Seminars

Fast Spectral Time-Domain PDE Solvers for Complex Structures: The Fourier-Continuation Method

Oscar P. Bruno

Professor
Department of Applied and Computational Mathematics
CalTech
Pasadena, CA

We present fast spectral solvers for time-domain Partial Differential Equations. Based on a novel Fourier-Continuation (FC) method for the resolution of the Gibbs phenomenon, these methodologies give rise to time-domain solvers for PDEs for general engineering problems and structures. The methods enjoy a number of desirable properties, including spectral time evolution essentially free of pollution or dispersion errors for general PDEs in the time domain, with conditional/unconditional stability for explicit/alternating-direction methods and high order of temporal accuracy. A variety of applications to linear and nonlinear PDE problems will be presented, including the diffusion and wave equations, the Navier-Stokes equations and the elastic wave equation, demonstrating the significant improvements the new algorithms can provide over the accuracy and speed resulting from other approaches.

Oscar Bruno received his Ph.D. degree from the Courant Institute of Mathematical Sciences, New York University. Following graduation, he held a two-year position as Visiting Assistant Professor with the University of Minnesota, and he then joined the faculty of the Georgia Institute of Technology (Georgia Tech), where he served as Assistant and Associate Professor. After a four-year period with Georgia Tech, in 1995 he joined the faculty of the California Institute of Technology (Caltech), where he has served as Professor in the Department of Applied and Computational Mathematics since 1998, and as Executive Officer of that department during 1998-2000. Dr. Bruno’s research interests lie in areas of optics, elasticity and electromagnetism, remote sensing and radar, overall electromagnetic and elastic behavior of materials (solids, fluids, composites materials, multiple-scale geometries), and phase transitions. Dr. Bruno has directed 37 graduate students and postdocs during his career, and his research efforts have resulted in the publication of more than 100 refereed articles, and have been acknowledged by his plenary presentations at many international conferences, his service on editorial boards of important scientific journals, including the SIAM Journal of Applied Mathematics, the SIAM Journal on Scientific Computing, and the Proceedings of the Royal Society of London, and his election to honorary societies, most notably the Council for the Society for Industrial and Applied Mathematics. Dr. Bruno is a recipient of the Sigma-Xi faculty award, the Friedrichs Award for an outstanding dissertation in mathematics, a Young Investigator Award from the National Science Foundation. and a Sloan Foundation Fellowship. Dr. Bruno is a SIAM Fellow in the class of 2013, and a National Security NSSEFF Vannevar Bush fellow, in the class of 2016.

Wednesday, August 28, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Pahlevan

Insights into the Nonlinear Evolution of Flame Fronts through Experiments and Models

Basile Radisson

Postdoctoral Scholar
Department of Aerospace & Mechanical Engineering
USC

Due to the exothermicity of combustion reaction, premixed flames are intrinsically unstable. Consequently, cellular patterns are formed on the flame front resulting in rich and complex dynamics. Combustion involves several hundreds of elementary chemical reactions occurring at a submillimetric interface, rendering a detailed description computationally intractable. Simplified models offer much insight into these complex dynamics. Through asymptotic analysis, the shape of the flame front can be described by a set of poles and their dynamic evolution in the complex plane. Here, we demonstrate for the first time the validity of such model in comparison to experimental flame evolution. A premixed flame propagates in a reactive mixture held between two vertically oriented ceramic glass plates separated by a 5mm gap. By extracting a flame front from this experiment, we demonstrate the feasibility of describing the shape of the front by a set of poles’ solutions. The flame front dynamics are well described for approximately ten times the characteristic time of the instability. Beyond this time, the comparison is limited by the sensitivity to initial condition. However, by studying statistical properties of the flame front (here statistics of cell sizes), we demonstrate that the pole description is still valid at long time. Moreover, these statistics satisfy a gamma distribution, characteristic of phenomena for which the elementary interaction rule is of additive nature and which results here from the pole to pole attraction. This analytical prediction of the cell-size distribution could be of great interest for the understanding of turbulent flame dynamics.

We will conclude this presentation by commenting on the role of gravity and the influence of burner walls.

Basile Radisson is a post-doctoral scholar in the Aerospace and Mechanical Engineering department at the University of Southern California since June 2019. Prior to joining USC, Basile received his PhD from IRPHE, Aix-Marseille Université, France, working on flame front dynamics under the supervision of C. Almarcha and B. Denet. Basile’s research interests lie in instability phenomena driven by geometric non-linearities. He is currently working on snap instabilities in fast elastic filaments under the supervision of E. Kanso.

Wednesday, September 4, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Kanso

Settling of Cohesive Sediment: Particle-resolved Simulations

Eckart Meiburg

Professor
Department of Mechanical Engineering
UC Santa Barbara
Santa Barbara, CA

We develop a physical and computational model for performing fully coupled, grain-resolving Direct Numerical Simulations of cohesive sediment, based on the Immersed Boundary Method. The model distributes the cohesive forces over a thin shell surrounding each particle, thereby allowing for the spatial and temporal resolution of the cohesive forces during particle-particle interactions.

We test and validate the cohesive force model for binary particle interactions in the Drafting-Kissing-Tumbling (DKT) configuration. Cohesive sediment grains can remain attached to each other during the tumbling phase following the initial collision, thereby giving rise to the formation of flocs. The DKT simulations demonstrate that cohesive particle pairs settle in a preferred orientation, with particles of very different sizes preferentially aligning themselves in the vertical direction, so that the smaller particle is drafted in the wake of the larger one. This preferred orientation of cohesive particle pairs is found to remain influential for much larger simulations of 1,261 polydisperse particles released from rest. These simulations reproduce several earlier experimental observations by other authors, such as the accelerated settling of sand and silt particles due to particle bonding, the stratification of cohesive sediment deposits, and the consolidation process of the deposit. This final phase also shows the build-up of cohesive and direct contact intergranular stresses. The simulations demonstrate that cohesive forces accelerate the overall settling process primarily because smaller grains attach to larger ones and settle in their wakes. An investigation of the energy budget shows that the work of the collision forces substantially modifies the relevant energy conversion processes.

Eckart Meiburg received his Ph.D. from the University of Karlsruhe. After a postdoc at Stanford, he became an assistant professor in applied mathematics at Brown. He then moved to USC as associate then full professor. He later moved to UC Santa Barbara.

Meiburg’s research interests are fluid dynamics and transport phenomena, primarily computational fluid dynamics. He uses highly resolved direct numerical simulations to investigate physical mechanisms governing the spatio-temporal evolution of a wide variety of geophysical, porous media, and multiphase flow fields. Some of his current interests are gravity and turbidity currents, Hele-Shaw displacements, double-diffusive phenomena in particle laden flows, and internal bores.

Meiburg has received a Presidential Young Investigator Award, a Humboldt Senior Research Award, and a Senior Gledden Fellowship (Institute of Advanced Studies, University of Western Australia). He is fellow of the American Physical Society and the ASME, was the 2012 Lorenz G. Straub Award Keynote Speaker (Univ. Minn.), gave the Ronald F. Probstein Lecture at MIT in 2018, and was Shimizu Visiting Professor at Stanford University.

Wednesday, September 11, 2017
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Bermejo-Moreno

Network-Based Characterization, Modeling, and Control of Fluid Flows

Kunihiko (Sam) Taira

Associate Professor
Mechanical and Aerospace Engineering
UCLA
Los Angeles, CA

The network of interactions among fluid elements and coherent structures gives rise to the amazingly rich dynamics of vortical flows. To describe these interactions, we consider the use of mathematical tools from the emerging field of network science that is comprised of graph theory, dynamical systems, data science, and control theory. In this presentation, we discuss ways to describe unsteady fluid flows with vortical-interaction, modal-interaction, and probability-transition networks. The insights gained from these formulations can be used to characterize, model, and control laminar and turbulent flows. We will also discuss some of the challenges of applying network based techniques to fluid flows and the prospects of addressing them through data-inspired techniques.

Kunihiko (Sam) Taira is an Associate Professor of Mechanical and Aerospace Engineering at UCLA. His research focuses on computational fluid dynamics, flow control, and network science. He received his B.S. degree from the University of Tennessee, and his M.S. and Ph.D. degrees from the California Institute of Technology. He is a recipient of the 2013 U.S. Air Force Office of Scientific Research and 2016 Office of Naval Research Young Investigator Awards.

Wednesday, September 18, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Luhar

Design & Implementation of a 3D-PTV System

Dana Dabiri

Associate Professor
William E. Boeing Department of Aeronautics & Astronautics
University of Washington
Seattle, WA

The dream of experimental fluid dynamicists is to be able to measure complex, three-dimensional turbulent flow fields globally with very high spatial and temporal resolution. While we are still far from fully realizing this dream, significant progress has been made towards this goal during the last three decades. Early quantitative measurement methods using Pitot tubes, Venturi tubes and later measurement methods, such as Hot Wire Anemometry (HWA) and Laser-Doppler Anemometry (LDA), by their nature, were measurement methods that provided instantaneous velocity signals at single-points through time. Early emphasis in turbulence research and its theoretical advancement therefore necessitated a statistical description of turbulent flow fields, which relied heavily upon measurements provided by these single-point measurement techniques. Since the early seventies, the discovery of the existence of three-dimensional coherent structures within turbulent flows using qualitative flow visualization methods (i.e. shadowgraphs, Schlieren systems, dye injection, etc.) has been of significant interest for turbulence researchers. While flow visualization techniques have been around since the days of Prandtl, it is only due to the advent of modern imaging, laser, and data acquisition technology has allowed for qualitative flow visualization to become quantitative. These advents have allowed for the development and advancement of are relatively new measurement technique, Particle Image Velocimetry (PIV) and Particle Tracking Velocimetry (PTV) in two dimensions, and more recently in 3 dimensions. Because of its ability to provide global two/three-dimensional kinematic information as well as its ability to map the evolution of coherent structures through time, PIV/PTV has become a powerful tool in studying, understanding, and modeling fluid flow behavior. In this talk, I will describe the particulars of the 3D Particle Tracking Velocimetry method we have developed and touch on some applications in microflows and LES studies.

Dana Dabiri is Associate Professor at the William E. Boeing Department of Aeronautics & Astronautics at the University of Washington, in Seattle. He received his B.S. in Mechanical Engineering at the University of California, San Diego in 1985; he received his M.S. in Mechanical Engineering at the University of California, Berkeley in 1987; and he received his PhD in Aerospace Engineering at the University of California, San Diego in 1992. He was a Post-doc at Caltech from 1992-1993, and continued at Caltech as a research scientist until the end of 2001. In 2002, he joined the faculty at the William E. Boeing Department of Aeronautics & Astronautics as an Assistant Professor, and was promoted as an Associate Professor in 2009. He serves as an associate editor for the Journal of Visualization since 2009. His work pursues developing novel ways for quantitatively visualizing the movements of fluids. Most recently, Professor Dabiri’s research activities have developed novel implementations of 2D and 3D digital particle tracking velocimetry (2DPTV & 3DPTV) system that allows for high-resolution measurements of fluid flows.

Wednesday, September 25, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Pahlevan

Navigating in a Turbulent Environment

Mimi Koehl

Professor
Department of Integrative Biology
University of California, Berkeley
Berkeley, CA

When organisms locomote and interact in nature, they must navigate through complex habitats that vary on many spatial scales, and they are buffeted by turbulent wind or water currents and waves that also vary on a range of spatial and temporal scales. We have been using the microscopic larvae of bottom-dwelling marine animals to study how the interaction between the swimming or crawling by an organism and the turbulent water flow around them determines how they move through the environment. Many bottom-dwelling marine animals produce microscopic larvae that are dispersed to new sites by ambient water currents, and then must land and stay put on surfaces in suitable habitats. Field and laboratory measurements enabled us to quantify the fine-scale, rapidly-changing patterns of water velocity vectors and of chemical cue concentrations near coral reefs and along fouling communities (organisms growing on docks and ships). We also measured the swimming behavior of larvae of reef-dwelling and fouling community animals, and their responses to chemical and mechanical cues. We used these data to design agent-based models of larval behavior. By putting model larvae into our real-world flow and chemical data, which varied on spatial and temporal scales experienced by microscopic larvae, we could explore how different responses by larvae affected their transport and their recruitment into reefs or fouling communities. The most effective strategy for recruitment depends on habitat.

Mimi Koehl, a Professor of the Graduate School in the Department of Integrative Biology at the University of California, Berkeley, earned her Ph.D. in Zoology at Duke University. She studies the physics of how organisms interact with their environments, focusing on how microscopic creatures swim and capture food in turbulent water flow, how organisms glide in turbulent wind, how wave-battered marine organisms avoid being washed away, and how olfactory antennae catch odors from water or air moving around them.

Professor Koehl is a member of the National Academy of Sciences and the American Academy of Arts and Sciences, and is a Fellow of the American Physical Society. Her awards include a MacArthur “genius grant”, a Presidential Young Investigator Award, a Guggenheim Fellowship, the John Martin Award (Association for the Sciences of Limnology and Oceanography, for “for research that created a paradigm shift in an area of aquatic sciences”), the Borelli Award (American Society of Biomechanics, for “outstanding career accomplishment”), the Rachel Carson Award (American Geophysical Union, for “cutting-edge ocean science”), and the Muybridge Award (International Society of Biomechanics “highest honor”).

Wednesday, October 2, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Kanso

Active Particles in Complex Fluids

Gwynn Elfring

Associate Professor
Department of Mechanical Engineering
and
Institute of Applied Mathematics
University of British Columbia
Vancouver, BC

Active particles are self-driven objects, biological or otherwise, which convert stored or ambient energy into systematic motion. The motion of small active particles in Newtonian fluids has received considerable attention, with interest ranging from phoretic propulsion to biological locomotion, whereas studies on active bodies immersed in complex fluids are comparatively scarce. In this talk I will discuss a theoretical formalism for understanding the motion of active particles in complex fluids and then discuss the effects of viscosity gradients, viscoelasticity and shear-thinning rheology in the context of biological locomotion and the propulsion of colloidal Janus particles.

Gwynn Elfring is an Associate Professor in the Department of Mechanical Engineering and the Institute of Applied Mathematics at the University of British Columbia, and currently a Visiting Associate in the Division of Chemistry and Chemical Engineering at the California Institute of Technology. His group at UBC conducts research on biological locomotion and fluid-body interactions in complex fluids and interfaces. Previously, he completed a Ph.D. at the University of California San Diego under the supervision of Eric Lauga and a postdoctoral fellowship with L. Gary Leal and Todd M. Squires at the University of California Santa Barbara.

Wednesday, October 9, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Kanso

Turbulent Flow Interactions with Complex Topography: From Permeable Walls to Barchan Dunes

Kenneth T. Christensen

Viola D. Hank Professor
Department of Aerospace & Mechanical Engineering
Department of Civil & Environmental Engineering & Earth Sciences
University of Notre Dame
Notre Dame, IN

This talk will highlight on-going studies of interactions between turbulence and complex topography. The latter is encountered in a multitude of engineering and natural flow scenarios and can lead to significant modifications of the overlying flow as well as unique coupling between the flow and the topography. This talk will focus on two specific flow-topography situations that are often observed in nature: flow overlying a permeable boundary (like a gravel river bed, for example) and flow over barchan dunes – crescent-shaped bedforms that form in aeolian and sub-aqueous environments with a limited sediment supply and relatively unidirectional flow. In the study of permeable boundaries, models were formed from cubic and body-centered arrangements of spheres wherein the surface exposed to the flow was either smooth or rough (the former meaning no spheres protruded into the flow, though the boundary was still permeable). For the barchan-dune work, fixed-bed barchan models were developed to reflect morphologies identified in nature when these dunes come in close proximity. In both cases, the models were fabricated in acrylic and deployed in a refractive-index-matched (RIM) flow facility whose working fluid has the same optical refractive index as the models. The optically-unimpeded access afforded by the RIM methodology, coupled with particle-image velocimetry measurements, provides unique views of the rich flow dynamics associated with both of these turbulence–complex topography interactions.

Kenneth T. Christensen is the Viola D. Hank Professor at the University of Notre Dame, with a joint appointment in the Departments of Aerospace & Mechanical Engineering and Civil & Environmental Engineering & Earth Sciences. He also presently serves as the Department Chair of Aerospace & Mechanical Engineering and recently completed a two-year term as a Provost Fellow at Notre Dame. He joined the ND faculty after ten-plus years on the faculty at the University of Illinois at Urbana-Champaign. Christensen directs a research group that pursues experimental studies of turbulence, geophysical flows and microfluidics and is a WPI Principal Investigator in the Carbon Dioxide Storage Division of the International Institute for Carbon-Neutral Energy Research (I2CNER) based at Kyushu University in Japan. He is a Fellow of AAAS, APS and ASME, an Associate Fellow of AIAA and has received the AFOSR Young Investigator Award (2006), the NSF CAREER Award (2007), the Francois Frenkiel Award for Fluid Mechanics from APS-DFD (2011) and the Gustus Larson Memorial Award from ASME (2016).

Wednesday, October 16, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Pantano

Neural-Inspired Sparse Sensing and Control for Agile Flight

Bing Brunton

Professor

Winged insects perform remarkable aerial feats in uncertain, complex fluid environments. This ability is enabled by sensation of mechanical forces to inform rapid corrections in body orientation. Curiously, machanoreceptor neurons do not faithfully report forces; instead, they are activated by specific time-histories of forcing. In a set of results that combine biomechanical modeling, sparse sensing mathematical theory, and neural encoding, we find that, far from being a bug, neural encoding by biological sensors is a feature that acts as built-in temporal filtering superbly matched to detect body rotation. Indeed, this encoding further enables surprisingly efficient detection using only a small handful of neurons at key locations. Our ongoing and future work explores biological strategies to use smart data as an alternative to big data, including understanding and designing neural-inspired sensors and actuator to achieve hyperefficient sensing and control for agile flight.

Thursday, October 17, 2019
11:00 AM
Biegler Hall, Room 112 (BHE 112)

host: Kanso

Aortic Stiffness, Pressure and Flow Pulsatility and Cardiovascular Disease

Gary F. Mitchell, MD

President
Cardiovascular Engineering, Inc.
Norwood, MA

The aorta provides a critical buffer between the heart and peripheral organs. Juxtaposition of the highly compliant aorta with stiff conduit arteries creates impedance mismatch and local wave reflection that limits transmission of potentially harmful pulsatile energy into the fragile microcirculation. Aortic wall stiffness increases progressively throughout the human lifespan. However, from young adulthood through midlife, pressure pulsatility actually falls as a result of aortic remodeling to a larger lumen area, which reduces characteristic impedance of the aorta. From midlife onward, aortic wall stiffening accelerates and characteristic impedance and pressure pulsatility increase markedly and contribute to the epidemic of wide pulse pressure (isolated systolic) hypertension. After this midlife transition, stiffness of the aorta exceeds that of the muscular arteries, leading to impedance matching and diminished wave reflection, which increases transmission of excessive pressure and flow pulsatility into the microcirculation. Excessive pulsatility in the microcirculation, particularly in high flow organs such as the brain and kidneys, causes microvascular damage, remodeling and dysfunction, leading target organ damage. In addition, aortic stiffening adds to load on the heart and may interfere with diastolic function, adding to risk for heart failure in late life. Successful interruption of the unfavorable aortic stiffening cascade likely will require early intervention and lifelong prevention, although attempts to reverse the process have been inadequately examined at present.

Gary F. Mitchell is a cardiologist and internationally acknowledged leader in the field of vascular stiffness and pulsatile hemodynamics. He received his medical degree from Washington University in St. Louis and completed his training in Medicine and Cardiology at Brigham and Women’s Hospital in Boston, where he served as a staff cardiologist until 1998. He left the Brigham in 1998 to become founder and president of Cardiovascular Engineering, Inc., which is an NIH-funded small business that designs and develops innovative devices that measure arterial stiffness. Dr. Mitchell and colleagues have used those devices to examine genetic and environmental correlates of arterial stiffness and the role that arterial stiffness plays in the pathogenesis of hypertension and target organ damage. He joined the Framingham Heart Study as a Framingham Investigator in 1999 and became a collaborator on the AGES-Reykjavik study in 2006 and the Jackson Heart Study in 2010. Using devices designed and built by Cardiovascular Engineering, Dr. Mitchell has performed detailed assessments of arterial stiffness and pulsatile hemodynamics in more than 20,000 research participants, including participants in all 3 generations of the Framingham Heart Study as well as participants in the AGES-Reykjavik study, the REFINE study and the Jackson Heart Study.

Wednesday, October 23, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Pahlevan

From Beating Hearts to Buzzing Wings: Flow Physics and Computation at the Intersection of Mechanics and Bioengineering

Rajat Mittal

Professor
Department of Mechanical Engineering
Johns Hopkins University
Baltimore, Maryland

The unceasing growth in computational power and the development of new software tools and numerical algorithms is opening up exciting areas of research, discovery & translation in mechanics and biomedical engineering. Consider the mammalian heart, which has been sculpted by millions of years of evolution into a flow pump par excellence. During the typical lifetime of a human, the heart will beat over three billion times and pump enough blood to fill over sixty Olympic-sized swimming pools. Each of these billions of cardiac cycles is itself a manifestation of a complex and elegant interplay between several distinct physical domains including electrophysiology, muscle mechanics, hemodynamics, flow-induced valves dynamics, acoustics, and biochemistry. Computational models provide the ability to explore such multi-physics problems with unprecedented fidelity and precision. In my talk, I will describe several projects that demonstrate the application of powerful computational tools to problems ranging from chemo-fluidics of clot formation to fluid-structure interaction in prosthetic heart valves. The talk will culminate with a brief discussion on a new project where we are using computational aeroacoustics to analyze wing-tone based communication in mosquitoes.

Rajat Mittal is Professor of Mechanical Engineering at the Johns Hopkins University (JHU) with a secondary appointment in the School of Medicine. He received the B. Tech. degree from the Indian Institute of Technology at Kanpur in 1989, and the Ph.D. degree in Applied Mechanics from The University of Illinois at Urbana-Champaign, in 1995. His research interests include fluid mechanics, computing, biomedical engineering, biofluids and flow control. He is the recipient of the 1996 Francois Frenkiel Award from the Division of Fluid Dynamics of the American Physical Society, and the 2006 Lewis Moody Award from the American Society of Mechanical Engineers. He is a Fellow of American Society of Mechanical Engineers and the American Physical Society, and an Associate Fellow of the American Institute of Aeronautics and Astronautics. He is associate editor of the Journal of Computational Physics, Frontiers of Computational Physiology and Medicine and the Journal of Experimental Biology.

Wednesday, October 30, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Pahlevan

Fabrication, Mechanical Characterization, and Modeling of 3D Architected Materials upon Static and Dynamic Loading

Carlos M. Portela

Postdoctoral Scholar
Department of Mechanical and Civil Engineering
California Institute of Technology
Pasadena, CA

Architected materials have been ubiquitous in nature, enabling unique properties that are unachievable by monolithic, homogeneous materials. Inspired by natural processes, man-made three-dimensional (3D) architected materials have been reported to enable novel mechanical properties such as high stiffness-to-density ratios or extreme resilience, increasingly so when nanoscale size effects are present. However, most architected materials have relied on advanced additive manufacturing techniques that are not yet scalable and yield small sample sizes. Additionally, most of these nano-and micro-architected materials have only been studied in the static regime, leaving the dynamic parameter space unexplored.

In this talk, we discuss advances in our understanding of architected materials by: (i) proposing numerical and theoretical tools that predict the behavior of architected materials with non-ideal geometries, (ii) presenting a pathway for scalable fabrication of tunable nano-architected materials, and (iii) exploring the response of nano-and micro-architected materials under three types of dynamic loading. We first explore the mechanics of lattice architectures with features at the micro-and millimeter scales, and discuss the effect of nodes (i.e., junctions) to obtain more accurate computational and theoretical predictive tools. Going beyond lattices, we propose alternative node-less geometries that exhibit extreme mechanical resilience at the nanoscale, and we harness self-assembly processes to demonstrate a pathway to fabricate them in cubic-centimeter volumes while maintaining nanoscale resolution. Lastly, we venture into the dynamic regime by designing, fabricating, and testing micro-architected materials that exhibit vibrational band gaps in the MHz regime as well as nano-architected materials with extreme energy absorption upon microparticle supersonic impact.

Carlos M. Portela is a postdoctoral scholar in Applied Physics and Materials Science at the California Institute of Technology. He received his Ph.D in mechanical engineering at Caltech, working with Professors Julia R. Greer and Dennis M. Kochmann (ETH Züaut;urich) on the fabrication, testing, and modeling of 3D architected materials with nanometer-to centimeter-scale dimensions. He obtained his M.S. in Mechanical Engineering at Caltech in 2016, and a B.S. in Aerospace Engineering and a B.A. in Physics from the University of Southern California (USC). Carlos is the recipient of the 2019 Gold Paper Award from the Society of Engineering Science (SES), the Centennial Award for the Best Thesis in Mechanical and Civil Engineering at Caltech, and his work has been featured in The National Nanotechnology Initiative Supplement to the President’s 2020 Budget (National Science and Technology Council). His research interests lie at the intersection of materials science and mechanical engineering, striving to provide pathways for architected materials to aid in tackling multi-disciplinary challenges.

Wednesday, November 6, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Pahlevan & Shiflett

Two Problems in Wall-Bounded Flow: Fluid Energy Extraction in Wind Farms, and Surfactant Effects in Superhydrophobic Drag Reduction

Paolo Luzzatto-Fegiz

Assistant Professor
Department of Mechanical Engineering
University of California, Santa Barbara
Santa Barbara, CA

In this talk, we consider two fluid problems directly linked to decarbonization efforts. In the first part, we investigate fundamental limits to the performance of large wind farms. Since wind turbines are often deployed in arrays of hundreds of units, wake interactions can lead to drastic losses in power output. Remarkably, while the theoretical “Betz” maximum has long been established for the output of a single turbine, no corresponding theory appears to exist for a generic, large-scale energy extraction system. We develop a model for an array of energy-extracting devices of arbitrary design and layout, first focusing on the fully-developed regime, which is relevant for large wind farms. We validate our model against data from field measurements, experiments and simulations. By defining a suitable ideal limit, we establish an upper bound on the performance of a large wind farm. This is an order of magnitude larger than the output of existing arrays, thus supporting the notion that large performance improvements may be possible.

In the second part of this talk, we examine flow past superhydrophobic surfaces (SHS). These coatings have long promised large drag reductions; however, experiments have provided inconsistent results, with many textures yielding little or no benefit. By performing surfactant-laden simulations and unsteadily-driven experiments, we demonstrate that surfactant-induced Marangoni stresses can be to blame. We find that extremely low surfactant concentrations, unavoidable in practice, can drastically increase drag, at least in laminar flows. To obtain accurate drag predictions on SHS, one must therefore solve the mass, momentum, bulk surfactant and interfacial surfactant conservation equations, which is not feasible in most applications. To address this issue, we propose a theory that captures how the near-surface dynamics depend on the seven dimensionless groups for surfactant. We validate our theory extensively in 2D, and describe progress toward 3D and turbulent models. Our theory significantly improves predictions relative to a surfactant-free one, which can otherwise overestimate drag reduction by several orders of magnitude.

Here are links to papers/resources that form the basis for this talk:

• https://doi.org/10.1103/PhysRevFluids.3.093802
• https://doi.org/10.1073/pnas.1702469114
• https://doi.org/10.1103/PhysRevFluids.3.100507
• https://doi.org/10.1103/APS.DFD.2017.GFM.V0098
• https://arxiv.org/abs/1904.01194

Paolo Luzzatto-Fegiz graduated with a BEng in Aerospace Engineering from the University of Southampton, where he received the Royal Aeronautical Society Prize for highest first-class degree and the Graham Prize for best experimental project in the School of Engineering Sciences. After a summer working with the ATLAS Magnet Team at CERN, he completed an MSc in Applied Mathematics at Imperial College, and an MS and PhD in Aerospace Engineering at Cornell University. His doctoral work received the Acrivos Award of the American Physical Society for outstanding dissertation in Fluid Dynamics at a U.S. university. He was awarded a Devonshire Postdoctoral Scholarship from the Woods Hole Oceanographic Institution, as well as a Junior Research Fellowship from Churchill College, Cambridge. He is currently an Assistant Professor in Mechanical Engineering at UCSB, where he has received the Northrop Grumman Teaching Award and a Gallery of Fluid Motion Award from APS-DFD. He co-invented a salinity sensor for oceanography that has been adopted by 20 institutions, and led the first microgravity experiment from NSF CBET, which successfully returned in January 2019 from the International Space Station.

Wednesday, November 13, 2019
3:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Refreshments will be served at 3:15 pm.

host: Luhar

Bermejo Moreno