جستجوی مقالات مرتبط با کلیدواژه « صفحه ای » در نشریات گروه « مکانیک »

Superhydrophobic surfaces have gained significant attention as a promising approach for drag reduction of submerged objects. Accurate evaluation and prediction of drag reduction induced by these surfaces require expensive experimental measurements, numerical simulations, or the development of reliable models and correlations. In this paper, a model is proposed for calculating the skin friction coefficient and drag reduction of superhydrophobic flat surfaces. Utilizing previous data on the skin friction coefficient of flat surfaces under no-slip boundary conditions, a model is developed to estimate the skin friction reduction and skin friction coefficient of these surfaces after applying superhydrophobic coatings. The validity of the model is verified by comparing its results with those of computational fluid dynamics (CFD) simulations of flow over a flat plate at different velocities. The results of the model and simulations indicate that for inlet velocities of 1, 5, and 25 m/s and a slip length of 50 μm, drag reductions of 15%, 41%, and 77%, respectively, are expected. Additionally, the skin friction reduction increases with increasing flow Reynolds number. The developed model is validated for flat surfaces and its ability to accurately estimate the skin friction coefficient and drag force of these surfaces is thoroughly examined. However, further investigations are required to assess the model's validity for curved surfaces and variable slip lengths.

Waste heat dissipation from spacecraft subsystems is crucial due to spatial limitations (no convective heat transfer, limited electrical power, lightweight, and high reliability). Radiators are often responsible for collecting and dissipating this waste heat into cold space. Panel radiators are widely used in various satellites, including scientific, communication, and remote sensing satellites. In scientific satellites, panel radiators are used to dissipate heat generated by scientific instruments such as cameras and spectrometers. In communication satellites, panel radiators are used to dissipate heat generated by power amplifiers. In remote sensing satellites, panel radiators are used to dissipate heat generated by sensors and processors. High efficiency, light weight, and high reliability are the advantages of using this equipment. The main challenge in using it is to provide sufficient heat dissipation area and uniformity of the surface temperature when radiating waste heat to the cold environment (space). The use of heat pipes in the panel radiator structure provides this uniformity. A heat pipe radiator consists of a sandwich panel with an embedded network of heat pipes. Increasing the number of heat pipes reduces the temperature gradient across the radiator surface but increases the radiator weight. Due to the importance of equipment lightness in space systems, optimization of the number of pipes and their geometric arrangement in the radiator should be such that maximum temperature uniformity on the surface and minimum radiator weight are achieved. The objective of this research is to optimize the performance of a radiator (maximum temperature uniformity on the surface) to achieve minimum weight while considering the weight and size constraints imposed by the system designer as requirements. Initially, a mathematical model is developed and solved numerically, and the effect of design parameters on the performance of a panel radiator, including face and core thickness, spacing between heat pipes, mass, and surface area, is comprehensively investigated. Based on the simulation results, considering the weight limitations and existing face and core thicknesses, the maximum allowable spacing between heat pipes is calculated to achieve maximum efficiency of the panel radiator. A network of heat pipes with this characteristic was produced and used in the panel sandwich. The results obtained from testing the manufactured panel radiator were compared with the design efficiency to validate the model. Based on the experimental results, an efficiency of 89% was obtained at a root temperature of 39°C. The error of this efficiency with the efficiency calculated from the theory is about 3%.

A method for increasing the accuracy of the initial alignment process of inertial navigation systems with a stable platform is presented through state feedback control in flight mode. In the presented method, the state feedback controller is designed by using the stable plate deviation angles and the sensor error which is extracted with the help of Kalman filter. To do this, while checking the observability of the system, by adding suitable flight maneuvers and expressing the equations of propagation of navigation error, in the form of a fixed piece system with time, it is possible to estimate the angles and errors of the sensors in the align phase. The groundwork is provided for the design of state feedback. Then, taking into account the stable platform motion equations and using the principle of separation of observer and controller design, a state feedback controller is designed. In the end, the simulation results of the proposed method show an increase in the accuracy of the alignment process and, consequently, an increase in the accuracy of the navigation, compared to the conventional output feedback method.

Endplates are one of the most important components of polymer electrolyte membrane (PEM) fuel cell that must apply uniform contact pressure distribution on the membrane electrode assembly (MEA). For this reason, these plates must have good bending rigidity. In this research, the bending behavior of composite sandwich plates has been investigated numerically in order to achieve a composite structure with high bending stiffness for use in the endplates of PEM fuel cells. For this purpose, the bending strength of composite sandwich panels with grid core was evaluated based on the type of material and relevant standards through numerical simulation. Numerical simulation has been performed in Abaqus software using finite element method. The effect of different angles of fibers in and three different geometries of square, triangular and diamond grid core for C/Sic and E-Glass Epoxy materials on the bending behavior of composite sandwich panels were analyzed. The obtained results indicate the better performance of C/Sic materials in the stress test, as well as the more suitable bending behavior of triangular mesh geometry with fiber angles (0-45-90).

In this research, design and simulation of a single capillary injector and three-hole circular injector plate of a 10N Hydrazine monopropellant thruster were performed. Ansys Fluent software was used to simulate the injector and injector plate . Volume of fluid (VOF) method was used to simulate such a flow and turbulence was simulated by k-e model. The characteristics of the injector and injector plate including mass flow rate and average velocity in the injector nozzle were calculated by changing the inlet pressure. The results showed that the injector and the injector plate have the ability to supply the desired mass flow rate of the monopropellant thruster at a known design pressure. In fact the capillary injector has replaced swirl injector with hollow cone spray used in the previous version of this thruster. The dimension of the chamber was significantly reduced by using the capillary injector, which reduces both the volume of the expensive iridium catalyst and weight of the thruster.

Today, pool boiling heat transfer is used in many industrial equipments and engineering applications. Pool boiling heat transfer compared to single-phase heat transfer has a higher heat transfer capacity due to the use of latent heat evaporation of liquids. Therefore, in this study, the pool boiling heat transfer on a flat vertical plate and plate with triangular and circular grooves have been investigated using numerical simulations. In studies, the effect of adding two types of triangular grooves, two types of circular grooves with different dimensions and four heating surface temperatures has been investigated. In order to simulate the two-phase flow in this study, the volume of fluid method was used and the governing equations were solved using the finite volume method. The obtained results showed that adding triangular and circular grooves on the heating surface will increase the average heat flux. In the triangular grooved surface case, the average heat flux which entered to liquid increased between 128% to 177%. By using circular groove, the average heat flux aguments about 69% to 105%. Also, the water vapor value in the triangular grooved surface case incresead between 160% to 340% and in the circular grooved surface case rises between 101% to 155%.

In this study, the effect of FG porous on the free vibration behavior of sandwich plate with piezoelectric face sheets and FG core is studied. A new hyperbolic shear deformation function is presented in this paper. The properties of the FG core varied along the thickness according to power law. The properties of the FG material change with the power distribution as a function of the changes in properties along the thickness. The structure of porous materials has cavities and pores. Therefore, in this research, a linear porous model and two types of nonlinear porous models are considered for the property change function. The governing differential equations are derived using the Hamilton principle. The obtained equations were solved using the Navier method for simple boundary conditions. The effects of important geometric and mechanical parameters including thickness to length ratio, length to width ratio, ratio of FG core thickness to piezoelectric face sheets and the effect of electrical potential on the natural frequency response of porous FG sandwich and piezoelectric face sheets have been investigated. To verification, the analytical results obtained in this study are compared with the results presented in the literature, and in this comparison, a good agreement was obtained, which shows the correctness theory, deriving and solving equations.

Heat transfer enhancement from a flat plate in which a thin blade is rotating at a certain speed in the vicinity of the plate has been investigated in the present study. For this numerical study, the finite volume method was used in which the rotation of the blade was simulated by the definition of sliding mesh and interface boundary within the computational domain. The equations of mass, momentum, and energy were solved in two-dimensional and transient form for airflow with constant physical properties using Fluent commercial software and the details of the flow and thermal fields were obtained at the top of the plate. The rotational Reynolds number of the blade has been changed in the range of 1360-5460 and the dimensionless parameter of blade-end-to-plate distance to the blade length (d/D) was ranged from 0.105 to 0.420. The results indicated that heat transfer from the plate is improved by increasing the rotational speed of the blade, especially on the front side of the rotating blade, while the effect of the d/D was effective only over a limited area of the plate central section

In this research, the method of simulating the rarefied gas flow on a flat plate and calculating aerodynamic properties have been investigated using direct simulation Monte Carlo or DSMC-Open Foam Software. One of the most efficient methods for simulating rarefied gas flow in nonequilibrium conditions is the direct simulation Monte Carlo or DSMC simulation method, in which the gas is considered as a particle. In this study, using DSMC-OpenFoam software, the results obtained from a flat plate at zero angle of attack are validated with the available results, with the accuracy of less than 15% error.  In the following as an innovation, the flow on the vertical plane and the calculation of aerodynamic forces as well as the effect of different angles of the flat plane on the satellite panel are investigated. It has been shown that the drag coefficient in the vertical plane mode is 10 times that of the horizon flat plane. The method and software introduced in this research is a powerful method for various simulations of drag on satellites under rarefied gas condition. ‌‌

The study of turbulent boundary layer trailing-edge noise is a fundamental issue in design and production of flying vehicles with minimum noise. Over the past decades, to reduce trailing-edge noise, various passive methods have been proposed, and the most recent strategy is the use of noise-reducing finlets. In the present study, to investigate the effect of noise reduction finlets on the turbulent flow field in the vicinity of the model trailing edge, a flat-plate model equipped with unsteady surface pressure transducers has been designed and built. Then, by installing a set of finlets with different lateral spacing on it, their effects on the flow field downstream of the finlets have been experimentally studied. Results show that the flow behavior downstream of the finlets is strongly affected by the spacing between the finlets. While the use of coarse finlets leads to a reduction in the mean velocity, turbulence intensity, and energy contents of low-frequency turbulent structures in the near-wall regions downstream of finlets, the flow behavior downstream of fine finlets are somewhat similar to the flow behind a backward-step. For fine finlets, the energy contents of turbulent structures in the near-wall regions are significantly reduced over the whole frequency range. he results of cross-correlation and coherence between surface pressure fluctuations and velocity field also showed that while the most important mechanism affecting surface pressure fluctuations for the coarse finlets is the exit low-momentum flow, for the fine finlets, vortical structures formed by flow separation have the most impact.

In this research, the entropy generation by steady two-dimensional boundary layer flow of a hybrid nanofluid on a porous vertical flat plate with the magnetic field is studied. The governing equations by using of similarity solution transformed to nonlinear ordinary differential equations and also effect of buoyancy, magnetic, the volume fraction of nanofluid are investigated particularly on speed, temperature, and dimensionless entropy generation. The results showed that in entropy generation with an increase of similarity parameter, the thermal profile and absolute value of thermal gradient are decreased and therefore dimensional thermal entropy generation is decreased and trends to zero and after a certain amount thermal boundary layer is disappeared and just the entropy of fraction loss and magnetic field are produced. Also, the investigation shows with increasing density of nano particles, the speed of nanofluid decreases by about 30 percent and increases the thermal profile about 40 percent. The results also show with increase the magnetics, the speed of nanofluid decreases about 15 percent and decreases thermal profile about 10 percent.

In this study, the effect of rectangular finlets on the trailing edge noise of a flat plate in an incompressible turbulent boundary layer with Mach number 0.06 has been studied numerically. The finlets are modeled upstream of the trailing edge of the flat plate with two spanwise spacing of 1.5 and 0.9 mm. Large eddy simulation (LES) approach with Lund inflow generation model in open source code of OpenFOAM has been used to simulate the turbulent boundary layer flow. Probe utility has been used for pressure and velocity data acquisitions. The finlets have increased the PSD of pressure fluctuations in the low to mid frequencies and decreased it at high frequencies between and downstream of the finlets. Also, the spanwise length scale of the pressure fluctuations increases but the eddy convection velocity is reduced and their effects has intensified by the reduction of the spanwise spacing. The mean velocity decreases but the turbulence intensity increases downstream of the finlets. The reduction of the spanwise spacing of the finlets results in the formation of a stronger shear layer and the extension of the shear sheltering area downstream of the finlets. According to the results of the far-field noise prediction with Curle analogy, finlets with the spacing of 1.5 mm have slightly reduced the noise about 0.3dB but the finlets with the spacing of 0.9 mm have increased the trailing edge noise of the flat plate, about 1dB.

The main purpose of this study is to investigate the effect of the pulsating of the inlet jet on the heat transfer rate short distances of the nozzle from the concave surface. For this purpose, three-dimensional simulation of flow and heat transfer of sinusoidal pulsed jets on the concave surface has been performed at distances of 0.5 times of nozzle diameter to 4 and for Reynolds numbers of 7000 and 14000. The results of the numerical simulation are in good agreement with the experimental results of the steady jet. The result shows that the effect of pulsating the flow with the sine function decreases at short distances between the jet and the concave surface. So that at a distance of 4 times of nozzle diameter, pulsating jet led to a 10% increase in the average Nu, while this value is equal to 5% for a distance of 0.5 times of nozzle diameter. It can be found that pulsating the flow decreases Nu at low frequencies, and then with increasing the frequency of the pulsed jet, the Nu number increases. Furthermore, with increasing the distance between the surface and the inlet jet, the Nu number decreases significantly. This rate of reduction is lower in comparison to the steady jet.

In this work, a plate heat exchanger with an inconsistent flow is investigated in different working conditions. Four different models in laminar and turbulent flows with rigid and elastic plates at different Reynolds numbers have been studied, and the effect of the separator plate elasticity on the heat transfer rate and the efficiency of the heat exchanger has been investigated. A sinusoidal profile is used at the entrance to the cold passage. The results of this study showed that the elasticity of the intermediate plate in both laminar and turbulent flow has raised the heat transfer rate. Of course, in cases where the middle plane was elastic, the friction coefficient increased relative to the rigid plate. The efficiency of the heat exchanger is defined and is obtained for the heat exchanger to consider both heat transfer rate and pressure drop. Results showed that in most cases, the efficiency of the heat exchanger with elastic separator is more than one. The results showed that the maximum efficiency is 5.2 which was observed in Re=3000 and the minimum efficiency is 0.9 which occurs in Re=200.

Composite materials are widely used in modern aerospace flight vehicles, especially because of their high specific strength and lightweight than other materials. Since the study of reliability and uncertainty of composite material design variables in aeroelasticity has received less attention, in the present work the reliability of the laminated composite plate due to uncertainty in variables including elastic model, Poisson coefficient, density, thickness, and length is examined. The composite laminated plate is symmetric with different boundary conditions subjected to supersonic airflow. The classical plate theory and the first-order piston theory are utilized to derive the equation of motion. The differential quadrature method has been used to discretize and analyze the aeroelastic equations. The governing equations after discretization are solved by calculating and analyzing eigenvalues and the occurrence of the flutter phenomenon for the laminated composite plate is obtained. To examine the reliability, the distribution of random variables as a normal distribution has been used. Finally, the Monte Carlo simulation method was used for five different boundary conditions to obtain the reliability of the plate flat threshold. According to the presented results, the reliability value of the composite plate flutter threshold for the full hinge boundary condition (SSSS) will be higher than other boundary conditions and the all-bound boundary condition (CCCC) will be lower than other boundary conditions. Also, according to the studies on the condition of the fiber angle of the composite plate, it can be concluded that increasing the angle of the fiber of the composite plate increases the reliability of the occurrence of the filter threshold.

The aim of the present work is to determine the transverse deflection of a plate on the granular material under the impact of a ball. Through the analytical solution, the granular bed is equivalent to the elastic bed. The granular materials behave elastically by with a damping reaction under the impact loading. There are suggestions by literatures for introducing these elastic and damping effects which are used in the present work. The impact force is an intensive short duration force which is considered as a function of time and impact position. The equations of motion are derived by Hamilton's principle and the stress and strain relations are written by the first order shear theory and the vibration response of the plate is derived. Moreover, an experimental apparatus is designed to measure the plate deflection. Experimental parameters include the plate type and thickness, ball speed, impact position and granular thickness. The experimental data are used to validate the theoretical evaluations. The comparison of the evaluated and measured data shows that the theoretical model is acceptably gives the plate deflection.

Optimization in the design and maintenance of many engineering systems, economic and even social has been used to minimize the cost or maximize profits. In the buckling analysis the hybrid composite plates with cutout, the effective parameters on buckling are the cutout geometry, fiber angle, cutout size to plate size ratio, bluntness of cutout corners, and rotation angle of cutout. Therefore, in this study, using genetic algorithm method an attempt has been made to introduce the optimum parameters to achieve the Maximum amount of critical buckling temperature of hybrid composite plate with polygonal cutouts in different boundary conditions and stacking sequences, which are subject to uniform temperature rise. The cutout in this study are circular, pentagonal, and hexagonal. The solving method used to analyze this study is the finite element based on the energy method. Also, the theory used in this paper is the first-order shear deformation theory. The results presented in this case show that by choosing the appropriate shape of cutout and the Optimal selection of parameters affecting buckling, the plate's resistance to thermal buckling can be increased. It was also found that stacking sequences and boundary conditions have a significant effect on the critical buckling temperature of a hybrid composite plate with cutout.

This paper deals with designing the navigation scheme of a gimballed inertial system. This design is introduced and proved in the form of two theorems. Most of the gimballed navigation schemes proposed in the literature have the drawback of estimating position rates for alignment commands. Not only the estimating position rates are the basic source of the position errors, but also, they make the alignment commands and their implementation more complicated. The major advantage of the proposed design is that it eliminates the errors resulting from the estimation of the longitude and latitude rates because the angular velocity commands of gyroscopes are proportional to accelerations’ integrals and independent of the system position. In this paper, the stabilized platform is modelled, the platform alignment procedure is determined, and the initial conditions of the navigation phase are calculated. The results of the navigation scheme are compared with the wander-azimuth scheme in four scenarios and the performance of the position-independent navigation scheme is evaluated in practical tests and its results are presented.

This study aims to investigate the influence of the plate uncertainty elastic modulus on free vibration response and buckling behavior. To this purpose, elastic modulus of plate is modeled as a random variable with a normal distribution. Spatial autocorrelation function is used for random fields. In this method, the correlation is dependent on the distance, as the points be far away from each other, the correlation is also reduced. Then, applying the powerful finite element method stochastic finite element relations were calculated using Monte Carlo simulation. To this purpose, a four-node Kirchhoff’s element was used with twelve degrees of freedom. For the analysis, random variable is simulated 5,000 times. At last, by numerical tests, the effects of uncertainty on elastic modulus are investigated on the natural frequencies and buckling loads of plate. The results of these tests show that the effect of uncertainty in elastic modulus of the plate has a different effect on the response of vibration and buckling of plate. So that these changes have low effect on free vibration responses of plate. But buckling loads are highly dependent on the elasticity coefficient.

In this paper, the hybrid front tracing method is used to simulate the rising of a single bubble in a quiescent liquid. This method is a new approach to capture of fluid interfaces in the constant grid and using a one field concept. A one field approximation is used, where one set of governing equations are only solved in the entire domain and different phases are treated as one fluid with variable physical properties. The interfacial effects are accounted for by adding appropriate source terms to the governing equations. The related unsteady incompressible full Naiver-Stokes equations were solved using a conventional finite volume method with a structured staggered grid. The interface was tracked explicitly in to connected marker points via hybrid front capturing and tracking method. A computer program was developed for numerical simulation of bubble movement in a viscous liquid in different gravity conditions, especially zero gravity conditions have been used. In addition to general studies of microgravity effects, the initiation of hydrodynamic Marangoni convection solely due to the variations of interface curvature (surface tension force) and thus generation of shearing forces at the interfaces was also studied. Recognizing this movement in the study of bubble dynamics problems, especially in the nucleate pool boiling in zero gravity is important. In this regard, the simulation results were obtained for different conditions and the role of the hydrodynamic Marangoni convection on bubble movement in the absence of buoyancy force was investigated in the obtained results. The simulation results on the inclined plate show that bubble behavior in according to a critical angle from sliding on the wall to departure from wall and finally fluctuate departure have a good agreement with experimental results.