Sharif Digital Repository

One of the main reasons of brain attack is stenosis in Carotid bifurcation. As a result, it's a major field of interest for a large group of scientists including medical doctors and engineers. In the engineering fields a large number of studies have been done on this topic based on Numerical Simulation. As blood boundary conditions in this region strictly depend on the rest of the body's vascular system, the problem is the definition of the boundary condition in this approach. In addition, because of lack of enough hardware sources simulation of the whole blood system is impossible; thus the rest of the system should be simulated based on radical methods with less details. In this project a... 

To days the heterogeneous material are used extensively in the engineering materials. Optimization ability is a key feature of these materials to reach desired properties. Heterogeneous materials are the materials that make up from the constituents of multiphase materials in lower length scale such as mesoscopic, microscopic or/and Nano scales. So the properties of these materials at each scale are depending on to several characteristics of heterogeneities such as geometry, material and packing. In these materials the effects of heterogeneities at the lower scales are very significant and the constitutive equations are different for each range of scale. The proper selection of this range... 

In this paper, a machine learning-based atomistic-continuum multi-scale scheme is introduced to model the materials' geometrically and materially nonlinear behavior. The kinematic and energetic consistency principles are employed to link the atomistic and continuum scales. In order to establish the kinematic consistency principle, the periodic boundary condition is implemented for the atomistic RVE. The Ni-based superalloy, including 0 to 3% porosity, is considered for the models. Several parameter analysis is done to distinguish the proper atomistic RVE to be used in multi-scale models. The data set, including the stress-strain samples, is generated through molecular dynamics analysis... 

Recently, many investigations are done in the field of the modeling of the cardiovascular system. For the purpose of understanding of this system, mainly the location of formation and development of atherosclerosis, these investigations are so vital. The multi-scale modeling is an appropriate method to reduce the cost of calculation and also, this method simplifies the complicated model of cardiovascular system. This research presents a multi-scale 1D-3D modeling of cardiovascular system. In this model, the three dimensional geometry of carotid bifurcation is coupled with a one dimensional full model of cardiovascular system. We used some user defined function in the Fluent software for... 

The ever-increasing growth of Nanotechnology has elevated the necessity for the development of new numerical and computational methods that are better capable of evaluating systems at this scale. The existing techniques, such as Molecular Dynamics Methods, in spite of being fully capable of evaluating nanostructures, lack the ability to simulate large systems of practical size and time scales. Therefore, in order to be able to provide a realistic simulation of a large model, simulation of which is limited by the computational cost of the current molecular dynamics methods at hand, Coarse-Graining technique has recently become a very effective and beneficial method which refers to the... 

One of the most widely used methods in the study of the mechanical behavior of fiber-reinforced polymers is modeling and simulation of a unit cell behavior. According to the arrangement of composite materials, the unit cell is selected in order to include and represent actual constructions of the material. In this study a numerical self-consistence method is proposed to estimate effective properties of Carbon-epoxy composite materials. In this method, in addition of two main phases i.e. matrix and fiber, a phase of composite properties is also considered surrounding the unit cell. First using analytical and semi-empirical methods, the properties are calculated and are converged after... 

The present thesis aimed to develop a thermal-hydraulic model for simulation of multi-stream plate-fin heat exchangers. In this study, a multi-scale point of view was employed for three-dimensional simulation of multi-stream plate-fin heat exchangers. This is a cost-effective method that reflects the effects of complex phenomena at the micro-scale in the macro-scale model. In this model, fluid and solid matrix temperature distributions were obtained in the three-dimensional space, considering conduction heat transfer through separating plates, side plates, cap plates, and transverse conduction through fins. The modular form of the model has facilitated layer-by-layer simulation of parallel... 

In recent decades, particle composite materials have a wide range of applications in engineering. Particle composites are a group of heterogeneous materials with different length scales and are characterized by particles that are randomly distributed in a matrix phase. Geopolymer concrete is a special type of concrete that its binder is made by reacting alumina and silicate carriers with an activating agent and in recent years with the expansion of its use has reduced the amount of cement consumption. In addition to the many advantages of geopolymer concrete, it has disadvantages in terms of setting time. That is why the use of cement has been proposed as a solution to the problem. This... 

In this thesis, we investigate interactions of a double-gyre in the North Atlantic and the earth’s Chandler wobble using a single-layer ocean model based on depth-averaged Navier-Stokes equations and multiple-scale spectral solutions to it. The overall transfers of energy and angular momentum from the double-gyre to the Chandler wobble are used to calibrate the turbulence parameters of the idealized ocean model and Smagorinsky eddy viscosity is used to estimate turbulent diffusion terms. Our model is tested against a multilayer quasi-geostrophic ocean model in turbulent regime, and base states used in parameter identification are obtained from mesoscale eddy resolving numerical simulations.... 

During the last decade, thanks to a combination of exploding computational power and improved physical insight into material behavior, continuum and atomistic simulations improved greatly. Both classes of methods are now used to solve problems, which are more complicated than ever with greater accuracy than ever before. Nevertheless, there still exist problems for which neither method alone is sufficient. In general, atomistic simulations cannot be used for such length scales due to the restrictions on the number of atoms that can be simulated, along with the time scales, which they can be simulated for. In contrast, continuum simulations tend to fail at the atomic scale, for example due to... 

In this project, two multiscale modeling procedures have been implemented to study the mechanical behavior of SWCNT/polymer composites. First, a new three-phase molecular structural mechanics/ finite element (MSM/FE) multiscale model has been introduced which consists of three components, i.e. a carbon nanotube, an interphase layer and outer polymer matrix. The nanotube is modeled at the atomistic scale using MSM, whereas the interphase layer and polymer matrix are analyzed by the FE method. Using this model, we have investigated the macroscopic material properties of nanocomposite with and without considering the interphase and compared the results with molecular dynamics (MD) simulations.... 

In this thesis, a coarse-grained multi-scale method for 2D crystallyn solids based-on finite element consepts has presented. In this method, both scales are atomic scale and similar to what we see in non-local QC method, the entire atomic structure will be intact. Accordingly, calculations of potential functions and forces in the domain will have the atomic accuracy. In the presented method to reduce the domain’s degrees of freedom, the classical finite-element meshing concept to mesh the elastic linear areas in the domain is used and the MD calculations will done on the mesh nodes. Therefore, degrees of freedom in the system will reduce and consequently, the computational cost will reduce.... 

In continuum mechanics, the constitutive models are usually based on the Cauchy-Born (CB) hypothesis which seeks the intrinsic characteristics of the material via the atomistic information and it is valid in small deformation. The main purpose of this thesis is to investigate the temperature effect on the stability and size dependency of Cauchy-Born hypothesis and a novel temperature-dependent multi-scale method is developed to investigate the role of temperature on surface effects in the analysis of nano-scale materials. Three-dimensional temperature-related Cauchy-Born formulation are developed for crystalline structure and the stability and size dependency of temperature-related... 

Nanostructured materials are a new kind of engineering materials which attracted researchers’ interest because of their interesting mechanical /physical properties, as well as controllable microstructural design ability for desired applications. These new materials are homogeneous at the macroscale but at the microstructural level, may have heterogeneities including common nanostructures. Because of multiscale nature of these materials, new multiscale methods should be developed and used for better understanding the behavior of them. Multiscale methods could be categorized into concurrent and hierarchical methods. In concurrent methods, the domain under study is explicitly divided into... 

This study considers blood flow in total cavopulmonary connection (TCPC) morphology, which is created in Fontan surgical procedure in patients with single ventricle heart disease. Ordinary process of TCPC operation reduces the pulmonary blood flow pulsatility; because of right ventricle being bypassed. This phenomenon causes a lot of side effects for patients. A cardiac surgeon has suggested that keeping main pulmonary artery (MPA) partially open, would increase pulmonary flow pulsations. MPA gets closed in ordinary TCPC operation. The purpose of current study is to verify the effects of keeping MPA partially open on pulmonary flow pulsations, by means of computational fluid dynamics (CFD).... 

In this thesis a mutliScale model based on the Cauchy-Born hypothesis and via usage of XFEM is proposed for crack modeling. By solving an example, the important of surface effects in the surface stresses region is shown. Considering not being able to model the surface effects with the Cauchy-Born method, the boundary Cauchy-Born method for modeling crack effects is used. Moreover, three Molecular Dynamics method for modeling crack will be proposed. According to the obtained results from these methods, it was deduced that for calculating the correct surface stresses in Molecular Dynamics the mutual interaction of upper and lower atoms of crack should be omitted. Finally, the validation of... 

In this thesis, a method has been exploited to couple the atomistic domain with the coarse-grained domain. Since molecular dynamics has a high computational cost when a large number of atoms exist, coarse-grained molecular dynamics was used in which a number of atoms are assumed as a bigger bid and interatomic potential is modified for bids so that the material’s mechanical properties remain constant. This method not only reduces the computational cost of calculating forces in molecular dynamics simulation but also, the time step used in Coarse-Grained Methods can be more than atomistic simulations as the frequency of occurring phenomena in CG scale is less than atomistic scale.The advantage... 

This study presents delamination in fiber-reinforced composite laminates by using multiscale modeling. The meso modeling is used to derive the relationship between microcrack density and damage parameters. Next the selected failure model is applied to analyze the macroscale modeling. The progress of failure terms and the reduction of fiber and matrix properties implemented into ABAQUS/Standard, which enables an individual to create a new material behavior through the user subroutine UMAT. In the following, the contours associated with each of the damage parameters are obtained in each of the damage mode. Then for a specific material, the relationship between microcrack density and damage... 

Due to high cost and ineffectiveness of molecular models a new method for coupling continuum models with molecular models is used. In this method, the continuum and molecular domains are overlapped. Comparing the results obtained from the concurrent simulations and molecular dynamic simulations proves the accuracy of the method used. The method is used for modeling single layered graphene sheets, stress contours are presented for multiscale and both static and dynamic simulations of concurrent. For multiscale simulations two different carbon nano tubes are investigated and strees-bond angle and strees-bond length are also presented  

In this research, multi-scale modeling of mixed-mode failure mechanism of quassi-brittle materials is presented. For modeling a realistic crack growth in heterogeneious media, crack initiation criterion, crack growth orientation and the macroscopic cohesive law are derived from a microscopic sample. As a microscopic crack initiation criterion, acoustic tensor is investigated and scaled acoustic tensor has been proposed by comparison its results with maximum principal tensile stress. For crack growth direction based on micro-scale, acoustic tensor and multiscale aggregating discontinuities has been investigated and multiscale aggregating discontinuities by comparison results of these methods...