MetaMesh: A hierarchical computational model for design and fabrication of biomimetic armored surfaces

Many exoskeletons exhibit multifunctional performance by combining protection from rigid ceramic components with flexibility through articulated interfaces. Structure-to-function relationships of these natural bioarmors have been studied extensively, and initial development of structural (load-bearing) bioinspired armor materials, most often nacre-mimetic laminated composites, has been conducted. However, the translation of segmented and articulated armor to bioinspired surfaces and applications requires new computational constructs. We propose a novel hierarchical computational model, MetaMesh, that adapts a segmented fish scale armor system to fit complex “host surfaces”. We define a “host” surface as the overall geometrical form on top of which the scale units are computed. MetaMesh operates in three levels of resolution: (i) locally—to construct unit geometries based on shape parameters of scales as identified and characterized in the Polypterus senegalus exoskeleton, (ii) regionally—to encode articulated connection guides that adapt units with their neighbors according to directional schema in the mesh, and (iii) globally—to generatively extend the unit assembly over arbitrarily curved surfaces through global mesh optimization using a functional coefficient gradient. Simulation results provide the basis for further physiological and kinetic development. This study provides a methodology for the generation of biomimetic protective surfaces using segmented, articulated components that maintain mobility alongside full body coverage.Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (Contract No. W911NF-13-D-0001)United States. Army Research Office (Institute for Collaborative Biotechnologies (ICB), contract no. W911NF-09-D-0001)United States. Department of Defense (National Security Science and Engineering Faculty Fellowship Program (Grant No. N00244-09-1-0064)

A Modular Approach to Large-scale Design Optimization of Aerospace Systems.

Gradient-based optimization and the adjoint method form a synergistic combination that enables the efficient solution of large-scale optimization problems. Though the gradient-based approach struggles with non-smooth or multi-modal problems, the capability to efficiently optimize up to tens of thousands of design variables provides a valuable design tool for exploring complex tradeoffs and finding unintuitive designs. However, the widespread adoption of gradient-based optimization is limited by the implementation challenges for computing derivatives efficiently and accurately, particularly in multidisciplinary and shape design problems. This thesis addresses these difficulties in two ways. First, to deal with the heterogeneity and integration challenges of multidisciplinary problems, this thesis presents a computational modeling framework that solves multidisciplinary systems and computes their derivatives in a semi-automated fashion. This framework is built upon a new mathematical formulation developed in this thesis that expresses any computational model as a system of algebraic equations and unifies all methods for computing derivatives using a single equation. The framework is applied to two engineering problems: the optimization of a nanosatellite with 7 disciplines and over 25,000 design variables; and simultaneous allocation and mission optimization for commercial aircraft involving 330 design variables, 12 of which are integer variables handled using the branch-and-bound method. In both cases, the framework makes large-scale optimization possible by reducing the implementation effort and code complexity. The second half of this thesis presents a differentiable parametrization of aircraft geometries and structures for high-fidelity shape optimization. Existing geometry parametrizations are not differentiable, or they are limited in the types of shape changes they allow. This is addressed by a novel parametrization that smoothly interpolates aircraft components, providing differentiability. An unstructured quadrilateral mesh generation algorithm is also developed to automate the creation of detailed meshes for aircraft structures, and a mesh convergence study is performed to verify that the quality of the mesh is maintained as it is refined. As a demonstration, high-fidelity aerostructural analysis is performed for two unconventional configurations with detailed structures included, and aerodynamic shape optimization is applied to the truss-braced wing, which finds and eliminates a shock in the region bounded by the struts and the wing.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111567/1/hwangjt_1.pd

Residual-Based Isotropic and Anisotropic Mesh Adaptation for Computational Fluid Dynamics

The accuracy of a fluid flow simulation depends not only on the numerical method used for discretizing the governing equations, but also on the distribution and topology of the mesh elements. Mesh adaptation is a technique for automatically modifying the mesh in order to improve the simulation accuracy in an attempt to reduce the manual work required for mesh generation. The conventional approach to mesh adaptation is based on a feature-based criterion that identifies the distinctive features in the flow field such as shock waves and boundary layers. Although this approach has proved to be simple and effective in many CFD applications, its implementation may require a lot of trial and error for determining the appropriate criterion in certain applications. An alternative approach to mesh adaptation is the residual-based approach in which the discretization error of the fluid flow quantities across the mesh faces is used to construct an adaptation criterion. Although this approach provides a general framework for developing robust mesh adaptation criteria, its incorporation leads to significant computational overhead. The main objective of the thesis is to present a methodology for developing an appropriate mesh adaptation criterion for fluid flow problems that offers the simplicity of a feature-based criterion and the robustness of a residual-based criterion. This methodology is demonstrated in the context of a second-order accurate cell-centred finite volume method for simulating laminar steady incompressible flows of constant property fluids. In this methodology, the error of mass and momentum flows across the faces of each control volume are estimated with a Taylor series analysis. Then these face flow errors are used to construct the desired adaptation criteria for triangular isotropic meshes and quadrilateral anisotropic meshes. The adaptation results for the lid-driven cavity flow show that the solution error on the resulting adapted meshes is 80 to 90 percent lower than that of a uniform mesh with the same number of control volumes. The advantage of the proposed mesh adaptation method is the capability to produce meshes that lead to more accurate solutions compared to those of the conventional methods with approximately the same amount of computational effort

Local scale air quality model system for diagnostic and forecasting simulations using the finite element method

Air pollution is an important topic with a great social impact; it is related with public health, environment and ecology, and climate change. Scientists have developed several models in the last thirty years, and regional air quality operational systems are used routinely by governments and agencies. Efforts have also been done to simulate the air quality in the local scale; main models are Gaussian and Puff models, that are based on a Lagrangian approach. In contrast with these models, in this thesis we have developed a system using an Eulerian approach. This model is specifically designed for regions with complex orography where the Lagrangian models have problems computing the trajectory of the particles. This model can be used for diagnostic or prediction simulations. Air quality operational systems depend on the orography, meteorological data, and emission data. Air quality models use processors to incorporate these data into the model. The data can come from numerical weather prediction systems, experimental data, or databases. In this thesis we have developed processors, specifically designed for the local scale, to incorporate these data into our system. To incorporate the orography, we have developed a mesh generation algorithm suitable for complex terrain discretization; it also allows to insert layers that can match the regional models. A wind field model has also been used; it can interpolate a three-dimensional wind field from some station measurements using a log-linear vertical wind profile, or can interpolate it from a numerical weather prediction system. Once an interpolated wind field is computed, a mass-consistent model is applied to ensure null divergence and impermeability in the terrain. The wind field is modified to take into account the injection of the pollutants into the atmosphere. Briggs studied the trajectory of the plume rise giving some empirical equations that will be used in our model. Briggs' equations describe the trajectory in a plane; our model will modify this trajectory adapting it to the ambient wind field. This modification allows the plume rise to surround the mountains or channel into the valleys. The transport and reaction of pollutants in the atmosphere is then computed using an splitting method, so the transport and the chemical reactions are computed independently. To solve the transport of pollutants we have used a finite element method stabilized using least squares. The chemical reaction is simulated using simplified models such as RIVAD, or more complex ones such as CB05. To obtain more accurate results we have used adaptation. An error indicator has been used to adapt the mesh to the solution. To adapt the mesh to the concentration distribution of all the species is very demanding, for this reason we have used a multimesh method where every chemical specie has its own mesh where we solve the transport and the chemical reactions are simulated in a common mesh. The system developed in this thesis has diagnostic and forecasting capabilities. For this reason we present two different applications. The first one is a diagnostic application in La Palma island (Spain), where wind measurements are given, and SO2 and NO2 emissions from a stack are considered. The topography of the island is real, from a digital elevation model, but the wind field measurements, and the stack location and emissions, are simulated. The second application is a forecasting application data from the CMAQ benchmark test. It is located in the surrounding of Pineville Kentucky. In this application we have used all the data from CMAQ and the chemical reaction model CB05.La contaminació atmosfèrica té gran impacte social; està relacionada ambla salut pública, l'ecologia, el medi ambient i el canvi climàtic. En els últims trenta anys, els científics han desenvolupat diversos models que els governs i les agències mediambientals utilitzen diàriament. També s'han fet esforços per simular la qualitat de l'aire en l'escala local. Els models principals són els models Gaussians i Puff, que es basen en una descripció Lagrangiana. En contrast amb aquests models, en aquesta tesi s'ha desenvolupat un sistema que utilitza una descripció Euleriana. Aquest model està dissenyat específicament per a les regions amb topografia complexa on els models Lagrangians tenen problemes calculant la trajectòria de les partícules. El model presentat en aquesta tesi pot ser utilitzat tant pel diagnòstic com per la predicció. Els sistemes operatius utilitzats actualment depenen de l'orografia, de dades meteorològiques i de dades d'emissió. Per tal d'incorporar aquestes dades, els models de qualitat de l'aire han desenvolupat diferents preprocessadors. Les dades poden venir dels sistemes numèrics de predicció meteorològics, de dades experimentals o de bases de dades. En aquesta tesi hem desenvolupat preprocessadors dissenyats específicament per a l'àmbit local, per tal d'incorporar aquestes dades al nostre sistema. Per incorporar l'orografia, hem desenvolupat un algoritme de generació de malles adequat per terreny complex; l'algoritme també ens permet inserir capes que poden coincidir amb la dels models regionals. S'ha desenvolupat un model de càlcul de camp de vent; a partir de les dades s'interpola un camp de vent tridimensional fent servir un perfil log-lineal vertical del vent, o s'interpola a partir dels resultats d'un sistema de predicció meteorològica. Quan s'ha calculat el vent interpolat, es fa servir un model de massa consistent per obtenir un camp de vent definitiu on s'ha imposat divergència zero i impermeabilitat del terreny. El camp de vent s'ha de modificar per tenir en compte la injecció dels contaminants a l'atmosfera. Briggs va estudiar la trajectòria de l'elevació de la ploma i va trobar equacions empíriques que utilitzarem en el nostre model. Les equacions de Briggs descriuen la trajectòria de la ploma en un pla vertical; el nostre model modificarà aquesta trajectòria adaptant-la al camp de vent ambiental. Aquesta modificació permet que l'elevació de la ploma rodegi les muntanyes o s'acanali a les valls. El transport i la reacció de contaminants a l'atmosfera es calcula utilitzant un mètode de "splitting", de manera que el transport i la reacció química es calculen de forma independent. Per resoldre el transport de contaminants, hem utilitzat el mètode d'elements finits estabilitzat amb mínims quadrats. La reacció química es simula mitjançant models simplificats com el model RIVAD, o més complexes com el CB05. Per obtenir resultats més precisos hem adaptat la malla a la solució utilitzant un indicador d'error. Haver d'adaptar la malla a la distribució de la concentració de totes les espècies contaminants és molt exigent i per aquest motiu hem utilitzat un mètode "multimesh" on cada espècie química té la seva pròpia malla on resolem el transport i les reaccions químiques es simulen en una malla comú. El sistema desenvolupat en aquesta tesi té capacitats de diagnòstic i pronòstic. Per aquesta raó es presenten dues aplicacions diferents. La primera és una aplicació de diagnòstic a l'illa de la Palma (Espanya), on es disposa de mesures de vent, i de les emissions de SO2 i NO2 d'una xemeneia. La topografia de l'illa és real, a partir d'un model digital del terreny, però les dades del camp de vent, la ubicació de la xemeneia i el valor de les emissions són simulades. La segona aplicació és una aplicació de predicció fent servir els resultats del benchmark del CMAQ. Es simula una zona a Pineville Kentucky. En aquesta aplicació hem utilitzat totes les dades del CMAQ i el model químic CB05