A stable and accurate control-volume technique based on integrated radial basis function networks for fluid-flow problems

Radial basis function networks (RBFNs) have been widely used in solving partial differential equations as they are able to provide fast convergence. Integrated RBFNs have the ability to avoid the problem of reduced convergence-rate caused by differentiation. This paper is concerned with the use of integrated RBFNs in the context of control-volume discretisations for the simulation of fluid-flow problems. Special attention is given to (i) the development of a stable high-order upwind scheme for the convection term and (ii) the development of a local high-order approximation scheme for the diffusion term. Benchmark problems including the lid-driven triangular-cavity flow are employed to validate the present technique. Accurate results at high values of the Reynolds number are obtained using relatively-coarse grids

Sonic and Photonic Crystals

Sonic/phononic crystals termed acoustic/sonic band gap media are elastic analogues of photonic crystals and have also recently received renewed attention in many acoustic applications. Photonic crystals have a periodic dielectric modulation with a spatial scale on the order of the optical wavelength. The design and optimization of photonic crystals can be utilized in many applications by combining factors related to the combinations of intermixing materials, lattice symmetry, lattice constant, filling factor, shape of the scattering object, and thickness of a structural layer. Through the publications and discussions of the research on sonic/phononic crystals, researchers can obtain effective and valuable results and improve their future development in related fields. Devices based on these crystals can be utilized in mechanical and physical applications and can also be designed for novel applications as based on the investigations in this Special Issue

Study on the Mechanical Properties of Carbon Nanotube Coated‒Fiber Multi-Scale (CCFM) Hybrid Composites

L'abstract è presente nell'allegato / the abstract is in the attachmen

Fabrication, Characterization and Modeling of Functionally Graded Materials

In the past few decades, a number of theoretical and experimental studies for design, fabrication and performance analysis of solar panel systems (photovoltaic/thermal systems) have been documented. The existing literature shows that the use of solar energy provides a promising solution to alleviate the shortage of natural resources and the environmental pollution associated with electricity generation. A hybrid solar panel has been invented to integrate photovoltaic (PV) cells onto a substrate through a functionally graded material (FGM) with water tubes cast inside, through which water flow serves as both a heat sink and a solar heat collector. Due to the unique and graded material properties of FGMs, this novel design not only supplies efficient thermal harvest and electrical production, but also provides benefits such as structural integrity and material efficiency. In this work, a sedimentation method has been used to fabricate aluminum (Al) and high-density polyethylene (HDPE) FGMs. The size effect of aluminum powder on the material gradation along the depth direction is investigated. Aluminum powder or the mixture of Al and HDPE powder is thoroughly mixed and uniformly dispersed in ethanol and then subjected to sedimentation. During the sedimentation process, the concentration of Al and HDPE particles temporally and spatially changes in the depth direction due to the non-uniform motion of particles; this change further affects the effective viscosity of the suspension and thus changes the drag force of particles. A Stokes' law based model is developed to simulate the sedimentation process, demonstrate the effect of manufacturing parameters on sedimentation, and predict the graded microstructure of deposition in the depth direction. In order to improve the modeling for sedimentation behavior of particles, the Eshelby's equivalent inclusion method (EIM) is presented to determine the interaction between particles, which is not considered in a Stokes' law based model. This method is initially applied to study the case of one drop moving in a viscous fluid; the solution recovers the closed form classic solution when the drop is spherical. Moreover, this method is general and can be applied to the cases of different drop shapes and the interaction between multiple drops. The translation velocities of the drops depend on the relative position, the center-to-center distance of drops, the viscosity and size of drops. For the case of a pair of identical spherical drops, the present method using a linear approximation of the eigenstrain rate has provided a very close solution to the classic explicit solution. If a higher order of the polynomial form of the eigenstrain rate is used, one can expect a more accurate result. To meet the final goal of mass production of the aforementioned Al-HDPE FGM, a faster and more economical material manufacturing method is proposed through a vibration method. The particle segregation of larger aluminum particles embedded in the concentrated suspension of smaller high-density polyethylene is investigated under vibration with different frequencies and magnitudes. Altering experimental parameters including time and amplitude of vibration, the suspension exhibits different particle segregation patterns: uniform-like, graded and bi-layered. For material characterization, small cylinder films of Al-HDPE system FGM are obtained after the stages of dry, melt and solidification. Solar panel prototypes are fabricated and tested at different water flow rates and solar irradiation intensities. The temperature distribution in the solar panel is measured and simulated to evaluate the performance of the solar panel. Finite element simulation results are very consistent with the experimental data. The understanding of heat transfer in the hybrid solar panel prototypes gained through this study will provide a foundation for future solar panel design and optimization

전기방사 공정을 통한 헬리컬 구조의 은나노 섬유의 제조

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부(하이브리드 재료), 2020. 8. 유웅열.This study aimed to fabricate silver nanofibers using new process parameters of electrospinning and develop transparent and stretchable electrodes for stretchable electronics using them. A series of research was carried out to achieve goals as follows. A multi-physics model for the simulation of gas-assisted melt-electrospinning (GAME) process was developed to understand the roles of process parameters. By numerically calculating the stresses acting on the jet during a single-nozzle GAME process, the shear viscous stress was identified as the main factor of jet stretch. The jet stretch ratio increased sharply when shear viscous stress reached the level at which jet sharpening occurred, leading to stable jet formation. This stress was defined as the critical shear viscous stress to determine stable spinnability. In addition, a multi-nozzle GAME was simulated, proposing a spinnability diagram for stable spinning. A new process was designed to fabricate helical fibers. Here, the effect of solidification behavior of the jet on the formation of intrinsic curvature and on the final morphology of electrospun fibers was investigated. Fiber morphology during electrospinning was observed to dramatically change from straight to helical due to rapid solidification of the jet. Investigation of the resulting jet morphologies revealed that fiber structure changed from straight to helical as the vapor pressure increased. A similar effect was observed with conductive solutions prepared by adding large amounts of metal ion to the polymer solution. Simulations revealed that the jet near the nozzle tip was subject to a strong electrical field due to increased charge density. The thickness of the emerging fiber was rapidly reduced with fast and simultaneous solidification, resulting in helical nanofibers. A mechanism was suggested that can describe the formation of helical fibers. Transparent and stretchable electrodes (TSEs) was fabricated using electrospun silver nanofibers. Here, a composite comprising shape memory polymer–TSE (SMP–TSE) using crosslinked polycyclooctene as a substrate was fabricated, which showed wrinkle-free deformation and switchable optical transparency. Because of its considerable elongation without residual strain and the shape memory behavior of polycyclooctene, in-plane buckled nanofibers were formed effectively. Due to these in-plane buckled nanofibers, the electrode maintained its resistance during 3,000 cycles of a bending test and 900 cycles of a tensile test. Furthermore, SMP–TSE was able to electrically control its temperature, optical transparency, elastic modulus, and shape memory behavior. Finally, SMP–TSE was demonstrated for a smart electrode that could control its optical and mechanical properties. Keywords: Electrospinning, Numerical simulation, Process parameters, Silver nanofibers, Transparent and stretchable electrode Student number: 2014-22539본 연구의 목적은 전기방사 공정에서의 새로운 공정 변수들을 이용하여 은나노섬유를 제작하고 이를 활용하여 투명·신축 전극을 제작하는 것이다. 이를 위한 일련의 연구들이 다음의 순서로 진행되었다. Gas-assisted 용융 전기방사 공정에 대한 multi-physics 모델링을 개발하였으며 이를 통하여 공정 변수들의 역할을 파악하였다. 단일 노즐을 이용한 공정에서 젯의 표면에 인가되는 응력을 수치해석을 통하여 분석함으로써 젯의 인장에 있어 점성 전단 응력이 주요하게 작용함을 밝혀냈다. 젯은 점성 전단 응력이 특정 값에 도달하였을 때에 안정적으로 형성되었으며, 젯의 인장률 또한 급격히 증가하였다. 이때의 응력을 안정적인 방사성을 판단하는 임계 점성 전단 응력이라 정의 하였다. 단일 노즐 공정뿐만 아니라 멀티 노즐 공정을 모델링하였으며 이로부터 안정적인 방사성을 판단할 수 있는 방사성 다이어그램을 제시하였다. 다음으로는 섬유의 구조를 헬리컬 구조로 제작하는 공정을 디자인 하였다. 우선, 젯의 초기 곡률과 젯의 위치에 따른 고화가 섬유의 구조에 미치는 영향을 조사하였다. 전기 방사된 섬유의 구조는 용매의 증기압이 증가함에 따라 발생하는 빠른 고화로 인하여 직선형태에서 헬리컬 구조로 변화하였다. 이는 금속 이온이 과량으로 첨가된 전도성 용액에 대해서도 유사한 결과를 보였다. 이에 대한 시뮬레이션 결과는 전하 밀도의 증가가 강한 전기장을 발생시켰으며, 이로 인하여 젯의 급격한 인장 및 고화가 발생하였음을 보여주며, 그러한 이유로 헬리컬 구조가 형성되었음 나타냈다. 이를 이용하여 헬리컬 구조의 섬유가 형성되는 메커니즘을 제시하였다. 마지막으로 전기방사를 이용해 제작한 은나노섬유를 이용하여 투명·신축 전극을 제작하였다. 전극은 형상기억 고분자인 crosslinked polycyclooctene을 기판으로 활용하였다. 제작한 전극은 잔류 변형이 없고 투명도를 제어할 수 있는 특성을 보였다. 큰 인장에도 잔류 변형을 보이지 않는 형상기억고분자 기판의 특성으로 인해 면내 굽힘 구조의 은나노섬유가 효율적으로 제작되었다. 이러한 특성으로 제작한 전극은 3,000회의 굽힘 평가와 900회의 인장평가에도 전도성을 유지할 수 있었다. 또한 제작한 전극은 전기적 자극을 통하여 온도, 투명도, 강성 및 형상기억 특성을 제어할 수 있다는 특징을 보였다. 제작한 전극을 활용하여 광학적 그리고 기계적 특성을 제어할 수 있는 새로운 형태의 스마트 전극을 시연 하였다. 핵심어: 전기방사공정, 전산모사, 공정변수, 은나노섬유, 투명·신축 전극 학번: 2014-22539Chapter 1. Introduction 1 1.1. Electrospinning 1 1.1.1. Introduction of electrospinning 1 1.1.2. Types of electrospinning 3 1.1.3. Parameters in electrospinning 6 1.1.4. Structures of electrospun nanofibers 16 1.1.5. Application of electrospun nanofibers 26 1.1.6. Limitation and perspective of electrospinning 44 1.2. Research objectives 47 Chapter 2. Numerical simulation of gas-assisted melt electrospinning 50 2.1. Needs for modeling of gas-assisted melt electrospinning 50 2.2. Methods 53 2.2.1. Gas-assisted melt-electrospinning process 53 2.2.2. Numerical simulation of single-nozzle GAME process 56 2.2.3. Calculation of electric field in multi-nozzle configuration 60 2.2.4. Numerical simulation of multi-nozzle GAME process 61 2.3. Results and discussion 62 2.3.1. Simulation of single-nozzle GAME process 62 2.3.2. Simulation of multi-nozzle GAME process 73 2.4. Summary 81 Chapter 3. Fabrication of inherently helical structure nanofibers 83 3.1. Needs for fabrication of helical nanofibers 83 3.2. Experimental 85 3.2.1. Preparation of dielectric solution for helical nanofibers 84 3.2.2. Preparation of conductive solution for helical nanofibers 86 3.2.3. Electrospinning and spinneret geometry 86 3.2.4. Characterization of Electrospun Fibers 87 3.3. Results and discussion 88 3.3.1. Effect of solvent vapor pressure on structure 88 3.3.2. Effects of solidification on structure 94 3.3.3. Numerical simulations of jet near nozzle 99 3.3.4. Further enhanced helical structrues 105 3.4. Summary 111 Chapter 4. Fabrication of a stretchable, wrinkle-free electrode with switchable transparency 113 4.1. Transparent and stretchable electrode 113 4.2. Experimental 116 4.2.1. Materials 116 4.2.2. Preparation of shape memory polymer substrate 117 4.2.3. Fabrication of free-standing silver nanofiber 117 4.2.4. Characterization of SMP–TSE 118 4.3. Results and discussion 119 4.3.1. Fabrication of free-standing silver nanofibers 119 4.3.2. Optoelectrical properties of silver nanofibers 128 4.3.3. Shape memory substrate 129 4.3.4. Shape memory polymer–transparent and stretchable electrode 137 4.4. Summary 143 Chapter 5. Conclusions 145 Chapter 6. Appendix 147 Reference 161 Korean abstract 189Docto

Research and technology highlights of the Lewis Research Center

Highlights of research accomplishments of the Lewis Research Center for fiscal year 1984 are presented. The report is divided into four major sections covering aeronautics, space communications, space technology, and materials and structures. Six articles on energy are included in the space technology section

Research and technology

The NASA Lewis Research Center's research and technology accomplishments for fiscal year 1987 are summarized. It comprises approximately 100 short articles submitted by staff members of the technical directorates and is organized into four sections: aeronautics, aerospace technology (which includes space communications), space station systems, and computational support. A table of contents by subject was developed to assist the reader in finding articles of special interest

Some new thermo-elastic solutions for cylindrical and spherical composites

In modern engineering applications, multilayered structures are extensively used due to the added advantage of combining physical, mechanical, and thermal properties of different materials. Many of these applications require a detailed knowledge of transient temperature and heat-flux distribution within the component layers. Both analytical and numerical techniques may be used to solve such problems. Nonetheless, numerical solutions are preferred and prevalent in practice, due to either unavailability or higher mathematical complexity of the corresponding exact solutions. Rather limited use of analytical solutions should not diminish their merit over numerical ones; since exact solutions, if available, provide an insight into the governing physics of the problem, which is typically missing in any numerical solution. Moreover, analyzing closed-form solutions to obtain optimal design options for any particular application of interest is relatively simpler. In addition, exact solutions find their applications in validating and comparing various numerical algorithms to help improve computational efficiency of computer codes that currently rely on numerical techniques. Although multilayer heat conduction problems have been studied in great detail and various solution methods including orthogonal and quasi-orthogonal expansion technique, Laplace transform method, Green’s function approach, finite integral transform technique are readily available; there is a continued need to develop and explore novel methods to solve problems for which exact solutions still do not exist. One such problem is to determine exact unsteady temperature distribution in polar coordinates with multiple layers in the radial direction. Numerous applications involving multilayer cylindrical geometry require evaluation of temperature distribution in complete disk-type. One typical example is a nuclear fuel rod, which consists of concentric layers of different materials and often subjected to asymmetric boundary conditions. Moreover, several other applications including multilayer insulation materials, double heat-flux conductimeter, typical laser absorption calorimetry experiments, cryogenic systems, and other cylindrical building structures would benefit from such analytical solutions. Then, object of the present thesis is to derive new thermo-elastic solutions for composite materials constituted by multilayered spheres and cylinders under time-dependent boundary conditions. These solutions are utilized for several engineering applications and we report some applications in last analyze chapters of present thesis. In follows, we will described the contents of thesis. In first chapters are reported the thermo-mechanical foundations and a summary of the formulation of thermo-elastic problems for isotropic material. In chapter X it is developed an analytical approach to find exact elastic solutions for multilayered cylinder composed of isotropic constituents and determining the analytical response in terms of displacements and stresses for all the De Saint Venant (DSV) load conditions, that is axial force, torque, pure bending and combined bending moment and shear actions. Successively, on the basis of the found analytical solutions, a homogenization procedure is adopted in order to obtain the overall constitutive elastic laws for multilayered cylinder, in this way deriving the exact one-dimensional model characterized by the axial stiffness, flexural rigidity, shear deformability and torsional stiffness relating beam’s generalized stresses and strains. By playing with the Poisson ratios of adjacent phases, some counterintuitive and engineering relevant results are shown with reference to unexpected increasing of overall stiffness of multilayered cylinder. In chapter XI it is presented an analytical elastic solution for multilayered cylinder constituted by transversally-isotropic n-phases, under radial pressure, axial force and torque. Then, by utilizing the homogenization theory, it is obtained the overall elastic stiffness of the equivalent homogeneous transversally-isotropic solid, establishing the constitutive elastic laws relating stresses and strains. In chapter XII it is developed an analytical approach to find exact elastic solutions for multilayered cylinder subjected to axial force, constituted by n orthotropic cylindrical hollow phases and a central core, each of them modelled as homogeneous and cylindrically anisotropic material. In chapter XIII it is reported an analytical solution for multilayered cylinder composed by hollow cylindrical monoclinic phases under axial force and torsion. In this chapter, we consider the chiral structure for each cylindrical layer. In particular the composite material is constituted by two hollow cylindrical monoclinic phases. The cylindrical monoclinic elastic property of multilayered cylinder is obtained by the particular chiral structure. In fact, we consider the two hollow phases constructed by right-handed and left-handed spiral helices whose long axes are all parallel. These helical spirals may be either touching or separated by a matrix material and are composed by elastic orthotropic material. In chapters XIV, XV, XVI are reported some thermo-elastic solution, for hollow cylinders, hollow spheres and plates, respectively. In chapter XVII we consider a steady-state thermo-elastic problem of multilayered cylinder with finite length. The thermal and mechanical loads applied on the cylinder are axisymmetric in the hoop direction and are constant in the axial direction. In order to obtain analytical solutions for temperature, displacements, and stresses for the two-dimensional thermo-elastic problem, the cylinder is assumed to be composed of n fictitious layers in the radial direction. The material properties of each layer are assumed as homogeneous. In chapter XVIII are determined the displacements, strains, and stresses from the general analytical solution of multilayered sphere composed by an arbitrary number of layers constituted by materials with generic modulus of elasticity, thermal expansion coefficient and thermal conductivity. Material properties are assumed to be temperature-independent and homogeneous in each layer. The multilayered sphere is considered as a classical composite material whose properties abruptly vary from one hollow sphere to the other. In chapter XIX are presented the most important standard fire curves: ISO 834, External fire curve, hydrocarbon fire curve, ASM119 and parametric fire curves (European Parametric fire curves, Swedish Fire Curves, BFD curves, CE 534 curve). Moreover in this chapter are reported the mechanical and thermal properties of steel and concrete at elevate temperature. In chapters XX and XXI, the one-dimensional quasi-static uncoupled thermo-elastic problem of a multilayered sphere and multilayered cylinder, with time-dependent boundary conditions are considered, respectively. The body forces and heat generation vanish. In both cases, the analytical solution is obtained by applying the method of separation of variables. In chapter XXII it is studied a spherical tank methane gas-filled exposed to fire characterized by hydrocarbon fire curve. The interaction between spherical tank and internal gas is studied. By applying a suitable simplified hypothesis on the mechanics of problem, we determine the analytical thermo-elastic solution for spherical tank. By applying the solution obtained, the increasing graded temperature of gas methane in spherical tank is determined. Finally, a numerical example is reported for a spherical tank exposed to hydrocarbon fire, showing the collapse temperature. In chapter XXIII, an industrial insulated pipeline is modelled as multilayered cylinder, subjected to mechanical and thermal loads. By using a multi-layered approach based on the theory of laminated composites, the solutions for temperature, heat flux, displacements, and thermal/mechanical stresses are presented. By applying the analytical thermo-elastic solution reported in Chapter XVII, a parametric analysis is conducted in order to analyze the mechanical behaviour of an industrial insulated pipeline composed by three phases: steel, insulate coating, and outer layer made of polyethylene to protect the insulation. In this model, parametric analyses are conducted by varying the Young’s modulus, Poisson’s ratio, thermal conductivity and linear thermal expansion coefficient of insulate coating. The analysis shows the maximum Hencky von Mises’s equivalent stress in steel phase and in insulate coating. Finally, it is presented a numerical example by considering three types of materials for insulate coating: (1) Expanded Polyurethane; (2) Laminate glass; (3) Syntatic foam. In chapter XXIV it is analyzed a cylindrical concrete specimen under axial force within Fibre Polymeric Reinforcing sheets. The elastic solutions found in Chapter XII are here extended to the post-elastic range. The evolution of the stress field when the core phase is characterized by an Intrinsic Curve or Schleicher-like elastic-plastic response with associate flow rule and the cylindrically orthotropic hollow phase obeys to is shown the elastic-brittle Tsai-Hill anisotropic yield criterion. The choice of these post-elastic behaviours is suggested by experimental evidences reported in literature for these materials, as well as the cylindrical orthotropy of the hollow phase intrinsically yields to consider several perfectly bonded FRP layers as an equivalent one, interpreting their overall mechanical response by invoking the theory of homogenization and the mechanics of composites. At the end, a numerical example application to cylindrical concrete specimens reinforced with Carbon FRP is presented, by furnishing a predictive formula – derived from the previously obtained analytical solutions - for estimating the overall collapse mechanism, the concrete ultimate compressive strength and the confining pressure effect. The results are finally compared with several experimental literature data, highlighting the very good agreement between the theoretical predictions and the laboratory measurements. In chapter XXV it is reported an analytical thermo-elastic solution in closed form for bi-layer hollow cylinder subjected to time-dependent boundary conditions. It is assumed that each hollow cylinder is composed by a homogeneous and thermo-isotropic material, characterized by different mechanical and thermal parameters, i.e. modulus of elasticity, thermal expansion coefficient and thermal conductivity. Moreover, these material properties in each hollow cylinder are assumed to be temperature-independent. In other words, the bi-layer hollow cylinder is considered as a classical composite material whose properties abruptly vary from one hollow cylinder to the other. In particular, it is obtained a new analytical solution for a bi-layer hollow cylinder, constituted by two phases: Ceramic ( ) and Metal ( ) subjected to heat flux on inner surface

Flow and thermal transport in additively manufactured metal lattices based on novel unit-cell topologies

The emergence of metal Additive Manufacturing (AM) over the last two decades has opened venues to mitigate the challenges associated with stochastic open-cell metal foams manufactured through the traditional foaming process. Regular lattices with user-defined unit cell topologies have been reported to exhibit better mechanical properties in comparison to metal foams which extend their applicability to multifunctional heat exchangers subjected to both thermal and mechanical loads. The current study aims at investigating the thermal-hydraulic characteristics of promising novel unit cell topologies realizable through AM technologies. Experimental investigation was conducted on four different topologies, viz (a) Octet, (b) Face-diagonal (FD) cube, (c) Tetrakaidecahedron, and (d) Cube, printed in single-cell thick sandwich type configuration in 420 stainless steel via Binder Jetting technology at same intended porosity. The effective thermal conductivity of the samples was found to be strongly dependent on the lattice porosity, however, no significant dependence on the unit-cell topology was demonstrated. Face-diagonal cube lattice exhibited the highest heat transfer coefficient and pressure drop, and consequently provided the lowest thermal-hydraulic performance. A procedure to incorporate the manufacturing-induced random roughness effects in the samples during numerical modelling is introduced. The numerical simulations were conducted on samples exhibiting the roughness profiles having statistically same mean roughness as the additively manufactured coupons and the results were compared to that obtained from the intended smooth-profiled CAD models that were fed into the printing machines. The analysis showed that inclusion of roughness effects in computational models can significantly improve the thermal performance predictions. Through this study, we demonstrate that additively manufactured ordered lattices exhibit superior thermal transport characteristics and future developmental efforts would require extensive experimentations to characterize their thermal and flow performance as well as local surface quality and AM-induced defect recognition. Experimental findings would also need to be supported by computational efforts where configurations which closely mimic the real AM parts could be modeled. A combined experimental-numerical framework is recommended for advancements in metal additive manufacturing-enabled enhanced heat transfer concepts

Structural, Magnetic, Dielectric, Electrical, Optical and Thermal Properties of Nanocrystalline Materials: Synthesis, Characterization and Application

This book is a collection of the research articles and review article, published in special issue "Structural, Magnetic, Dielectric, Electrical, Optical and Thermal Properties of Nanocrystalline Materials: Synthesis, Characterization and Application"