جریان آرام

The dynamic response of a natural laminar flow airfoil undergoing harmonic small-amplitude pitching oscillations is investigated using Large-eddy simulations with a chord-based Reynolds number of Re = 750;000. Throughout the pitch cycles, large changes in the transition point are seen as well as trailing-edge separation. This leads to a nonlinear response of the aerodynamic forces. Although the nature of the flow is highly nonlinear, the development of the boundary layer over the airfoil surface can be modeled using a simple phase-lag concept which suggests a quasi-steady evolution of the boundary layer. Based on this phase-lag assumption, a simple new empirical model is developed which agrees very well with the measured experimental data. With the aid of this model, the primary source of non-linearities in the unsteady aerodynamic forces is identified to be the quasi-steady term, which can be evaluated from the static airfoil characteristics. The strength of this remains unchanged with the variation of reduced frequency, but the harmonic term's strength, which models the unsteady effects, increases with increasing frequency.

The development of microchannel manufacturing technology has led to a growing interest in using them as heat exchangers. Microchannels are used to control the temperature of equipment and components that generate a high amount of heat flux. Heat transfer can be further increased by dispersing particles (nano-sized particles) with a low volume fraction into the base fluid (nanofluid). A nanofluid can change the thermophysical properties of a base fluid and improve its thermal performance. The purpose of this study was to examine the heat transfer characteristics of oil-based nanofluid within wavy microchannels in series and parallel arrangements of microchannels. In order to examine the performance of each microchannel separately and to make it easier to draw their diagrams, they are named 1 and 2. Experiments were performed on TiO2 and SiO2 oil-based nanofluids in volume fractions of 0.05 and 0.1, flow rates of 0.5, 1.0, and 1.5 lit/min, and inlet temperatures of 40°C, 45°C, 50°C, 55°C. The results show an increase in the Nusselt number of the base fluid up to 41.8% in the series arrangement and also a decrease in the surface temperature in the series arrangement compared to the parallel arrangement. Also, TiO2 and SiO2 nanofluids with a volume fraction of 0.1 caused the highest increase in heat transfer, up to 56% and 52.7% in parallel arrangement and up to 45.8% and 42% in series arrangement compared to the base fluid, respectively. The pressure drop of the test section in series arrangement was up to 82.1% higher than parallel.

Flow-induced vibration is the effective factor in mechanical destruction of structures that are exposed to fluid flow. In this study, random vortex- boundary element methods (RVM-BEM), is used to simulate two-dimensional laminar fluid flow around four one/two-degrees-of-freedom cylindrical cylinders in a rectangular arrangemen. Hydrodynamic force coefficients, streamlines and cylinder displacements were plotted. The vorticity distribution is separated into blob-vortexes and its changes are studied by tracking these vortexes in the Lagrangian approach by considering two mechanisms of convection and diffusion in each time step. Satisfying no-slip boundary condition, vortex sheets were created on boundary. The cylinder vibrations were modeled as a system of mass, spring, and damper. The results showed that for 1 and 2DoF compared to the stationary cylinder, the average drag coefficient changes are 0.84 and 0.97, respectively. The rear cylinders vibrations amplitude were less than in the front cylinders. The y-amplitude was three times larger than x-amplitude. The maximum x-amplitude vibration of 1DoF cylinders was 1.51 times larger than 2DoF ones. Solving flow over 2DoF single cylinder by BEM(no need for homodis mapping or considering the vortexe images) with a similar solution in Ansys-Fluent software, showed 25% reduction in runtime and 2.3% increase in calculations accuracy.

Cooling is one of the most important challenges in saving energy and increasing the productivity of many industries. The first serious obstacle in compressing and making heat transfer devices more efficient is the weak properties of heat transfer fluids. The energy efficiency of heat transfer equipment depends on the heat flux changes created in them. The use of common fluids such as water and ethylene glycol as cooling fluids in the cooling system of various heat transfer devices is not the answer to the removal of very high heat flux (tens of megawatts per square meter). Today, nanofluids are made as new heat transfer fluids by adding nanoparticles to fluids that have low heat transfer and by creating changes in the density, specific heat and viscosity of these fluids in order to increase thermal conductivity and improve heat transfer performance. In this research, the results of laboratory research on heat transfer in nanofluids under laminar, turbulent flows and pool boiling are discussed and investigated.

Solving many important industrial problems requires knowing the values of the heat transfer coefficient of passing of one or more fluid streams in different equipment, systems or pipes. In the present study, a numerical model has been developed to simulate compressible fluid flow at the inlet of a hot pipe with different angles to the horizon. In this zone, the hydrodynamic and thermal boundary layers of the fluid flow are developing. Due to the turbulence of the fluid flow due to the interaction of heat and fluid flow inside this tube, a three-dimensional turbulent model was used for this simulation. For this purpose, continuity and compressible Navier-Stokes equations, Reynolds stress model, and turbulent and compressible energy equation have been solved simultaneously. Then, using a set of numerical runs by the concepts of experimental design and optimization methods, a predictive formula for the Nusselt number for these flows has been obtained. Finally, the ability of this formula has been investigated using a set of laboratory data.

This study aimed to investigate the heat transfer of water/Fe3O4 nanofluid in a square cross-sectional channel with dimensions of 80 cm ⨯1 cm ⨯1 cm under the influence of a uniform heat flux perpendicular to the laminar flow of ferrofluid in the presence of a magnetic field. Firstly, the production of ferrofluid with concentrations of 0.5 vol.% and 1vol.%, their quality, and the quality of the production method was investigated. The results of zeta potential and vibrating-sample magnetometer tests show the good quality and stability of the produced ferrofluid. The thermophysical properties of the made ferrofluid are compared and evaluated with existing experimental correlations. The heat transfer of the produced ferrofluids under the influence of heat fluxes of 134-546 Watts is investigated in the absence of an external magnetic field. Then, the effect of the external magnetic field on the heat transfer at 0.5 vol.%, under the influence of a total heat flux of 1258.2 Watts is investigated. The magnitude of increase of heat transfer coefficient compared to pure water, without external field, for ferrofluid with 1 vol.%, under total heat fluxes of 134, 545, and 321.3 Watts, are 30%, 48%, and 38% respectively. At a heat flux of 1258.2 Watts, the heat transfer coefficient in the presence of an external magnetic field increases by 3.16% at 0.5 vol.% compared to the absence of a magnetic field.

A major part of the flow field has no complicated behavior in many laminar & turbulent flows which can use large mesh and simplified governing equation for numerical simulation. In order to reduce memory and CPU time, the flow field was decomposed to several blocks with different size and dimension in each block. A two dimensional backward facing step was considered in this research that used three different type of blocking in numerical modeling. Computer memory and CPU time consumption in addition to numerical result of main fluid parameters. Consideration showed that, suitable combination of used blocks led to the results with the same accuracy as model was employed only one block and experimental results, in addition to the reduction of memory and CPU time consumption.

The two-dimensional numerical simulation of the spray flame formed in a laminar counterflow configuration has been performed to investigate flame regimes and its effect on changes in important species and radicals. Ethanol is used as a liquid spray fuel and the skeletal mechanism of ethanol/air with 40 species and 180 preliminary reactions is used for combustion reactions. The Sauter mean diameter of fuel droplets and randomly injected into the air inlet are considered using a UDF code, and the motion of the droplets is calculated using the Lagrangian approach. The results show that the predominant flame regime is Non-Premixed in the high equivalences ratio, high strain rates and large droplet diameters. Also, the combustion reactions are suppressed in the center areas of the flame, due to the reduction of the oxidant fraction and the OH mass fraction, and the flame index is zero, which indicates the internal group of droplets in this area.

In the present paper, the optimal geometry of a two-dimensional multi-channel with one inlet and three outlets is investigated at Re=10. In this study, the topology optimization based on a porosity method using lattice Boltzmann simulation is adopted to find the optimal layout by computing the sensitivity analysis of an objective function. In contrary to previous studies of the channel with constant width of the outlets, the average velocity of the outlets is maintained as the same while the energy dissipation is reduced by 26.04% in comparison of the results, showing the adavntages of changing in width of ducts rather than the change in their average velocities. In this case, the geometry conditions for the inlet duct including width and position of the duct, play an important role in the final design as the main goal of this research. Assuming that the duct width in the outlets changes linearly, the numerical results showed that the equality of inlet width with the largest outlet caused the lowest power loss for the flow. The final geometry was getting much smooth when the inlet was located in front of the outlet with the largest width however it cannot be definitely optimum. In order to obtain the lowest energy loss, the inlet should be parallel to the space between the two outlets with medium and large widths. A reduction of 23.18% of loss was found in this case rather than the inlet was set in front of the duct having the smallest size.

In this paper, the effects of tripwire in upstream of flow over a circular cylinder in a 2D channel and Re=100 has been investigated. The dimensions of the channel are 20D×50D. The distance between cylinder and wire is considered from 1.5D to 6D, and the relative size of wire is considered from 0.1D to 0.9D. The governing equations are continuity, momentum, and energy for 2D, incompressible, unsteady and vicious conditions. The no-slip condition for both cylinder wall, constant temperature for cylinder, and adiabatic wall for wire have been considered in this paper. The followed equations have been solved by the back-ward differentiate method and the CFD software - COMSOL MULTYPHISIC. The results show that the higher diameter of the wire and horizontal and vertical distances of the wire causes a higher decrease in values of cylinder’s drag and heat transfer. At all, It is found that existing of wire in upstream, decreases the drag and heat transfer of the cylinder but oscillating wire with the amplitude of 0.25D and relative distance and diameters of 3D and.3D causes increasing in discussed parameters relative to the stationary state of them.

One of the effective factors in decreasing the efficiency of boilers during the operation period is sediment. The sediment sits as a layer on the boiler's heat transfer surfaces and reduces the boiler's thermal efficiency due to the thermal resistance it creates. For this reason, it is very important to determine the amount of sediment and to accurately measure it in boiler pipes.In this paper, it is assumed that no information is available on the distribution of sediment in boiler pipes, so the sediment profile is estimated using temperature simulation on the inner wall of boiler pipes as well as conjugate gradient method. In this method, the error rate was calculated by estimating the function of multi-criteria sediment profile  , which indicates the high accuracy of the conjugate gradient method in sediment estimation. The amount of sediment exceeded a specified value.

Thermal and hydrodynamic behavior of a laminar flow of water within a horizontal curved Vipertex tube with mixed convection heat transfer, in the range of low Grashof numbers, has been numerically studied. The curved horizontal Vipertex tube has geometry of 180o, fixed radius of centerline curvature of 2R/D=6.62, roughness height e/D=0.1, and a constant heat flux is exerted on the walls. The three-dimensional governing equations were using a finite volume method. To solve the problem, the computational fluid dynamics of ANSYS Fluent The results reveal that not only Grashof number and the buoyancy forces arising from it, but the mutual effects of the centrifugal and the buoyancy forces affect the thermal and hydrodynamic characteristics such as axial velocity contours, secondary flow vectors, temperature contours, heat transfer coefficient, and skin friction coefficient. So that, for a given Reynolds number, increasing due to more interaction between buoyancy and centrifugal forces, results in the Vipertex tube. Therefore, the buoyancy forces decrease and lead to the lower heat transfer coefficient, but in smooth curved Grashof number leads to the higher heat transfer coefficient. Nevertheless, the Vipertex curved tube in of Grashof and Reynolds, in each Grashof and Reynolds equally, has a higher heat transfer than a smooth curved pipe. The results also indicated that the skin friction coefficient in these types of tubes can be up to 3.5 times higher than that of smooth one with a Grashof increase.

In this study, a linear parabolic trough concentrating photovoltaic/thermal system has been simulated and the effects of using Al2O3/ethylene glycol-water 50:50 nanofluid with different nanoparticle shapes including platelet, cylindrical, blade and brick shapes from energy and exergy standpoints in the laminar and turbulent regimes have been numerically investigated. The proposed model has been validated using existing experimental results where good agreement was observed. The results indicated that using nanoparticles of cylindrical shape in laminar regime and brick-shaped one in turbulent regime lead to the best system performance compared to others. In addition, applying nanofluid in laminar regime is more effective compared to turbulent regime

In this study, the effect of chaotic advection on laminar mixing is analyzed experimentally and numerically. Mixer includes two circular rotors and a circular stator in the way that rotational speed of each rotor could be controlled by time. This kind of mixer is widely used in the food and pharmaceutical industry. The performance of the mixer was investigated experimentally with dye injection and image recording. Moreover, the effect of chaotic flow on mixing numerically is studied qualitatively and quantitatively with Lagrangian fluid particle tracing, calculating the stretching rate of fluid elements and Poincare sections. The experimental results indicate that as a result of applying sinusoidal perturbation on rotational velocity of rotors, weak mixing zones will disappear with the passage of time, and fluid particles will be distributed uniformly in the surface of the mixer. Numerical simulation shows that the mixer flow is sensitive to initial condition when rotational velocity of rotors is variable, and this is one of important factors in chaotic flow. Further, calculation of stretching rate of fluid elements indicates that average exponential value of fluid elements stretching in the chaotic flow is two times more than non-chaotic one. This research, based on scientific concepts and theoretical application of chaos in fluid mechanics, denotes that the efficiency of low-speed mixers in highly viscous fluid mixing is increased without any increase in the consuming energy.

In this work, the effect of a controller wire on the hydro-thermal flow characteristics around a circular cylinder in a channel was studied. The two-dimensional unsteady and incompressible flow with heat transfer was investigated by solving the Navier-Stokes and energy equations using control volume methodology. The second-order upwind scheme is used to discretize convective terms. The discretized equations are solved by PISO algorithm. The angular position for the wire investigated from 0° to 180° from the forward stagnation point and the diameter ratio for wire and the cylinder is set at 0.05 to 0.4, in laminar Reynolds numbers. The surface Nusselt number of cylinder is increased by adding a wire on the cylinder surface between 90ᵒ and 180ᵒfrom the forward stagnation point. In addition, as the Reynolds number decreases, the effects of adding a wire on the main cylinder are becoming stronger.

In this study, the laminar forced convection heat transfer of water-coper nanofluid is numerically investigated within a parallel plate channel. Fixed temperature heat sources with the specified sizes and distances are embedded on the walls of the channel. The entry and exit sections of the channel as well as the sections between the heat sources are thermally insulated. The fluid flow with uniform velocity and temperature enters the channel. The channel length is considered large enough, so the flow in the channel output is assumed fully developed. The aim of this research is the numerical investigation of the effects of the Reynolds number, the solid volume fraction and the number of the heat sources on the flow field and heat transfer rate. For this purpose, the governing equations are discretized by finite difference method based on the control volume formulation and are solved using the SIMPLE algorithm. In order to validate the computer program, the results of this study have been compared with the results of the previous numerical studies. This comparison has confirmed the accuracy of the performance of the computer program. The results show that the rate of heat transfer increases by increasing the solid volume fraction and Reynolds number. The results also show that, heat transfer rate increases when the heat sourse is divided into smaller sections and these sections are distributed on the channel wall.

Given the need to increase the efficiency of heat transfer in thermal systems, especially systems using nanofluids in microscale and nanoscale heat transfer equipment ideas to improve their performance is very good.In present study, the flow and heat transfer of Water-Cu nanofluid in micro-tube with slip regime with constant wall heat flux numerically simulated with low Reynolds numbers. Slip velocity and temperature jump boundary conditions are also considered along the microtube walls, for first time. The results are presented as the profiles of temperature and velocity. Nusselt number and pressure drop coefficient calculated in interance and full developed region. The effect of slip and using nano particle considerd.
It is observed that Nusselt number increases with slip velocity coefficient and pressure drop coefficient decreases; att intrance region the Raynolds of flow has effect on Nusselt and pressure drop coefficient,too.
Likewise observed nano particle adding to water has low effect to increases Nusselt number and pressure drop coefficientt.

In this study, accelerating motion of a sphere in incompressible fluid has been nvestigated by the Navier-Stokes equations. The motion is set in a small time interval, having the constant velocities out of it.The laminar and unsteady flow has been solved in various Reynolds numbers. In a short moment the Reynolds number is rised which cause large variations in the drag force. This method is applicable to other accelerating motions such as oscillation and also spherical submarines.

In this research, free convection heat transfer over vertical sinusoidal wall in constant temperature has been performed, using Navier-Stokes equations in laminar flow regime with finite volume method. Also, a new pressurebased model for the convective fluxes is offered, which minimizes the need for artificial dissipation. The first and second-order versions of the suggested model are unable to calculate the convective fluxes with a good accuracy.Numerical results are well adapted with the numerical or experimental results of other researchers. In this study, the variable parameters are Reyleigh number and the ratio of domain length to the sinusoidal wave length. The results show that the local heat transfer coefficient variation frequency for space is equal to the wall frequency. Meanwhile,with increasing the Reyleigh number, the average heat transfer coefficient increases.