Sharif Digital Repository

In the present study, we investigate the effect of the hemodynamic factors of the blood flow on the cerebral aneurysms. To this end, a hypothetical geometry of the aneurysm in the circle of Willis, located in the bifurcation point of the anterior cerebral artery (ACA) and anterior communicating artery (ACoA) is modeled in a three-dimensional manner. Three cases are chosen in the current study: an untreated thin wall (first case), untreated thick wall (second case), and a treated aneurysm (third case). The effect of increasing the aneurysm wall thickness on the deformation and stress distribution of the walls are studied. The obtained results showed that in the second case, a reduction in the... 

Buoyancy-induced fluid flow, which is responsible for most forms of macro-segregation and channel-type segregates in castings, is not directly controllable. If left uncontrolled, natural convection will contribute to non-uniform distribution of alloy constituents and grain structure during solidification of a casting. Non-uniform distribution of chemical composition and physical structure in an alloy casting can significantly affect the reliability of mechanical components. Therefore, materials with acceptable defects can be produced only by trial-and-error and their acceptability is determined by costly inspections. We present a novel technique to control the formation of chimneys and... 

Scale precipitation due to the mixing of incompatible injected water with formation brine and its subsequent deposition in porous media is an unpleasant phenomenon in water injection projects that can lead to severe injectivity and productivity decline. As a result of the complexity of geochemical reactions, modelling scale precipitation and deposition is a challenge. This paper presents a coupled geochemical and fluid flow model to simulate reactive flow in porous media which models pressure difference increase resulting from scale formation during water injection into porous media. To simulate chemical reactions during scale formation and subsequent rock permeability decline, PHREEQC... 

The paper investigates the effects of fluid flow on the static and dynamic behaviors of electrostatically actuated carbon nanotubes using nonlocal elasticity theory. The influences of various parameters of fluid flow including fluid viscosity, velocity, mass and temperature on the mechanical behaviors of the carbon nanotubes under static and step DC voltages are studied using this theory. The results computed from the nonlocal elasticity theory are compared with those estimated using the classical elasticity theorem and the outcomes demonstrate the applicability of the electrostatically actuated carbon nanotubes as nano sensors and nano actuators in nanofluidic systems. The nanosystem can be... 

A mathematical model is proposed for evaluating flow behaviour under hot deformation conditions. The effects of dynamic recovery and recrystallisation as well as temperature and strain rate variations are considered in the model by means of Bergstrom's approach and the additivity rule for strain. To verify the model, hot compression tests for three grades of steel together with upsetting experiments are carried out. Comparison between experimental and theoretical results confirms the reliability of the model. © 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining  

Fully coupled flow-deformation analysis of deformable multiphase porous media saturated by several immiscible fluids has attracted the attention of researchers in widely different fields of engineering. This paper presents a new numerical tool to simulate the complicated process of two-phase fluid flow through deforming porous materials using a mesh-free technique, called element-free Galerkin (EFG) method. The numerical treatment of the governing partial differential equations involving the equilibrium and continuity equations of pore fluids is based on Galerkin’s weighted residual approach and employing the penalty method to introduce the essential boundary conditions into the weak forms.... 

In this study, we perform numerical simulations to investigate the thermal and flow characteristics of a parabolic trough solar collector equipped with a porous receiver tube and internal longitudinal fins. The heat transfer medium is a synthetic oil-Cu-Al2O3 hybrid nanofluid. We examine the thermal characteristics of the nanofluid in response to variations in several system parameters. We find that at Reynolds numbers between 5 × 103 and 5 × 105, increasing the volume fraction of Cu nanoparticles can increase the temperature gain at the exit of the receiver tube by 6.4%. Furthermore, the temperature gradient in the cross-section of the collector increases as the direct normal solar... 

A comprehensive study was performed to analyze the unsteady laminar flow characteristics around a porously covered, a fully porous, and a solid squared section cylinder located in the middle of a plane channel. In order to simulate fluid flow inside porous media and porous–fluid interface accurately (minimizing modeling error), the porous region was analyzed in pore scale, using LBM. Additionally, to minimize the LBM-related compressibility error through the porous region, a multi-block multiple relaxation time lattice Boltzmann method (MRT-LBM) was used. Also, to decrease CPU time, a Navier–Stokes flow solver, based on finite volume method and SIMPLE algorithm, was coupled with MRT-LBM to... 

Hybrid FVM-LBM schemes are developed in the past few years to use capabilities of both Navier-Stokes based finite volume method (FVM) and lattice Boltzmann method (LBM) to solve macro-meso multiscale problems. In this scheme, the major task is to develop some lifting relations that reconstruct distribution functions in LBM sub-domain from macroscopic variables and their derivatives. The macroscopic variables are computed using Navier-Stokes based FVM in macroscale sub-domain, while distribution functions are computed using LBM in mesoscale sub-domain. The pioneer works in this area used the single relaxation time (SRT) version of LBM. However, it is known that the numerical stability and... 

During the low salinity waterflooding (LSWF) of a viscous asphaltic oil reservoir, fluid-fluid interactions have a large influence on the fluid flow, pore-scale events, and thus oil recovery efficiency and behavior. In-situ water-in-oil (W/O) emulsion formation is a consequence of crude oil and brine interfacial activities. Despite the published studies, the pore-scale mechanisms of W/O emulsion formation and the role of injected brine salinity, injection rate, and pore-scale heterogeneity on emulsion formation and stability requires a deeper understanding. To address these, a series of static and dynamic micro-scale experiments were performed. The salinity dependent oil-brine interactions... 

One of the most important challenges in mobile Ad-hoc networks is simulation. The simulation of these networks based on the accessible simulation techniques, which are based on packet, is a demanding and time-consuming task. Moreover, with the complexity of the network and the increase in the numbers of the nodes, it is likely to take a long time. This is because these simulators, one by one, regard the acts of all moving packets in one part of the network, and process these acts as a series of events. Since the number of the events is high in this method, the simulation takes time. Nowadays, in wired networks, several methods have been suggested to lessen the time of simulation; one example... 

In the current research, a three-dimensional photovoltaic thermal system integrated with phase change material system with nanofluids is investigated. The working fluids involved in this study include nano-magnesium oxide, multiwall carbon nano tube and hybrid (mixture of nano-magnesium oxide and nano-multiwall carbon nano tube) nanofluids dispersed in pure water. After comparing single-phase model and mixture model, the mixture model is used in the study and fluid flow regime in the collector is assumed to be laminar, fully develop, uniform and incompressible, to model the nanofluid in the system. A parametric analysis is conducted to examine the effect of various parameters such as working... 

A two impinging-jets contacting device (TIJCD) for liquid-liquid extraction processes was proposed and tested through the standard test system recommended by the European Federation of Chemical Engineering (EFCE), toluene-acetone-water, as a typical example of liquid-liquid extraction processes. The results obtained for the overall volumetric mass transfer coefficient (capacity coefficient) were more than three times higher than those reported in the conventional contactors and in an air-driven two impinging-streams contactor. These experimental results clearly indicate the greater performance capability of TIJCD relative to those of conventional contactors. To have a perfect study on the... 

A numerical study has been carried out to investigate the fluid flow structure and convective heat transfer due to a circular jet impinging on a rotating disk. The temperature distribution and convection heat transfer coefficient on the disk are calculated. Flow is considered to be steady, incompressible and turbulent. k-e RNG model is used to model the turbulent flow. Two new criteria are introduced and used to evaluate the performance of cooling process which are maximum temperature difference on the disk and the average temperature of the disk. The first parameter shows the uniformity of temperature distribution in the disk and the second shows the effect of both thermo physical... 

Recently a great attention has been given to the oscillatory flow modeling in the pulse tube cryocoolers. In this paper multi dimensioning effects of the fluid flow in the pulse tube are investigated. A complete system of governing equations is solved to report the flow field, friction coefficient and Nusselt number in the pulse tube. Harmonic approximation technique is employed to derive an analytical solution. In this respect, mass, momentum and energy balance equations as well as the equation of state for ideal gas are transformed by implementing the harmonic approximation technique. The present model is able to predict the behavior of the two dimensional compressible oscillatory flow in... 

Valveless piezoelectric micropumps are in wide practical use due to their ability to conduct particles with absence of interior moving mechanical parts. In this paper, using finite element method, an extended numerical study on fluid flow through micropump chamber and diffuser valves is conducted to find out the optimum working conditions of micropump. In order to obtain maximum generality of the reported results, an analytical study along with a dimensional analysis is presented primarily, to investigate the main dimensionless groups of parameters affecting the micropump net flux. Consequently, the parameters appeared in the main dimensionless groups have been changed in order to understand... 

In the present research a numerical investigation has been made to evaluate the results of analytical approach in the analysis of micro channel heat sink (MCHS) which is increasingly used in the cooling of high heat dissipating electronic devices. In this regard the effects of geometrical and flow parameters affecting the absolute thermal resistance of MCHS have been analyzed. The effect of using different fluids in the value of the thermal resistance is considered as well. The accuracy of analytical method in predicting heat transfer and fluid flow regimes in micro channel is checked in comparison with numerical method. The effects of hydraulic and thermal entrance lengths of these systems... 

Free surface flows are frequently encountered in hydraulic engineering problems including water jets, weirs and around gates. An iterative solution to the incompressible two-dimensional vertical steady Navier-Stokes equations, comprising momentum and continuity equations, is used to solve for the priori unknown free surface, the velocity and the pressure fields. The entire water body is covered by a unstructured finite element grid which is locally refined. The dynamic boundary condition is imposed for the free surface where the pressure vanishes. This procedure is done continuously until the normal velocities components vanish. To overcome numerical errors and oscillations encountering in... 

This paper presents a numerical multiscale formulation for analysis of the transient heat and fluid flow in deformable heterogeneous porous media. Due to the heterogeneity of the media, the direct numerical simulation of the micro-structures leads to high computational costs. Hence, the multi-scale method can provide an efficient computational procedure. To this end, the first-order computational homogenization is adopted for two-scale simulation of THM problems. The governing equations of the problem contain a stress equilibrium equation, a mass continuity equation and an advection–diffusion equation in a fully coupled manner. Accordingly, the proper virtual power relations are defined as a... 

Two-dimensional stagnation-point flow of viscoelastic fluids is studied theoretically assuming that the fluid obeys the upper-convected Maxwell (UCM) model. Boundary-layer theory is used to simplify the equations of motion which are further reduced to a single non-linear third-order ODE using the concept of stream function coupled with the technique of the similarity solution. The equation so obtained was solved using Chebyshev pseudo-spectral collocation-point method. Based on the results obtained in the present work, it is concluded that the well-established but controversial prediction that in stagnation-point flows of viscoelastic fluids the velocity inside the boundary layer may exceed...