Seminars

Seminars

Seminars are held on Tuesdays at 3:00 pm in 206 Thomas. Please e-mail comments, questions, and requests to be added to the seminar e-mail list to Maria Koeper.

October 13, 2008
11:00 a.m., Beckman Institute Auditorium
Garrett Reisman, NASA Astronaut
From Caltech to the Space Station and Back
He will give a presentation on his three month stay aboard the International Space Station.

Reisman was born in Morristown , New Jersey , but considers Parsippany , New Jersey his hometown where he graduated from Parsippany High School . He received two Bachelor of Science degrees, one in economics and another in mechanical engineering and applied mechanics, from the University of Pennsylvania in 1991. Reisman received a Master of Science in mechanical engineering from the California Institute of Technology in 1992, where he continued his post graduate studies to receive his doctorate degree in mechanical engineering in 1997.

In July 1998, Reisman was selected by NASA for the 19th group of astronaut candidates as a mission specialist. Reisman's assignments have included working on the space station robotic arm, the next generation space shuttle cockpit and living in the Aquarius underwater habitat as a crewmember of the NASA Extreme Environment Mission Operations program.

Reisman served with both the Expedition 16 and the Expedition 17 crews as a flight engineer aboard the station. He launched with the STS-123 crew aboard the space shuttle Endeavour on March 11, 2008 and returned to Earth with the crew of STS-124 aboard the space shuttle Discovery on June 14, 2008. During his 3 month tour of duty aboard the station, Dr. Reisman performed one, 7 hour spacewalk and executed numerous tasks with the station robotic arm and the new robotic manipulator.

October 21, 2008
Guillaume Blanquart, Stanford University
Modeling Soot Formation in Gas Turbine Engines: An Example of Multi-Physics and Multi-Scale Fluid Mechanics
Because of its high energy density, easy transportability, and still relative abundance, combustion of petroleum-based liquid fuels will remain the principal source of energy for transportation as reported by the International Energy Outlook (2007 Report). The combustion of these liquid fuels in gas turbine or reciprocating engines results in very complex flows where several physical phenomena interact with one another over a large range of scales. These multi-physics and multi-scale problems are among the most challenging fluid mechanics problems of the present century and the limiting factors in the development of more efficient uses of energy. The formation of pollutants, such as soot particles, from the combustion of these fuels is a representative example of the large diversity of scales and physical processes encountered. Predicting soot formation requires combining several key ingredients. First, a detailed chemical kinetic mechanism was developed for high temperature combustion of a wide range of engine relevant hydrocarbon fuels with an emphasis on the prediction of soot. This chemical model is then coupled with a statistical multivariate soot model based on three independent quantities, which describes in a consistent framework the size, the geometry, the chemical composition, and the reactivity of soot particles. The integration of these models in numerical simulations of complex reactive flows, where the turbulent motion is strongly affected by the combustion processes, requires accurate and robust numerical methods. This task is realized by using new high order fully conservative numerical schemes for the simulations of turbulent reacting flows in complex geometries. Finally, these different key elements are combined in a detailed study of soot formation in a dump combustion chamber using Direct Numerical Simulation. This simulation is aimed at understanding the effects of turbulence on the formation and growth of soot particles. The simulation results show that turbulence has a strong impact on the local statistical distribution of soot particles. Furthermore, the soot fields exhibits very complex structures resulting from a very complex turbulence-combustion interaction.

October 24, 2008
11:00 p.m., Lees Kubota Hall - Note special date, time and location
Jorg Imberger, University of Western Australia
Physical Limnology: A Review
Over the last 30 years, limnology has become a mature field with most of the energy flux paths now well established and incorporated into 3D models. The energy from the wind and the sun enters a lake via the free surface, the river inflow may form overflows, intrusions or underflows and the selectivity of a withdrawal flow depends strongly on the thermal stratification in the lake. The talk is structured to follow the energy flux from the wind, to surface waves and surface layer turbulence, to basin scale barotropic lake seiching and internal waves, to high frequency free internal waves and free gyres, to the benthic boundary layer to finally the intermittent turbulence field in the water column; this is generated by non linear wave breaking, Kelvin Hemholz billowing and Holmboe shear instabilities, depending on the relative placement of the shear and density field gradients. Inflows and outflow dynamics is then briefly reviewed. Next, I show how this symphony of motions adds to sustain a weak vertical mass flux and a horizontal dispersion; the latter being critically dependent on the topology of the horizontal residual circulation and the presence of unsteady stagnation points.

I will conclude with an illustration of how the full complexity of these motions can be captured with a 3D model that also beautifully illustrates the competitive nature of the various components of the motion in determining the net mixing in a lake and I show how this competition is a strong function of the lake and stratification properties.

November 4, 2008
David A. Boyd, California Institute of Technology
Extreme Heat in Extremely Small Particles: Resonant Electromagnetic Heating and Peculiar Heat Conduction in Nanostructures
Heat is essential for many chemical, physical, and biological processes and is generally considered in the context of macroscopic phenomena. However,the ability to localize heat on the nanoscale could in turn allow such processes to be localized on the nanoscale allowing the possibility of unprecedented spatial and temporal control. At the nanoscale, materials can have fundamentally different behavior than their bulk counterparts. For example, a resonant electronic phenomenon arises in noble metal nanoparticles that allow the particles to strongly absorb and scatter visible light even though the characteristic lengths of the particles are much smaller than the wavelength of the light used to excite them. We have recently demonstrated that it is possible to heat metallic nanostructures attached to a planar substrate to very high temperatures with only a low-power, visible laser. The nanoparticles are heated because this resonant mechanism is lossy, and the absorbed energy is quickly converted into heat. However, if one assumes classical heat conduction from the particles to the surrounding medium, our results are quite surprising: the heat generated by the absorbed light should be quickly transferred to the supporting substrate. The applicability of Fourier's law of heat conduction has recently been called into question with regards to nanoscale structures. In this talk, I will discuss our experimental results, as well as applications of nanoscale heating to material deposition and fluid transport, and how they support a non-classical mode of heat conduction from very small structures.

November 11, 2008
Michele Guala, California Institute of Technology
Key Scales and Mechanisms in Fluid Particle Systems
The interactions between a fluid and one or more particles often occur at different scales and involve different physical mechanisms. Experimental work in basic and environmental fluid mechanics is presented, with the aim to provide an overview on the possible level of coupling between particle and fluid motions.

The main focus is at the scales at which the interaction takes place, i.e. at the scale at which key modeling assumptions are required. Starting with a case study where the flow field is driven by the particle motion (particle wall collision), we will approach the complexity of particle turbulent interaction, and we will end our excursion with the case limit of material elements, passive by definition. Despite of the different feedback effects on the flow, we stress that particles are essentially Lagrangian objects and thus deserve to be studied experimentally in a time resolved, possibly three dimensional, domain.

December 2, 2008
Michael Bergdorf, ETH
Multiresolution Particle Methods for Flow Simulations
Particle methods have proven themselves as a robust means of addressing transport problems ranging from turbulent flow to the deformation of computational geometry.

The formulation of hybrid particle-mesh methods, which pragmatically uses particles for advection, and the mesh for the "rest", has lead to a wealth of generalizations and extensions, enabling e.g. the massively-parallel simulation of turbulent flows using billions of particles.

Based on this foundation of hybrid particle-methods, we design a particle method formulation with multiresolution capabilities, leading to a framework of methods which are able to address and exploit the presence of different interacting scales.

I will demonstrate the capabilities and limitations of this Lagrangian wavelet-based particle method and its application to interface tracking problems. In this context, I will show how these methods may have to adapt and compromise in order to take full advantage of the trends in computing hardware, such as many-core systems and multilayer parallelism.

December 12, 2008 (Thesis Seminar)
2:30 p.m., Room 306 Thomas
Or Yogev
Computational Evolutionary Embryogeny
Evolutionary and development processes (embryogeny) are used to evolve the configurations of three­dimensional structures to achieve specified performances. The combination of evolutionary and developmental processes is used in natural systems, but has not yet been applied to the design of continuous three­dimensional load­supporting structures. Beginning with a single artificial cell containing information analogous to a DNA sequence, a structure is grown according to the rules encoded in the sequence which are regulated by an environment. Each artificial cell in the structure contains the same sequence of growth and development rules, and each artificial cell is an element in a finite element mesh representing the structure of the mature individual. Rule sequences are evolved over many generations through selection and survival of individuals in a population. Modularity and symmetry are visible in nearly every natural and engineered structure. Understanding of the evolution and expression of symmetry and modularity are emerging from recent biological research. Initial evidence of these attributes is present in the phenotypes that emerge from the artificial evolution, although neither characteristic is imposed nor selected­for directly. The computational embryogeny approach presented here shows promise in synthesizing novel configurations of high­performance systems, and may advance system design to a new paradigm, where current design strategies have difficulty producing useful solutions.

January 6, 2009
Peter Varkonyi, Budapest University of Technology and Economics, Hungary
Shape Evolution of Abrading Particles
Individual particles of granular matters are often shaped by abrasive processes. Abrasion is widely believed to generate round shapes, however the outlines of real worn sand grains or rocks may range from smooth coin-like plates to irregular polyhedra. My aim is to show that increasing asphericity, or even the development of surface singularities can naturally arise in simple, idealized abrasion models, free from anisotropic effects, macroscopic random perturbations or inherent asymmetries of the models themselves. I consider random pairwise collisions of sets of arbitrary convex particles. The rate of abrasion is assumed to depend on the average intensity of collisions on their surfaces. As main result, I demonstrate that this model reproduces various shapes including spheres, 'coins', and polyhedra, resembling beach pebbles and ventifacts (stones exposed to wind-driven sand).

Hence, it provides a novel link between seemingly different abrasion processes, which were examined separately in the past. The presented work was done in collaboration with G. Domokos (Univ. of Cambridge), A. Á. Sipos (Budapest Univ. of Technology), and Gy. Szabó (Univ. of Szeged).

January 27, 2009
Melissa Green, Princeton University
Three-Dimensionality of Bio-Inspired Unsteady Flow Fields
The locomotion of fish and aquatic animals is achieved by the oscillation of fins and flukes, which creates highly three-dimensional, unsteady flow fields that are not yet well-understood.   The principal non-dimensional parameter presently used to describe these flows is the Strouhal number, St = fA/U, which depends on the frequency of oscillation (f), the width of the wake (A), and the freestream velocity (U.)  In previous work on two-dimensional foils, wake structure and thrust performance have been shown to scale with this parameter, but it does not include considerations of three-dimensionality, which become important in the study of low-aspect ratio propulsors.  

In the present work, Digital Particle Image Velocimetry (PIV) was used to investigate the wakes of rigid pitching panels with a trapezoidal panel geometry, chosen to model idealized fish caudal fins.  A Lagrangian coherent structure (LCS) analysis is employed to investigate the formation and evolution of the panel wake.  The LCS analysis, which employs calculations of the Direct Lyapunov Exponent (DLE) has several advantages over  Eulerian methods, including greater detail and the ability to define structure boundaries without relying on a preselected threshold.  In this way, we are able to interrogate the interactions of the structures within the wake and more fully understand the mechanisms of wake evolution.

February 24, 2009
Lees Kubota Lecture Hall
Per Peterson, University of California, Berkeley
Future Nuclear Energy: Advanced High Temperature Reactors
Nuclear power currently generates 70% of all non-fossil electricity in the United States, with the remainder being produced dominantly by conventional large hydroelectricity plants. Major efforts are now underway to deploy new nuclear power plants to expand U.S. nuclear electricity generation capacity. This seminar explores longer-term technology options for nuclear energy production, including the transition to higher operating temperatures, increased passive safety, and advanced fuel cycles including the potential future role of thorium as a reactor fuel. The engineering issues that underlie these future technologies are outlined and discussed.

March 3, 2009
Joanna M. Austin, University of Illinois at Urbana-Champaign
The Role of Thermochemistry in Hypersonic Shear Flows
In high enthalpy hypersonic flight, thermochemical relaxation times are typically comparable to flow residence times, leading to nonlinear coupling between chemical reactions, vibrational excitation, and fluid mechanics. The gas chemical composition and internal energy depart significantly from equilibrium, affecting for example the planetary entry dynamics of both natural and man-made objects. Experimental data in hypervelocity flows are scarce, in part because creating high enthalpy conditions in ground test facilities is extremely challenging and flight tests are expensive.

A new expansion tube facility capable of test gas Mach numbers from 3.0 to 7.1 has been built at Illinois and carefully characterized with experimental measurements and numerical simulations. Two canonical shear flows are being examined in the high enthalpy free stream: triple-point generated free shear layers and boundary layer flows.

Initial experiments identified an opposing wedge configuration used to generate a Mach reflection with associated triple-point shear layers.

The experimental configuration is chosen to give well-characterized inflow and boundary conditions. In addition, a Mach reflection results in a shear layer that separates a gas stream that has passed through a normal shock from a gas stream that has passed through two oblique shocks, leading to dramatically different temperatures and degree of dissociation across the shear layer. Key diagnostic tools include spectroscopic measurements confirming the presence of dissociated NO behind the normal shock, flow visualizations, and temperature measurements benchmarked against calculations using detailed and reduced kinetic mechanisms.

The experimental work is complemented by spatial linear stability analysis. This study is the first linear stability analysis of a hypersonic shear layer to include detailed modeling of molecular effects. An existing molecular-molecular energy transfer rate model is extended to higher collisional energies. Non-equilibrium model results are compared with calculations assuming equilibrium and frozen flow over a range of (frozen) convective Mach numbers from 0.341 to 1.707.

Non-equilibrium effects appear in the creation of nitrous oxide due to dissociation. Dissociation and vibration transfer effects on the perturbation evolution remain closely correlated at all convective Mach numbers.

March 10, 2009
2:00 p.m.
Mo Samimy, The Ohio State University
Active Control of High-Speed and High Reynolds Number Jets for Noise Mitigation Using Plasma Actuators
Jet noise has been an environmental problem since the advent of jet engines. However, the problem has become more severe due to the ever increasing number of flights, the encroachment of residential establishments around airports, and increasingly stricter environmental regulations. Scaling analysis from over 50 years ago showed that the jet noise scales with the eighth power of jet exhaust velocity and with the second power of jet exhaust nozzle diameter (U^8 D^2 ) while the thrust generated by the jet scales with the second power of both jet exhaust velocity and nozzle diameter (U^2 D^2 ). Therefore, increasing the jet exhaust nozzle diameter and thus reducing the jet exhaust velocity has been a simple solution for noise mitigation. An added advantage of larger diameter and lower velocity engines is their better fuel efficiency. The aircraft jet engine industry has used this concept to its fullest extent, and as a result we now have jet engines with an overall diameter of over 9 ft. However, engines larger than these cannot be accommodated on an aircraft. Furthermore, large engines are not suitable for supersonic commercial and high performance military aircraft due to the increased drag penalty with the increased size. A class of high amplitude and high bandwidth plasma actuators called localized arc filament plasma actuators (LAFPAs) has recently been developed at OSU and used for jet noise mitigation with promising results. The control relies on high bandwidth and high amplitude of the actuators to selectively excite various instabilities of the jet to achieve either mixing enhancement or noise mitigation. The lecture will briefly review the actuators and the instabilities of jets and will present and discuss some results on the application of the actuators in subsonic and supersonic jets.

March 17, 2009
Pinaki Chakraborty, University of Illinois at Urbana-Champaign
Turbulent Friction on Rough and Smooth Walls: It is All in the Spectrum (Even in 2D)
In an arch-famous diagram published in 1933, Johann Nikuradse plotted six log-log curves evincing the dependence on the Reynolds number (Re) of the friction coefficient (f) of the turbulent flow in six pipes of fixed roughness. (The roughness of a pipe is the ratio r/R, where r is the size of the roughness elements that line the interior wall of the pipe, and R is the radius of the pipe.) Nikuradse's diagram is rich in distinctive but barely understood features, including a pronounced "hump" where f peaks shortly after the transition to turbulence; a "smooth regime" governed by Blasius's empirical scaling, f _~Re-^1/4; shallow "bellies" where f attains local minima at intermediate values of Re; and a "rough regime" governed by Strickler's empirical scaling, f _~(r/R)-^1/3. Here I will derive an expression of f by assuming that the eddies that transfer momentum between the rough walls and the flow are governed by the phenomenological spectrum of turbulence (Kolmogorov's spectrum for the inertial range with corrections for the energetic and dissipation ranges). I will show that the resulting expression for f is in minute qualitative agreement with Nikuradse's diagram, including all of the distinctive features listed above. I will also describe ongoing experimental work aimed at confirming the existence of close ties between turbulent friction and the turbulent spectrum in 3D flows. Next, I will turn to 2D turbulent flows. I will predict theoretically, for the first time, the dependence of f on Re and r/R for 2D turbulent flows. For the inverse energy cascade, the predictions are the same as for 3D flows. For the enstrophy cascade, I will predict the modified Blasius scaling f _~Re-^1/2 and the modified Strickler scaling f _~r/R. To verify these theoretical predictions, I will invoke a set of computational simulations performed recently by Guttenberg and Goldenfeld. I willl also describe ongoing experimental work on turbulent friction in 2D flows in soap films.

March 19, 2009 (Thesis Seminar)
1:00 p.m.
Erin Koos
Rheological Measurements in Liquid-Solid Flows
The behavior of liquid-solid flows varies greatly depending on fluid viscosity, particle and liquid inertia, and collisions and near-collisions between particles. Shear stress measurements of these flows were completed using a coaxial rheometer specially designed to minimize the effects of secondary flows. Experiments were performed for a range of Reynolds numbers, solid fractions and ratio of particle to fluid densities. Using results from these experiments, an effective viscosity is defined for these liquid-solid flows. With neutrally buoyant particles, the effective viscosity is monotonic but a non-linear function of the solid fraction. Non-neutrally buoyant flows require an additional correction to account for variations in particle mixing.

April 21, 2009
Marcel Ilie, University of California, San Diego
Numerical and Physical Aspects of Coupled Phenomena in Fluid Dynamics
Recent improvements in the processing power of computers make the application of large Eddy simulation (LES) to turbulent flows more feasible. LES, because of it slower computational cost, is a promising alternative method to direct numerical simulation (DNS) for the prediction of high Reynolds-number flows.

The present research concerns the challenges associated with the numerical computation and physical aspects of turbulent flows encountered in aeronautical and environmental engineering. Two different research studies are presented.

The first concerns the perennially challenging problem of blade-vortex interaction (BVI) phenomenon encountered in rotorcraft. One of the main issues associated with the numerical prediction of BVI is the inherent dissipation of the turbulence models which severely affects the characteristics of the vortex. LES approach overcomes this issue and provides a good prediction of the aerodynamic coefficients. A strong (two-way) coupling, LES based, aeroelastic model is proposed. The model provides a good prediction of the aeroelastic response of airframe structure to the aerodynamic forces. Aerodynamic studies of helicopter blade-vortex interaction (BVI) phenomenon, using LES, identified several factors influencing the BVI phenomenon, such as vortex characteristics (size and strength), blade-vortex vertical miss distance, angle of attack, leading edge icing and the aeroelastic response of the blade.

The second concerns particle dispersion in the atmospheric boundary layer. Two different numerical approaches are presented: Lagrangian particle tracking using LES, and an Eddy Interaction Model (EIM) using Reynolds-averaged Navier-Stokes (RANS) simulations. We examine the influence of flow Reynolds number and particle characteristics (shape, size, density) on particle dispersion. Both flow Reynolds number and particle characteristics influence particle dispersion and deposition.

The results of the present studies will be presented in detail.

April 28, 2009
Arnab Samanta, Postdoctoral Scholar, California Institute of Technology
Finite-Wavelength Scattering of Incident Vorticity and Acoustic Waves at a Shrouded-Jet Exit
We consider a round jet shrouded for a finite downstream distance by a sharply-terminated concentric cylinder. As the vortical disturbances supported by a vortex-sheet model of the jet pass the sharp edge of the shroud exit some of the energy is scattered into acoustic waves. We quantify scattering into radiating acoustic modes by obtaining the far-field directivities. Of greater interest in the present study, however, is the scattering into upstream propagating acoustic modes, which is a potential mechanism for closing the resonance loop in the high-amplitude "howling" resonances that have been observed in various shrouded jet configurations over the years. We develop a model for this interaction at the shroud exit. The jet is represented as a uniform flow separated by a cylindrical vortex sheet from a concentric co-flow within the cylindrical shroud. A second vortex sheet separates the co-flow from an ambient flow outside the shroud, downstream of its exit. The Wiener--Hopf technique is used to solve the scattering problem and compute reflectivities at the shroud exit. The focus here is on wavelengths comparable to the shroud exit diameter for which resonances are observed. For some conditions it appears that the reflection of finite-wavelength hydrodynamic vorticity modes on the vortex sheet defining the jet could be sufficient to reinforce the shroud acoustic modes in a way that facilitate resonance. The analysis also gives the reflectivities for the shroud acoustic modes, which would also be important in establishing resonance conditions. Interestingly, it is also predicted that the shroud exit can be "transparent" for ranges of Mach numbers, with no reflection into any upstream propagating acoustic mode. This is phenomenologically consistent with observations in certain experiments that indicate a peculiar sensitivity of resonances of this kind to, say, jet Mach number.

May 4, 2009 (Thesis Defense)
9:00 a.m., 104 Watson
Leonard Lucas
Uncertainty Quantification Using Concentration-of-Measure Inequalities
This work introduces a rigorous uncertainty quantification framework that exploitsconcentration–of–measure inequalities to bound failure probabilities using a well-definedcertification campaign regarding the performance of engineering systems. The frameworkis constructed to be used as a tool for deciding whether a system is likely to perform safelyand reliably within design specifications. Concentration-of-measure inequalities rigorouslybound probabilities-of-failure and thus supply conservative certification criteria, in additionto supplying unambiguous quantitative definitions of terms such as margins, epistemicand aleatoricuncertainties, verification and validation measures, and confidence factors.This methodology unveils clear procedures for computing the latter quantities by meansof concerted simulation and experimental campaigns. Extensions to the theory includehierarchical uncertainty quantification and validation with experimentally uncontrollablerandom variables.

May 5, 2009
Francisco G. Emmerich, Universidade Federal do Espirito Santo, Brazil
Tensile Strength and Fracture Toughness of Brittle Materials Considering and Connecting Microstructure and Atomicity
We address the fracture properties of brittle materials under tension by using a force-atomistic approach: we analyze the forces that act in the solid down to the smallest dimensions between the atoms, unit cells, or grains, observing the minimum characteristic length scale around the point where the fracture begins, and discussing from fundamental principles the criterion for brittle fracture initiation. We take into account the forces due to the applied stress and the material cohesion forces, particularly at the crack tip, where the local hyperelasticity of the material plays a governing role. We connect microstructure and atomicity by using the concept of a total stress concentration factor, equivalent to a local resultant force, which can be obtained through the interaction of two multiplicative terms. By using an experimentally proved maximum tensile-stress criterion of bond rupture, based on the satisfaction of the static equilibrium condition given by Newton's second law up to the beginning of the rupture, we obtain a general expression for the tensile strength, which can be simplified through an effective local cohesive stress. By using the approximation of the stress concentration factor obtained from the concept of equivalent ellipse, we obtain expressions for the tensile strength and fracture toughness. Thus, we explain in a unified framework from fundamental principles a set of established experimental results of brittle fracture of materials under tension, including the dependence of the tensile strength on the crack tip radius of curvature, and some scatter in reported values of fracture toughness and cleavage surface energy. This work can be useful to make more realistic predictions of fracture properties of brittle materials taking into account microstructure and atomicity. It also points out some inadequacies in Griffith energy approach and discusses possible dangers in terms of structural safety.

May 5, 2009 (Thesis Defense)
4:00 p.m., 104 Powell Booth
Feras Habbal
An Optimal Transport Based Meshfree Numerical Method for Simulating Fluid Flows
This presentation develops a novel meshfree numerical method for simulating fluid flows. This method is devised by extending the Benamou-Brenier differential formulation of optimal transport to a multifield variational formulation characterizing fluid flows including viscosity, equations of state, and general boundary conditions. Here the governing principle lends itself ideally to discretization by a combination of conforming interpolation of the velocity field and pointwise sampling of the local material state. A standard Riemann benchmark test is conducted to verify the proposed numerical scheme. In order to highlight the ability of integrating this meshfree method with traditional grid-based numerical methods, such as the finite element method, an illustrative simulation of a fluid-structure interaction is presented. This simulation consists of a compressible Newtonian gas-filled balloon impacting a surface at high speed. This example was inspired by the success of NASA's airbag deployment for the final phase of entry, decent and landing of past Mars missions.

May 27, 2009 (Thesis Defense)
3:30 p.m.
Julia Braman
Safety Verification and Failure Analysis of Goal-Based Hybrid Control Systems
The success of complex autonomous robotic systems depends on the quality and correctness of their fault tolerant control systems.  A goal-based approach to fault tolerant control, which is modeled after a software architecture developed at the Jet Propulsion Laboratory, uses networks of goals to control autonomous systems.  The complex conditional branching of the control program makes safety verification necessary.  Three novel verification methods are presented.  In the first, goal networks are converted to linear hybrid automata via a bisimulation.  The converted automata can then be verified against an unsafe set of conditions using an existing symbolic model checker such as PHAVer.  Due to the complexity issues that result from this method, a design for verification software tool, the SBT Checker, was developed to create goal networks that have state-based transitions.  Goal networks that have state-based transitions can be converted to hybrid automata whose locations' invariants contain all information necessary to determine the transitions between the locations.  An original verification software called InVeriant can then be used to find unsafe locations of linear hybrid systems based on the locations' invariants and rate conditions, which are compared to the unsafe set of conditions.  The reachability of the unsafe locations depends only on the reachability of the states of the state variables constrained in the locations' invariants from their initial conditions.  In cases where this reachability condition is not trivially true, the software efficiently searches for a path to the unsafe locations using properties of the system.  The third verification method is the calculation of the failure probability of the verified hybrid control system due to state estimation uncertainty, which is extremely important in autonomous systems that rely heavily on the state estimates made from sensor measurements.  Finally, two significant example goal network control programs are verified using the three techniques presented.

May 27, 2009 (Thesis Defense)
Room 306 Thomas
Jennifer Franck
Large-Eddy Simulation of Flow Separation and Control on a Wall-Mounted Hump
Active flow control techniques such as synthetic jets have been successful in increasing the performance of naturally separating flows on post-stall airfoils, bluff body shedding, and internal flows such as wide-angle diffusers. However, the development of accurate predictive tools for unsteady separation and control remains a challenge, especially at high Reynolds numbers. This thesis presents a compressible large-eddy simulations (LES) of turbulent flow over a wall-mounted hump at Re = 500,000 with active flow control. The flow is characterized by an unsteady, turbulent recirculation region along the trailing edge of the geometry. Control is applied just before the natural separation point via steady suction and zero-net mass flux oscillatory forcing, which is shown to be effective in decreasing the size of the separation bubble and pressure drag. LES results are validated against previously performed experiments by Seifert and Pack and those performed for the NASA Langley Workshop on Turbulent Flow Separation and Control. Three test cases are explored to determine the effect of explicit filtering and the Smagorinsky subgrid scale model on the average flow and turbulent statistics. The flow physics and the control effectiveness are investigated at two Mach numbers, M=0.25 and M=0.6, and the effects of compressibility on the baseline and controlled flow are discussed. In addition, two-dimensional direct numerical simulations (DNS) of the wall-mounted hump flow are performed. Two-dimensional results are shown to exhibit different baseline flow features than the 3D LES, but the controlled results give an indication of the most receptive actuation frequencies. Finally, the flow physics and effectiveness of two different regimes of the reduced actuation frequency, F+=O(1) and F+=O(10), are explored with LES, and compared with previous experimental and computational investigations.

June 5, 2009 (Thesis Defense)
10:00 a.m.
Francesco Ciucci
Continuum Modeling of Mixed Conductors: a Study of Ceria
In order to design, optimize, and characterize Solid Oxide Fuel Cell (SOFC) electrodes, it is very useful to have models that aid in interpreting experimental results. Samarium Doped Ceria (SDC) electrodes are currently of great interest for SOFC applications. For example, ceria-containing anodes can be operated directly on hydrocarbons without coking, and in addition can be used at lower temperatures than Ni/YSZ cermets thank to their mixed conducting behavior under sufficiently reducing conditions.

In this work, I first present a linear, time-independent model for the study of an SDC thick sample. This model allows the computation of species concentrations, electric potential and currents under small bias conditions. A regular perturbation of the drift diffusion equations and Poisson's equation is used to derive the model for the electrochemical behavior of bulk of the material. I also include the kinetics of reactions occurring at the SDC-gas surface where the SDC is exposed to a spatially uniform hydrogen-water-argon mixture at fixed total pressure. The linear response to a small voltage input is computed at various hydrogen and water partial pressures. The numerical procedure allows for fast computations and for the direct determination of fast and rate limiting steps.

The model is subsequently extended in order to study the time dependent case, i.e. impedance spectroscopy conditions, where a small sinusoidal current is imposed to the SDC system initially at equilibrium. This enables the calculation of impedance spectra and comparison to experiments in frequency space. The underlying assumption that diffusivities are constant within the sample is then relaxed; polarization resistance and impedance spectra are calculated for a wide variety of cases .

Finally the time dependent model is used to study the impedance spectra of an SDC thin film deposited on top of a purely ionic conductor and the model is shown to work as well for cathode materials such as Lanthanum Manganite.