Joel Saa-Seoane
PhD Candidate
Office: Room 31-215 (map)
77 Massachusetts Avenue
Cambridge, MA 02139
MIT E-mail: jsaa(@)mit.edu  
G-mail: joel.saa(@)gmail.com
Cell: (857) 600-7833

Numerical Simulation of Wave Phenomena on Heterogeneous materials
The simulation of wave phenomena in heterogeneous media has been a very active field of research. Depending on the specific problem that we are interested in solving, different techniques are here introduced. Firstly, Hybrid DG methods (or HDG) are developed for these kind of problems. These methods are fully implicit, unstructured, and high-order accurate in both space and time; yet they are computationally attractive because the only globally coupled unknown is the numerical trace of the velocity field. Since the numerical trace is only defined on the element faces and single-valued, the HDG methods may have significantly less global degrees of freedom than other DG methods using implicit time integration. Another attractive feature of the HDG methods is that they yield optimal convergence. The figures below show wave fields obtained using this numerical methodology.


                          1d Bandgap problem                                                                  Planar wave interaction with an object on a surface and a cylinder

However, some of the problems that we are particularly interested in solving have geometric details and solution patterns that belong to very different scales of size. Moreover, repetition of patterns is often very common in these problems; even infinite repetition when periodic. The approach that is here introduced is called a MultiScale CG method, or MSCG and it defines the subproblems at a subdomain level, in contrast to classical HDG or HCG. This subtle extension is crucial because it provides further reduction in the global degrees of freedom and better efficiency in parallelization. Moreover, it renders this approach very attractive for problems with repeated, piecewise-constant, or periodic coefficients, since in such cases the number of local subproblems can be significantly reduced by a judicious choice of subdomains and associated approximation spaces. With these technology, problems like the ones shown in the figures below can be accurately solved with an extremely high efficiency. The methodology has been developed for 2d and 3d applications.

 
                      Superlensing                        2d (above) - 3d (below) photonic waveguide                           Two dimensional Photonic Sharp Bend

          

Discrete Design Optimization of Structured Materials for wave problems
The design of materials is often based on physical and mathematical intuition. However, when finite domains are considered and manufacturability concerns want to be addressed, they might not be enough. Recently, several optimization techniques have successfully been extended and applied for design problems. Semidefinite or convex optimization techniques have already been used for the design of materials, as well as Nonlinear programming tools and Adjoint methods. Nevertheless, all these methods do not impose the discrete nature of the design variables and very often lead to continuous distributions of materials. In fact, what we seek is only using a finite pool of materials that are available. To that end, a local search discrete optimization procedure has been developed. Some results can be seen in the figures below.


                                          Solution and scattered fields of the cloaking problem. Without cloak and with the optimal distribution.
Photonic Bandgap Optimization
Photonic crystals have been a topic of high interest within the last few years, specially since in 1987 the 2d bandgap in periodic structures were found and its existence was proved. The symmetries that the crystal inherits from its periodicity give it some unique properties. Maximizing the gap opened between some two consecutive bands in the dispersion for both the TE and the TM polarization at the same time warrantees that for the frequencies given in the gap, there will be no k in which it will propagate; so it will exponentially decay.
It is then crucial to analyze the properties of such symmetries from a group theory perspective. A very interesting property that photonic crystals have comes from the Bloch's theorem if there is translational symmetry in a given lattice. With all, periodic structures exhibiting electromagnetic wave band gaps, or photonic crystals, have proven very important applications as device components for integrated optics including frequency filters, waveguides [see figure], switches, and optical buffers.
Shape optimization is used in order to get the unit lattice solution and bandgaps of over 40% (in terms of gap-midgap ratio) has been found for some square and triangular lattices. An initial guess must be given which is evolved iteratively until it gets into an optimal solution. The figure in the left shows one optimized structure with its band diagram representation and a gap-midgap ratio of over 30%.
Most of this work is part of the research worked out by Abby Men for her Ph.D Thesis together with the advise of Professors Robert Freund, Pablo Parrilo and Jaume Peraire.


  Some lowest-ω E field modes


Straight waveguide in a hexagonal lattice for some ω in the gap


Material Design Optimization for the Heat Equation
Consider for instance a one dimensional beam under heat transfer phenomena. It might be useful to maximize its inner structure such that the transferred heat, at a given time and for a given amount of material, is maxima, or minima. That is, we seek to minimize (or maximize) the time that it will take to the whole beam to get the prescribed temperature given as a source in the two extrema of the beam which is the same as maximizing (or minimizing) the heat transfer by the sources into the beam.
This kind of problems are governed by a system of ODE's which can be discretized using the finite element method. In the end, it turns out to be a convex optimization problem which gives a continuous distribution of material. However, it turns out that it would even be far more useful to get a discrete solution in which we can choose between one or another material for each discrete element. The discrete problem is, nevertheless, a nonconvex optimization problem, NP hard actually.
Using a subspace approximation one can obtain, always after choosing a good enough starting solution, the optimal discrete distribution of the two materials that optimizes the heat transfer. The attached figures show how the solutions are and how better the solution is from the naive constant solution.


Discrete vs. continuous optima.


Optima difference for each sol vs. volume constraint.

CETICA

The project Cetica, from Ciudad Eco-Tecno-Lógica, in Spanish Ecotechnological city has been one of the 16 admitted projects for the Spanish CDTI, the Industrial and Technological Development Center at the 2010 call. It is a project that tries to promote steel versus concrete when planning a civil structure and it is being developed by a great amount of Industrial Companies and Research laboratories at Universities. At the UPC-BarcelonaTECH and more exacly at the CIMNE, the International Center for Numerical Methods on Engineering we are developing an optimal collapse simulator for two and three-dimensional steel frames (and trusses).
The project of this Optimal Collapse Simulator for steel structures is led by a team of principal investigators (PIs) from three institutions:

        Prof. A. Huerta (UPC-BarcelonaTECH),
        Prof. J. Peraire (MIT),
        Prof. J. Bonet (Swansea University). The aim of the project is to obtain a new methodology for the calculation of the collapse state in structures. It is based on the lower bound theorem of the limit analysis and solved by optimization techniques. The lower bound theorem asserts that possible solution states fulfils equilibrium and do not violate the yield constraints. In this work, the collapsed state is found by means of searching the generalized stress distribution, subjected to the equilibrium and yield constraints, that maximizes the external loading multiplier. Apart from the general procedure based in generalized stresses, the main novelty lies on the adaptive linearisation of the yield quadratic forms as well as the formulation for the plastic flow in the space of bending moments and axial forces. Also, in order to obtain nonnegative variables a new strategy that avoids to introduce too many variables is implemented. This technique involves just one additional variable.


Boundgap decreases as adaptivity takes part.


Moment distribution for the collapse load.


EE - Engineering Education

I have been involved in many projects regarding Engineering Education since I just started my way in the University. As a Teaching Assistant for Calculus I was interested in improving the quality of teaching and learning, specially nimathematics. Within these last few years, I have worked on developing materials and contents on Calculus but including innovative techniques. I have presented some papers and publications and many presentations and communications in congresses regarding this topic. The faculty leading these works was mainly M.Rosa Estela, from UPC-BarcelonaTECH.
From 2006 we have been working on developing mathematical contents for Moodle, the most used virtual platform worldwide. Among these contents that we have developed there are theory laboratories, in which students can interact with the concepts, interactive examples, questionnaires... The last thing we are implementing now, is to think the questions for questionnaires in a way that we can program them and so use them randomly. That is a very good way to evaluate students.

     Important Hold the right click button to move the figure.

Regarding this improvement of teaching, we have also filmed some lectures and uploaded them to the Moodle platform webpage and we have even written a book in which we put all this content available, not only for our course students but also for everyone who want to reach them, both instructors and students.
Let's also say that all the programmed contents are done with the mathematical software Wiris, which is specially great to represent figures and it has been included in the Moodle platform in order to be able to create this contents directly on Moodle without attempting to use html, so it couldn't be more userfriendly. That's what instructors like most!

Further Information
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