Sharif Digital Repository

The fabrication of micron-sized instruments MEMS have been in fast progress in different fields of science and technology such as those of chemical processes in chemical engineering, propulsion in aerospace industries, microchip cooling and inkjet printers in electrical industries, and medical appliances in medical and biomedical science. They have forced the manufacturers and researchers to work and think in fabricating such small-scale sizes. However, the complexity of fabricating MEMS with moving parts has also promoted the manufacturers to think of alternative ways. For example the use of moving parts to pump fluid in such devises causes such difficulties and the researchers could adopt... 

Multi stage Pulse Tube Cryocoolers (PTCs) have found the interest of researchers due to its reliability, long life and absence of moving part. This thesis represents a comprehensive numerical simulation method to investigate one and two stage PTC. All components of the PTC are modeled employing the nodal analysis technique to discretize the mass, momentum and energy conservation equations. SUTPTC code has been developed in cryocooler laboratory of Sharif University of Technology (SUT) to analyze and optimize one and two stage cryocooler performance. The SUTPTC code has been validated with the existing experimental data. Employing the proposed code, the effect of precooling temperature, the... 

In this study, the solution of compressible flows is performed using finite volume lattice Boltzmann method (FVLBM). A model associated with 13 discrete velocity vectors and 2 energy levels is used and the Boltzmann transport equation is solved using a cell-centered finite volume on structured meshes. The values of distribution functions on each cell faceare determined by averaging from their values at the two control points located on the center of two neighboring cells. The fourth-order Runge-Kutta time-stepping scheme is applied to discretize temporal derivative term. The second- and fourth-order numerical dissipation termsareadded to the algorithm to stabilize the solution when solving... 

One of the most important reasons of death in today’s world is the diseases related to cardiovascular system. Hypercholesterolemia is one of the most common forms of these diseases. There are a lot of experiments for diagnosing this disease, which are classified into two groups: invasive and noninvasive experiments. One of the well-known noninvasive experiments in this area is the photoplethysmography in which the changes of the photoplethysmography signal can reveal some valuable information about the cardiovascular system’s health.The objective of this study is to simulate the photoplethysmography experiment considering the clinical conditions of the experiment. In this work, the obtained... 

In the present study, the simulation of incompressible Electro-Magneto-hydrodynamic flows is performed using a finite volume lattice Boltzmann method (FVLBM). The Boltzmann transport equation is solved using a cell-centered finite volume method on structured meshes. A central difference scheme is used to discretize the spatial derivatives and the fourth-order numerical dissipation term is added to stabilize the solution. To discretize the temporal derivative, the fourth-order Runge-Kutta time stepping scheme is applied. The standard collision-streaming lattice Boltzmann method has been used to simulate EMHD flows in the literature, however, it has several deficiencies such as the... 

Numerical simulations of participating media need careful solution of the radiation transfer equation (RTE) along the fluid flow governing equations. Evidently, the accuracy of achieved solutions highly depends on the accuracy of radiation transfer calculations. Despite numerous efforts performed in the previous researches, the RTE calculation still faces with several challenges from both accuracy and computational cost perspectives. The main objective of this research is to present new contributions in overcoming such challenges. So, a finite-volume (FV) solver is suitably developed to solve the RTE numerically. This extended solver is then coupled with an existing FV flow solver. The... 

In the present study, the thermal behavior of human teeth during laser radiation has been discussed. The high laser power, as well as its low emission, can allow deviations from the Fourier heat transfer model. To overcome this problem, the non- Fourier heat transfer method, dual phase lagged (DPL), has been used. Wisdom considered in this study to evaluate the thermal behavior of tooth. Using the finite volume method, the DPL equations are solved and the thermal response of the teeth is obtained during laser irradiation. The effect of delay times, which are the parameters of the DPL model and depart from the Fourier model, is fully demonstrated on the temperature behavior of the tooth, and... 

In recent years, many advanced Computational Fluid Dynamics (CFD) methods have been developed for accurate and efficient simulation of fluid flow. The application of these methods shows that for sufficient accuracy, the mesh resolution has to be reasonably high. Unfortunately, conventioanal single-grid Navier-Stokes solvers suffer from poor convergence characteristics. For this problem we used Multigrid method to speed up the convergence rate. In this section a code was developed (based on the conventional CFD algorithm) which can solve Navier-Stokes equations on structured grids. It also simulates the free surface by VOF method. Agglomeration multigrid method is used in present study for... 

Pulse Tube Refrigerator (PTR) has an exceptional feature among all similar refrigerators, because of its unique characteristics such as long lifetime, compact size and no moving parts in low temperature. Despite of all these advantages because of its low efficiency, many researchers are trying to optimize performance of this refrigerator. Most of these efforts are based on one-dimensional simulation of the system. In this thesis, a two-dimensional analysis is performed to study the nature of flow field in an Inertance Tube Pulse Tube Refrigerator (ITPTR). This analysis is based on a numerical modeling of an entire ITPTR. To reach this purpose the two-dimensional form of mass, momentum and... 

Helium gas is the working fluid in common Pulse Tube Refrigerators (PTRs) due to its brilliant properties in reaching cryogenics temperature as low as 4 K. Generally ideal gas equation of state has been used to evaluate thermodynamic performance of He. However using ideal gas equation of state for He in 10 bar and 5 K causes considerable error on estimation of density. The objective is to use real gas equation of state for temperatures near zero Kelvin. This approach makes an accurate prediction of PT cooling capacity and COP. In this thesis the objective is to realize the most relevant equations of state describing the proper behavior of He in the regions of operating point of PTR. Later on... 

In recent years, oil and gas reservoirs are one of the most valuable natural resources and are the main economical basis of the country. Therefore any research which can help to optimize the efficiency and enhance their recovery is of great importance. In the past 30 years, reservoir simulation has evolved from a research field to one of the most flexible tools in reservoir engineering. Simulation is usually more quick, cost effective and reliable than other methods in predicting reservoir performance. Because of complexity of the great amount of computations, research in oil reservoirs field is usually done by mathematical/computer programs, named as simulators. For this reason, various... 

In this study, a new finite-element-volume (FEV) arbitrary Lagrangian-Eulerian (ALE) method is suitably extended to simulate the three-phases flows air, liquid water droplets and ice in ice accretion, i.e., over flying object surfaces. This methods benefit from the advantages of both finite-volume and finite-element methods. This method is developed for the first time to simulate three phases turbulent flows. Since the ice formation and growth needs grid movement consistant with ice boundary movement, we have used ALE approach to fulfill this requirment. In this regard, we use the linear spring analogy approach to move the hybrid triangular-rectangular mesh suitably. Facing with a chaotic... 

In this study, 2-D compressible viscous and inviscid flows are simulated by using a finite volume Lattice Boltzmann method. Two different models, namely, the Qu model and Watari model are employed for compressible flows simulations. The first model includes 13 discrete velocity vectors and 2 energy levels in which the Maxwellian function is replaced with a simple function for describing the distribution function that is suitable for inviscid flow simulations. The second model is a thermal multi-velocity model with isotropic tensors up to seventh rank that is suitable for compressible viscous and inviscid flow simulations with arbitrary specific heats ratio. In both the models, lattice... 

Modern technology is accompanied by miniaturized devices. With increasing the use of micro satellites and micro airial vehicles (MAVs), finding new power-generating resources is inevitable. Batteries were always an option however high efficiency, long life-span and environmental consideration have encouraged the researches to focus on micro combustion. On the other hand, hydrocarbon fuel contains about 100 times more energy per unit mass than lithium-ion batteries. Micro combustion is defined as a combustion which is occurred bellow quenching distance and old literatures were uncertain about occurrence of such combustion. In this paper we present the fundamental concepts of micro combustion... 

In this study we investigated the stress effects on the fractured block in formation and its impact on flow through the matrix. At first the effect of various stresses on reduction of sample's pore volume and thereby reduction the permeability have been studied. The equations used to describe the elasticity relations in saturated matrix block is provided by Bayot theory. In this theory, the system of related equations to stress and strain changes have been developed in matrix. We have used finite element method in COMSOL software to solve the equation system created by elasticity relations in formation. After solving the equations, volume and volumetric strain changes which are caused by the... 

It is about half century passed from the invention and application of the polymer flooding. Although there are successful reports from the using polymer flooding, but there are problems that in some cases this process fails. For more insight on the problems and in order to improve
the efficiency of this process the conceptual understanding of the process is required. More conceptual understanding from this process, requires investigation in pore scale. In the presenting investigation, the flow of Newtonian and non-Newtonian fluids in the single phase and two phase form and in pore scale are simulated using finite volume method. Because, the finite volume method is used in very... 

Rapid variation in permeability with strong anisotropy are common features in subsurface reservoirs. When formulating a finite-volume pressure equation scheme, continuous normal flux and pressure are key physical constraints that must be imposed at control-volume interfaces, across which strong discontinuities in permeability can occur.
The derivation of algebraic flux continuity conditions for full-tensor discretization operators has lead to families of efficient locally conservative flux-continuous control-volume distributed (CVD) finite- volume schemes for determining the discrete pressure and elocity fields in subsurface reservoirs [5, 9, 11, 13].
When applying these...