection, and cracking patterns (number of cracks), were evaluated to investigate the flexural behavior of two HPFRCCs. It was found that the increase in fiber volume fraction not only promotes the flexural behavior from deflection softening to deflection hardening, but also improves all mechanical properties. Deflection capacity gains the most from deflection hardening behavior. Besides, substantial increase in load carrying capacity and energy absorption is also achieved. It was observed that HPFRCC with deflection hardening behavior exhibits multiple cracking in the post cracking behavior.

Hydroforming is a suitable solution method for removing complex parts’ forming problems such as stepped rectangular parts with uniform thickness distribution and higher quality. Optimization of the initial dimensions of the sheet in the hydroforming process has a large effect on producing products with lower price but higher quality. In this paper, optimizations of the sheet initial dimensions for two different geometries have been studied through sensitivity method. The same curves of formed sheet and final sheet has been seen at the end of the optimization step. In the optimization step, the ABAQUS software has been used to perform the simulation process. The examined parameters in this study are the effect of optimizing of the sheet initial dimensions on the thickness distribution and the maximum forming pressure. It has been concluded that the maximum thinning occurring in the punch radius and in the wall region is reduced by optimizing of the sheet initial dimensions at each stage. It must be mentioned that, investigating the maximum forming pressure shows that by the optimization step, material flow improves so, the maximum pressure will decrease.

In this paper, a high sensitive uncooled microcantilever infrared detector is designed and simulated. The detector consists of absorbing, bi-material and isolator regions, and has two layers suspended structure made of Silicon dioxide (SiO2) and Aluminum (Al) with a 1mm and 200nm thickness, respectively. Absorbing was increased by reflecting IR flux in absorber layer by coting Al under it, and bending of detector was increased by elongating bi-layer legs in absorber layer. Finite element analysis method was used to simulate thermal and mechanical behaviors. Temperature and displacement changes at the end of tip detector (farthest point from the support leg) were 3.651oC and 940nm, correspondingly, at the 100pW/mm2 boundary conditions for constant heat flux on the absorber. In this detector, the calculated heat transfer coefficient, power, temperature, body temperature, displacement and thermo-mechanical sensitivity are 9.7×10-3, 667.2mW-1, 32.8K/(pW.mm-2), 2.75nm/K, 9.34nm/(pW.mm-2) and 284nm/K, respectively. Those parameters are improved 16, 41, 17, 41, 41 and 2.38 times compared to the same Si3N4/Au detector.

One of the methods for increasing convective heat transfer is by utilizing twisted tapes in heat exchangers which enhances heat transfer in constant volume with increasing augmentation. In the past research, the combined effects of nanofluid and twisted tapes were not studied. In this research paper, the heat transfer performance of water/TiO2 nanofluid in a heat exchanger with twisted tape insert is evaluated. The studied parameters are pitch of twisted tape, mass flow rate, and concentration of nanofluid. The constant wall temperature is employed and the range of Reynolds number is from 3000 to 22000. Experiments were performed for the volumetric concentration range of 0 £f £0.5. The results confirm that heat transfer was enhanced by adding nano-particles. Also, the results were repeated by adding twisted tapes with lower pitches. The maximum heat transfer appraisal indicates a 103.45 percent increase for the concentration of 0.5 and the insert of the twisted tape with (H/D=5).

Aerodynamic characteristics of an airfoil are highly affected by the behavior of the boundary layer. This behavior, and the related phenomena, depends on such different parameters as Reynolds number, angle of attack, unsteady motion, local Mach number, and compressibility. In this paper a series of static and dynamic (pitching motion) tests at pre stall angle of attacks were performed in a high speed wind tunnel to study the steady and unsteady behavior of the compressible boundary layer on a Supercritical Airfoil. Some static tests were performed at Mach numbers of 0.4, and 0.5 whit maximum angles of attack of 6⁰ and oscillation amplitudes of 1o and 3o and oscillation frequencies of 3 and 6Hz in Sinusoidal pitching motions. Measurements involved pressure distribution and shear stress variations using multiple hot film. The effects of compressibility, free stream Mach number, mean angle, reduced frequency and oscillation amplitude were investigated. Results show delay in transition in pitching motion with respect to steady conditions, and asymmetry between transition and relaminarization during pitching motion.

When a wing is placed near a wall surface, two phenomenons occur. The lift force is increased and the drag force is reduced which will finally lead to high lift to drag ratio. This phenomenon is known as the effect of surface or ground effect. In this study, a two dimensional simulation has been developed to investigate the effect of oscillatory flow around a NACA 4412 airfoil near a wall surface. The effect of wing distance from the surface, the amplitude and frequency of the oscillating flow have been analyzed on the aerodynamic coefficients. The lift coefficient is increased due to the air compression and trapping between the underside of wing and the wall. The drag coefficient is reduced because of wall jet at the trailing edge which increases the back pressure. According to the results obtained in the unsteady case, by reducing the amplitude of free-stream velocity, the lift coefficient is enhanced and the drag coefficient is reduced. Also the drag coefficient decreases due to increasing frequency of the oscillating flow.

In this study attempt is made to study convection heat transfer from a new type of perforated fins with cross perforations using experimental measurement. Using the present experimental measurements, thermal performance of such perforated fins are determined. Comparison of convection heat transfer from the present array of perforated fins with the previous studies of perforated fins illustrates better heat transfer performance of the new arrangement and its higher convection heat transfer coefficient. Quantitatively, using the present fin arrangement increases the heat transfer coefficient and the fin effectiveness for about 50% with respect to the previous perforated arrangement. Comparison of the fins surface temperature measured by thermograph imager depict higher surface temperature for the middle fin. In addition, the present experimental data can be used to validate numerical simulations of perforated fins.

The forced convective heat transfer of a nanofluid composed of water and FMWNT nano-particles in a two-dimensional microchannel was numerically investigated. The bottom wall of the microchannel was fully insulated. The upper wall was insulated only at the entrance, the rest of the upper wall was subjected to a constant heat flux of q0”. A constant magnetic field with a strength of B0 was also applied on it. The slip velocity boundary condition was considered along the walls of the microchannel. Navier-Stokes equations were discretized and then solved numerically using a computer code. Results were presented in the form of velocity profiles, temperature, and the Nusselt number. In the present work, the effect of magnetic field on the slip velocity of the fluid adjacent to the microchannel wall was studied for the first time. The use of a nanofluid composed of water and carbon nanotubes (FMWNT), as the working fluid in a microchannel, could be another novelty of the present study. It was seen that stronger magnetic field corresponded to more amount of slip velocity.

Water management plays an important role in the performance of polymer fuel cell. Humidifying the reactant gases before entering the fuel cell and adjusting the wet content are among the most important ways for water management. In this study, a membrane humidifier of a gas-gas type has been modeled. The governing equations included mass conservation as well as energy and vapor transfer equations solved by numerical method and validated by experimental data available in relevant scientific articles. Then, the effect of operating parameters such as flow rate, temperature and relative humidity of the inlet gases and geometric parameters such as thickness, diameter and number of tubes on performance of humidifier have been studied. The results show that, in the same inlet mass flow rate in both wet and dry side, outlet wet gas temperature, heat transfer rate and vapor transfer rate in the counter flow are greater than the parallel flow, therefore the results for this humidifier are provided. In counter flow humidifier, by increasing inlet dry gas temperature, the rate of heat transfer and vapor transfer rate will decrease and also increasing relative humidity of inlet dry gas will reduces the vapor transfer rate and has negligible impact on outlet wet gas temperature and heat transfer rate. Finally, increasing the thickness, length and number of membrane tubes can have negligible impact on the heat transfer rate as well.