Seminars

Seminars are held Wednesdays, at 3:30 pm, in Seaver Science Library, Room 150 (SSL 150), unless otherwise noted. Refreshments are served at 3:00 pm. Call (213) 740-8762 for further information. Note—several seminars this semester are not at the regularly scheduled day, time, and place.

Archive of Seminar Announcements:

2017 | 2016 | 2015 | 2014 | 2013 | 2012 | 2011 | 2010 | 2009 | 2008 | 2007 | 2006 | 2005 | 2004

Keynote Lecture Series Archive

Spring, 2018

Entrapment, Escape, and Diffusion of Active Particles in Complex Environments

Saverio Spagnolie

Assistant Professor
Department of Mathematics
University of Wisconsin at Madison
Madison, WI

The swimming kinematics and trajectories of microorganisms and active synthetic particles are altered by the presence of nearby boundaries, be they solid or deformable, and often in perplexing fashion. When an organism’s swimming dynamics vary near a boundary, a natural question arises: is the change in behavior fluid mechanical, biological, or perhaps mediated by other physical laws? We will explore the hydrodynamic interactions between active particles and nearby surfaces, which can result in entrapment or escape depending on the propulsive mechanism used by the swimming body and its size (through the strength of Brownian fluctuations). If the confining geometry is regular, the swimming dynamics can settle towards a stable periodic orbit or can be chaotic depending on the nature of the scattering dynamics. Yet more stunning effects are achieved by large suspensions of active particles swimming en masse when bounded by freely moving interfaces. Applications are envisioned in bioremediation and sorting of active particles or microorganisms, and the work may speak to the behavior of biofilms and motile suspensions in heterogeneous or porous environments.

Saverio Spagnolie is an assistant professor in mathematics at the University of Wisconsin-Madison with a courtesy appointment in chemical and biological engineering. His research interests include fluid dynamics, soft matter, biolocomotion and numerical analysis. He is the founder of the Madison Applied Mathematics Laboratory, an interdisciplinary research lab with a focus on fluid-structure interactions. Before arriving in Madison, Saverio received a Ph.D. in mathematics at the Courant Institute of Mathematical Sciences, then held postdoctoral positions in the Mechanical/Aerospace Engineering department at UC San Diego and in the School of Engineering at Brown University.

Wednesday, January 10, 2018
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Using Coupled Fire/Atmosphere Modeling to Advance Wildland Fire Science and Assist Decision Makers

Rod Linn

Senior Scientist
Los Alamos National Laboratory (LANL)

Advancements in computing and numerical modeling have generated new opportunities for the use of coupled fire/atmosphere models in wildfire research. Models such as FIRETEC attempt to represent the interaction between dominant processes that determine wildfire behavior, including: convective and radiative heat transfer, aerodynamic drag and buoyant response of the atmosphere to heat released by the fire. Such models are not practical for operational faster-than-real-time fire prediction due to their computational and data requirements. However, their process-based model-development approach creates an opportunity to provide additional perspectives concerning aspects of fire behavior that have been observed in the field and in the laboratory; allow for sensitivity analysis that is impractical through observations and pose new hypothesis that can be tested experimentally. Specific examples of the use of FIRETEC in this fashion include: 1) investigation of the 3D fire/atmosphere interaction that dictates multi-scale fireline dynamics; 2) the influence of vegetation heterogeneity and variability in wind fields on predictability of fire spread; 3) the interaction between ecosystem disturbances such as insect attacks and potential fire behavior. Additionally, couple wildfire/atmosphere modeling opens new possibilities for understanding the sometime counterintuitive impacts of fuel management and exploring the implications of various prescribed fire tactics. Results from these studies highlight critical roles coupled fire/atmosphere interaction, which is directly affected by the structure of the vegetation in the vicinity of the fire. Vegetation structure not only impact the amount and distribution of combustible material, but it also influences the winds and turbulence that control the convective heating and cooling of unburned fuels Certainly there need to be continued efforts to validate the results from these numerical investigations, but, even so, they suggest relationships, interactions and phenomenology that should be considered in the context of the interpretation of observations, design of fire behavior experiments, development of new operational models and even risk management.

Rod Linn is a senior scientist in the Computational Earth Science group at Los Alamos National Laboratory (LANL), where he studies and models a wide range of atmospheric phenomenon using computational physics. Linn has led much of the development and application of the FIRETEC computer program for predicting wildfire behavior. Dr. Linn received his doctorate from New Mexico State University.

Wednesday, January 17, 2018
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Thermal Ignition of Gaseous Mixtures: Experiments and Simplified Modeling

Stephanie A. Coronel

Postdoctoral Scholar
California Institute of Technology
Pasadena, CA

In recent decades, there have been significant advances in flow visualization techniques, including the development of sophisticated optical diagnostics that yield quantitative information. In spite of the existence of newer diagnostics, older and less expensive techniques can still be used today to quantitatively measure parameters of interest in flows. This talk focuses on interferometry, an imaging technique that dates back more than a century but which has only recently been applied to visualization and measurement of ignition of gaseous reactive flows. In addition to interferometry, I discuss the image processing algorithms required to extract temperature from images of optical phase difference (directly measured through interferometry). The technique and algorithms are applied to ignition of reactive mixtures by moving hot particles, an explosion hazard that is present in the nuclear, aircraft, and industrial sectors. Spatio-temporal temperature measurements of the flow (Re < 200) indicate that ignition preferentially occurs in the region of flow separation of the particle. Furthermore, a simplified model of reactive flow adjacent to a hot surface indicates that ignition occurs some distance away from the surface. At this location, heat release in the gas from the chemical reactions exceeds heat losses back to the surface.

Wednesday, January 24, 2018
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

host: J.A. Domaradzki

Intracardiac Blood Flow Quantification in the Clinical Setting. Ready for Prime Time?

Juan Carlos del Alamo

Professor
Department of Mechanical and Aerospace Engineering
University of California San Diego
La Jolla, CA

Recent advances in imaging technology and computational fluid dynamics now allow physicians to obtain non-invasive robust measurements of intracardiac blood flow in the clinical setting. These advances have revealed that blood flow inside the heart chambers is characterized by the formation of unsteady vortex structures, generated during filling, that eventually last until the chambers are emptied.

This talk will summarize our recent efforts toward understanding how these flow patterns contribute to the function of the left ventricle (the main pumping chamber of the heart). We will show that the normal ventricular flow patterns: 1) Contribute to efficient filling of the ventricle. 2) Efficiently redirect the transit of blood towards the ventricle’s outflow tract. 3) Minimize the number of cardiac cycles that blood stays in ventricular transit, thereby reducing the risk of intraventricular blood clotting.

We will also illustrate how intraventricular flow quantification can be translated to the clinical setting in order to characterize and optimize the impact of clinical interventions and cardiac device implantation (e.g. bi-ventricular pacemakers and ventricular assist devices). In addition, we will provide examples of clinical studies in which we use intraventricular flow analysis to predict the risk of intracardiac blood clot formation and stroke, both in patients with regularly beating hearts and in patients with atrial fibrillation.

Juan Carlos del Alamo received a B.S., M.S. and Ph. D. in Aerospace Engineering at the Polytechnic University in Madrid. He was a Fulbright postdoctoral fellow at Harvard University and UC San Diego, where he received training in experimental cell mechanics and cardiovascular flows. Prof. del Alamo’s lab at UCSD focuses on biological fluid mechanics and cardiovascular physiology, with particular emphasis on cellular biomechanics and non-invasive characterization of intracardiac flows. This research has been recognized with a US Geological Survey Director’s Award (2010), the NSF CAREER Award (2011), the Hellman Family Fellowship (2012), and the William Parmley Award from American College of Cardiology (2015).

Monday, January 29, 2018
11:00 am
Hughes Aircraft Electrical Engineering Center, Room 132 (EEB 132)

Refreshments will be served at 10:45.

host: J.A. Domaradzki

—REMARKABLE TRAJECTORY LECTURE SERIES—

Catching The Wave

Larry Redekopp

Professor Emeritus
Department of Aerospace & Mechanical Engineering
University of Southern California
Los Angeles, CA

In this lecture we will engage in a “jet-ski ride” over and through a variety of wave phenomena explored during the span of my research career. Various hydrodynamic and gas dynamic wave types will be considered encompassing a range of scales extending from 10+7m to 10-4m, and several requisite mathematical contributions will be noted. Particular wave contexts and attendant applications vary from aerodynamics to planetary atmospheres to ocean physics to lake hydrodynamics to ink-jet printers, and involve both stable and unstable dynamics. This excursive “wave rider” tour will conclude with brief comments relating to career perspectives on educator experience, administrative involvements, professional contributions, and personal philosophy.

Wednesday, January 31, 2018
Reception 12 NOON – 12:45 PM
Lecture 12:45 PM – 2:00 PM
The Vineyard Room, International Academy Building/Davidson Conference Center

Experimental Mechanics Across Multiple Time and Spatial Scales

Kara Peters

Professor
Department of Mechanical and Aerospace Engineering
North Carolina State University
Raleigh, NC

This presentation will provide an overview of research programs at NCSU, focused on the development of new experimental mechanics techniques to collect information simultaneously across multiple time and spatial scales. This information can then reveal how failure and energy dissipation phenomena are coupled between these scales. Many of these techniques have been specifically created for extreme environments, e.g. high temperature or high-energy dynamic events. I will first discuss current research at NCSU towards integrating sensor networks into aircraft structures for monitoring of the airframe during fabrication, service and repair. Embedding sensors into the airframe structures can provide information not obtainable from surface mounted sensor systems, however also presents issues of interpretation of sensor signals and potential changes to the airframe durability. Example projects to be discussed include high-speed, full spectral interrogation of fiber Bragg grating sensors for damage assessment; optimization of sensor networks embedded in the airframe structure; and remote bonding of fiber Bragg grating sensors for Lamb wave detection in structures. The unique direction of these research activities is that they bridge the gap between the material and structural mechanics and optics/photonics communities to enable these new measurement techniques. Afterwards, I will present a general overview of other research areas in my group, for example high-speed polarization imaging for the measurement of collagen fiber realignment in the tendon-to-bone insertion region during dynamic impact events.

Kara Peters is a Professor in the Department of Mechanical and Aerospace Engineering at North Carolina State University. She received her PhD in Aerospace Engineering from the University of Michigan in 1996. For her dissertation work, she received the Ivor K. McIvor Award for Applied Mechanics at the University of Michigan. Following her PhD, Dr. Peters worked as Post-Doctoral Researcher in the Laboratory of Applied Mechanics at the Ecole Polytechnique Fédérale de Lausanne (Swiss Institute of Technology at Lausanne). Dr. Peters is a member of the ASME Adaptive Structures and Material Systems Technical Committee and was the chair of the SPIE Smart Structures and Materials Symposium in 2010 and 2011. She is an Associate Editor of the journal Smart Materials and Structures and on the editorial board of Measurement Science and Technology. Currently, Dr. Peters is serving as a rotator as the Program Manager of the Mechanics of Materials and Structures Program at the National Science Foundation.

Wednesday, February 7, 2018
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

host: J.A. Domaradzki

Flame Hole Dynamics Applied to the Modeling of Turbulent Nonpremixed Combustion

Carlos Pantano

Professor
Department of Mechanical Science and Engineering
University of Illinois at Urbana-Champaign
Urbana, Il

Turbulent diffusion flames can be quenched in regions of high strain owing to increased heat loss away from the reaction zone. These chemically inert regions are sometimes called flame holes (Dold et al. 1991). Turbulent flames with extinction are relevant in modern combustors where the flame temperature is kept low to reduce pollutant formation or in lifted jet flames used for thermal protection of the burner liner. Modeling the dynamical behavior of flame holes, without incorporating a detailed chemical-transport description, requires new numerical methods that describe the evolution in time of the flame boundary (or rim) on the moving stoichiometric surface. The kinematics of the flame rim is normally approximated as that of a two-dimensional edge flame whose speed of propagation is controlled by the local strain conditions. The computational challenge is the efficient numerical evolution of the flame rim using a state field defined on a two-manifold (of varying shape, and possibly multiply connected). In this talk, I will describe recent progress on the numerical and physical modeling of flame holes as it applies to turbulent nonpremixed flames with extinction. Special emphasis is made to achieve high-order of accuracy, flexibility, and robustness, while maintaining relatively low computational cost.

Carlos Pantano received his Bachelor degree in Industrial Engineering with specialization in Electrical Engineering from the University of Sevilla in Spain. He received a Masters in Applied Mathematics from Ecole Centrale Paris in France, and a Masters and PhD in Mechanical Engineering from the University of California San Diego. He held a Senior Postdoctoral position in Engineering from 2000 to 2001 at the Office National d’Etudes et de Recherches Aerospatiales in France and then moved to the California Institute of Technology as a senior post-doctoral associate and later as a senior research scientist until 2006. Currently, he holds the rank of Professor in Mechanical Engineering at Illinois. Professor Pantano received the Presidential Early Career Awards for Scientists and Engineering (PECASE) in 2006. He is currently an Associate Fellow of the American Institute of Aeronautics and Astronautics (AIAA) and member of American Physical Society (APS), Society for Industrial and Applied Mathematics (SIAM), and the Combustion Institute.

Thursday, February 15, 2018
11:00 AM
Location TBD

Refreshments will be served at 10:45 am.

host: J.A. Domaradzki

Tutorial on Machine Learning and Neural Networks, Part I

Kevin Chen

Center for Communications Research
La Jolla, CA

Tuesday, February 20, 2018
1:00 PM
Laufer Library (RRB 208)

host: Eva Kanso

Tutorial on Machine Learning and Neural Networks, Part II

Kevin Chen

Center for Communications Research
La Jolla, CA

Thursday, February 22, 2018
1:00 PM
Laufer Library (RRB 208)

host: Eva Kanso

 

host: J.A. Domardzki

New Results on Self-Excitation in Circulatory and Parametrically Excited Systems

Peter Hagedorn

Professor
Mechanical Engineering
Technische Universität Darmstadt
Darmstadt, Germany

In mechanical engineering systems, self-excited vibrations are in general unwanted and sometimes dangerous. There are many systems exhibiting self-excited vibrations which up to this day cannot be completely avoided, such as brake squeal, the galloping vibrations of overhead transmission lines, the ground resonance in helicopters and others. Most of these systems have in common that in the linearized equations of motion the self-excitation terms are given by non-conservative, circulatory forces and/or parametric excitation. The presentation will discuss some recent results in linear and nonlinear systems of this type.

Self-excited vibrations have of course been mathematically modelled and studied at least since the times of van der Pol. The van der Pol oscillator is a one degree of freedom system; its linearized equations of motion correspond to an oscillator with negative damping. Sometimes also other self-excited systems present negative damping, which can be made responsible for self-excited vibrations. In all the engineering systems mentioned above however, the self-excitation mechanism is mainly related to the interaction between different degrees of freedom (modes), and the linearized equations of motions contain circulatory terms. This together with parametric resonance is the main excitation mechanism discussed in this paper. Destabilization by ‘negative damping’ will not be considered. Also stick-slip phenomena are not in the focus of this presentation; they also do not seem to play an important role in all the examples given above.

The systems analyzed in this presentation therefore are characterized by the M, D, G, K, N matrices (mass, damping, gyroscopic, stiffness and circulatory matrices, respectively) which may all be time-dependent. In the unstable case, additional nonlinear terms do of course limit the vibration amplitudes. Different types of bifurcations relevant for these systems have recently been studied in the literature.

In the first part, MDGKN-systems with constant coefficients will be discussed. For a long time it has been well known, that the stability of such systems can be very sensitive to damping, and also to the symmetry properties of the mechanical structure. Recently, several new theorems were proved concerning the effect of damping on the stability and on the self-excited vibrations of the linearized systems. The importance of these results for practical mechanical engineering systems will be discussed. It turns out that the structure of the damping matrix is of utmost importance, and the common assumption, namely representing the damping matrix as a linear combination of the mass and the damping matrices, may give completely misleading results for the problem of instability and the onset of self-excited vibrations.

The second case considered deals with MDGKN-systems with time-periodic coefficients. The stability of these systems can be studied via Floquet theory. A typical property of parametric instability behavior is the existence of combination resonances. However, if parametric excitation in the system is simultaneously present in the K and the N matrices and/or there are excitation terms which are not all in phase, an atypical behavior may occur: The linear system may then for example be unstable for all frequencies of the parametric excitation, and not only in the neighborhood of certain discrete frequencies. Such atypical parametric instability happens even for M, D, G constant and zero mean values for the matrices K(t) and N(t). This was recently observed at the linearized equations of motion for a minimal model of a squealing disk brake. It turns out, that an even much simpler example of such a situation was given about 70 years ago by Lamberto Cesari, but seems to have fallen into oblivion. Until recently it was thought that such out of phase terms in the parametric excitation would not occur in engineering systems. In the presentation it is shown that they may indeed occur for example in the model of a squealing brake and probably in many other mechanical engineering systems, as long as there is slip with friction between solid bodies.

In the unstable case, additional nonlinear terms do of course limit the vibration amplitudes. Different types of bifurcations relevant for these systems are studied using normal form theory, in particular for the ‘Cesari equations’ with additional nonlinearities.

Wednesday, March 21, 2018
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

host: Firdaus Udwadia