Scientia Iranica
Volume:23 Issue: 5, 2016

The impact of a slit placed along a circular cylinder diameter, which is parallel to the incoming flow, is numerically investigated in order to nd a geometric modi cation that can help drag reduction, as well as vortex suppression, without any additional energy consumption. The drag reduction is achieved by diverting part of the fluid in the front stagnation region into the low pressure zone at the back of cylinder through the slit. The Reynolds number based on the cylinder diameter, D, ranges from 60 to 250. The e ect of the slit width ratio (s=D) on the drag and lift coecients, the Strouhal number, and the wake flow feature of the cylinder is presented. Favorable reduction of drag and lift coecients is observed with the rising of slit width ratio. The vortex shedding generated from the slit alters the wake flow thoroughly and drives the vortex shedding derived from the cylinder surface to further downstream.

In the present paper, conceptual duct shape design for kinetic energy extraction with hydrokinetic turbines is discussed. The goal is to nd a single-passage axisymmetric geometry that holds stable flow with maximum kinetic energy ux at duct throat. For nding the optimum duct shape, the fluid flow was numerically simulated in a wedge shaped space with Flow-Simulation Software. In a multi-stage conceptual design, tabulated con gurations were employed to study each geometrical characteristic separately. These include curvature of pro le camber, trailing edge shape, pro le tip shape, and duct exit cross sectional area. The revolved pro le of each duct consists of a well constrained composite curve with few degrees of freedom. The Sketcher environment of SolidWorks Software provides a feasible method of rebuilding constrained curves. Duct shape optimization was performed based on successive flow simulation and approximation of optimum geometric dimension at optimum flow condition. The drag coecients were compared with available experiments. Based on the numerical simulations with needle shaped leading edge, the duct throat velocity can be increased. Inversely, the flow blockage can reduce the kinetic energy flux at duct throat. The optimum duct shape has shown the greatest frictional drag coecient and the minimum flow separation.

The Di erential Quadrature Method (DQM) is one of the most powerful approximation methods for analyzing the free vibration of rectangular plates. It is easy to use and straightforward to implement. However, in spite of its many advantages, the conventional DQM has some limitations in determining the natural frequencies of rectangular plates involving free corners. This is because it is very dicult to implement the free corner boundary condition in conventional DQM. As a result, the method may exhibit some convergence problems and this may lead to erroneous and oscillatory results for natural frequencies of rectangular plates involving free corners. To overcome this diculty, this paper presents a simple DQM formulation in which all the natural boundary conditions, including the free corner boundary condition, are implemented in an easy manner. Its accuracy and eciency are demonstrated through the vibration analysis of rectangular plates with di erent combinations of free edges and free corners. Numerical results prove that the proposed method can produce much better accuracy than the conventional DQM while exhibiting a monotonic convergence behavior with respect to the number of sampling points. Furthermore, unlike the conventional DQM, solutions of the proposed method are not very sensitive to the sampling point distribution.

Heat transfer and hydrodynamics of air flow in a constricted microchannel are numerically investigated by considering the e ect of geometry, stagnation pressure and temperature at the inlet, and applied heat flux at the walls. Kurganov-Tudmor method is used to solve the governing equation; Maxwell and Smoluchowski approaches are used to model the rarefaction e ects of compressible micro flow. This study shows that using slip model is necessary in both high-speed and low-speed flows, especially at the downstream of the constricted portion. Also, throat height reduction results in more rarefaction and compressibility at the constricted region.

In the recent years, di erent mathematical models have been suggested for maneuvering of displacement vessels that are capable of estimation of vessel maneuvers with acceptable precision. These mathematical models are based on determined hydrodynamic coecients and their accuracy depends on the known coecients used to solve the mathematical model. System identi cation methods are developed to calculate these coecients utilizing input and output data obtained from di erent sources. In this research, a 4.36-m model of KRISO Container Ship (KCS) displacement vessel has been manufactured by berglass, and the maneuver turning tests have been carried by selfpropulsion method. A 3DM-GX1 sensor, together with a protractor and Global Positioning System (GPS), has been used to measure the yaw and rudder angle, position, linear accelerations, and angular velocities of the vessel in di erent times. The hydrodynamic coecients in the mathematical model are determined by the Extended Kalman lter method. Then, the mathematical model is solved and di erent maneuvers are simulated by coecients calculated from the experiments. Simulations are validated by model tests. The mathematical model and hydrodynamic coecients presented in this paper can be applied for optimization of ship maneuvering performance and course control purposes

An algorithm for aerodynamic preliminary design of axial compressors has been developed. The aerodynamic design modules are meanline design and annulus sizing, velocity triangle determination, blade design, and axial compressor three-dimensional geometry generator. The hub and tip radii are computed through the meanline method and the velocity triangles are determined through radial equilibrium and vortex equations. Using incidence and deviation angles correlations of NACA 65, DCA, and NACA 63- A4K6 pro les, the blades sections are designed on the compressor streamlines. The three-dimensional geometry of the designed compressors is the output of the developed preliminary design algorithm, which is generated through the developed geometrical code. This code generates the pro le coordinates of blades sections and stacks them over a guideline curve and assembles them on the compressor axis. In di erent sections of this article, the aforementioned modules are explained, the developed algorithm is presented, the required design variables are introduced, and the design constraints are investigated. Finally, the aero-dynamical and geometrical speci cations of the redesigned NACA 8-stage compressor achieved through the developed algorithm are presented and compared with the ones from original compressor.

In this study, the nonlinear oscillations of micro/nano beams, modeled by Timoshenko beam theory and actuated by suddenly applied electrostatic forces, are investigated. The e ects of electrostatic actuation, residual stress, mid-plane stretching, and fringing eld are considered in modeling. In order to develop the governing equations and the boundary conditions, the Hamilton''s principle is employed. After combining governing equations, the Galerkin''s decomposition method is used to convert the governing nonlinear partial equation to a nonlinear ordinary di erential equation. The Homotopy Analysis Method (HAM) is used to present semi-analytical solutions to the strongly nonlinear behavior of system. To verify the present model, in special limiting cases, the results are compared with numerical results; and in low values of beam thickness, the results are compared with those obtained with the assumption of Euler-Bernoulli beam theory, which are available in literature. Some numerical results are presented to investigate the e ects of high thicknesses and di erent values of residual stress on the nonlinear frequency and the midpoint de ection of the beam.

The exceptional mechanical performance and outstanding material properties of biological structures have attracted much attention during recent decades. Finite Element (FE) Method is one of the most useful tools for investigating the e ects of di erent parameters on the mechanical behaviour of such complex structures. This method, however, presents a major challenge for engineers and scientists, because many biological structures have complex shapes and geometries that are almost impossible to be modelled by using traditional modelling techniques. In this paper, we present a simple modelling method which is applicable for two- and three-dimensional (2D and 3D) geometric modelling of planar structures that are mainly made of one or two materials. The results show that the proposed method, which employs a Digital Image Processing (DIP) technique, is successfully able to develop precise FE models of natural structures considering their complex shapes and corrugated patterns.

A clear understanding of force-frequency e ect is essential for enhancing the accuracy of quartz crystal resonators and sensors. Several analytical models have been developed to predict the frequency change of piezoelectric crystals like quartz due to forcefrequency e ect. According to these models, frequency change is due to initial mechanical stresses caused by the application of diametrical forces. Usually, the anisotropy and piezoelectricity of quartz are neglected in determination of stress bias and in force-frequency equation. In this research, we quanti ed the e ect of anisotropy and piezoelectricity of ATcut quartz on the force-frequency coecient. To this end, besides using the mathematical models, a nite element code, based on linear and nonlinear Lagrangian formulations, was developed. The FEM showed more reliable results than di erent analytical approaches. We found that the force-frequency coecient is more sensitive to the anisotropy than to the piezoelectricity of quartz. By considering the anisotropy, the standard error of the force-frequency model decreased from 12.4 E-15 (ms/N) to 5.75 E-15 (ms/N). On the other hand, anisotropy can modify the shape of force-frequency curve at force azimuth angles close to 0.

In this paper, the conjugate heat transfer for water-Al2O3 nano fluid flow in double pipe heat exchangers was numerically modeled. The important parameters such as temperature distribution, local heat transfer coecient, pressure drop, and the heat transfer rate in inner and outer fluids were evaluated and compared. All the obtained results were simultaneously analyzed for parallel and counter flows, laminar and turbulent flows, and the presence or absence of nano fluid. The nano fluid flow was modeled by employing a twophase mixture method. The ndings indicate that parallel or counter flows have a more signi cant e ect on the heat transfer performance in the laminar flow than the turbulent one. The results also show that for warming a cold fluid, the most e ective mechanism is to use nano fluids in the tube containing the warm fluid. Similarly, for cooling a warm fluid, the most ecient method is to use nano fluids in the tube containing the cold fluid (using the nano fluid in the other tube).

electric eld. In this work, a wire-cylinder corona charger is presented, redesigned, and aerodynamically optimized. An initial 2D axisymmetric geometry of the charger was employed for the simulations using the Computational Fluid Dynamics (CFD) commercial code FLUENT 6.3.26. Through successive attempts, a new geometry was obtained by streamlining the walls to eliminate the undesired vortices produced in the flow eld of the previous ones. The process optimized the charger by minimizing losses and dilutions of the particles. For electrical simulations of the charger, a new numerical algorithm was designed based on the steady-state corona discharge to work with segregated solvers to satisfy governing equations. The algorithm was validated using a one-dimensional semianalytic solution of corona discharge. Tracing particles for the optimized geometry, the percentage of losses was calculated 6%, whereas the loss in the old geometry was more than 30%. The average charge and charge distributions induced on particles were also calculated with evaluation of the residence times in the charger.

In this investigation, nite element technique and experimental method were utilized for analyzing the residual stresses created during the welding process of plates. In this way, a welded specimen was prepared with controlled welding parameters. Hole drilling technique was used to measure the residual stresses in some points of the welded specimen. To evaluate accuracy of the FE modeling, a comparison between experimental data and FE results was utilized and showed a good agreement. According to this study, the procedure of the FE modeling with acceptable accuracy was extended. This improved model was employed to investigate the in fluence of the groove shape on the remained stresses of welded plates. It was shown that the weld groove shape can a ect the distribution of the remained stresses in thick welded plates, but it had no signi cant e ect on peak stresses.

The steady-state viscous flow and heat transfer in the vicinity of a nonaxisymmetric stagnation point of an in nite stationary cylinder with non-uniform normal transpiration, U0(''), and constant wall temperature are investigated. The impinging free stream is steady and with a constant strain rate, k. A reduction in Navier-Stokes and energy equations is obtained by use of appropriate similarity transformations. The semisimilar solution of the Navier-Stokes equations and energy equation has been obtained numerically using an implicit nite-di erence scheme. All the solutions above are presented for Reynolds number, Re = ka2=2, ranging from 0.01 to 100 for di erent values of Prandtl number and for selected values of transpiration rate function, S('') = U0('')=ka, where a is cylinder radius and  is kinematic viscosity of the fluid. Dimensionless shear stresses corresponding to all the cases increase with increase in Reynolds number and transpiration rate function. The local coecient of heat transfer (Nusselt number) increases with the increasing transpiration rate function and Prandtl number.

This research deals with the performance and cost assessment of a Kalina cycle integrated with Parabolic-Trough Solar Collectors (PTSC) using advanced exergy and exergoeconomic based methods to identify the improvement potential and the interaction among system components. The exergy destruction rate and the total operating cost within the components are divided into endogenous/exogenous and unavoidable/avoidable parts. Results indicate that the avoidable exergy destruction cost rate ( _C AV D ) of the entire system is only 29%, of which 32% is related to the components and 68% is due to the interaction between them. Also, the endogenous part of the exergy destruction cost for the entire system is notably high (87%) and the contribution of _C AV D in the entire system is 32%, of which 68% is related to the endogenous part. Furthermore, 84% of the investment cost rate is associated with endogenous cost rate ( _Z EN) and 84% of the avoidable investment cost rate ( _Z AV D ) is endogenous. It is revealed that the auxiliary heater and PTSC have the highest modi ed improvement potential with values of 66.9% and 59.5%, respectively.