MEAM Seminar Series Spring 2017
For Fall 2016 Seminars, click here.
Seminars are held on Tuesday mornings beginning at 10:45 am in Wu and Chen Auditorium, in Levine Hall (unless otherwise noted).
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Xiang I. A. Yang, Postdoctoral Researcher, Stanford University
"The Fluctuating Wall Stress in Wall-Bounded Turbulence"
Measuring, simulating and modeling of the fluctuating wall shear stress at Reynolds numbers of practical relevance are of interest to many fields including sand storm modeling, LES (large-eddy simulation) wall modeling, SWBLI (shock wave boundary layer interaction) modeling etc. In order to take direct measurement of the wall stress, the wall-turbulence community is making a collective effort, building a long pipe at Bologna. We, on the other hand, focus on numerical simulating and theoretical modeling of the fluctuating wall stress. Because DNS limits the Reynolds number, to investigate the behavior of the fluctuating wall stress at high Reynolds numbers, we use LES. With the integral wall model modeling the near wall turbulence, the probability distribution function of the wall stress from the wall-modeled large-eddy simulation agrees well with that in a filtered DNS. The sub-grid wall stress, resulting from the unresolved near wall fluid motions, is then to be modeled. Admitting the fact that the wall stress is the end result of the momentum cascading process, modeling the stress that results from the small-scale near wall turbulence requires a model for the momentum cascade. In this context, the hierarchical random additive model is relevant. Assumptions and predictions of this analytical model are discussed. We then present empirical evidence of the scaling laws that are permitted by this model. Those studies focus on canonical boundary layer flows with no roughness nor wall heat transfer. As real-world engineering often has to tackle rough walls, last, we present a systematic LES study of cuboidal roughness roughened turbulent boundary layers and an analytical rough wall model for the flow sheltering in the roughness layer.
Xiang Yang is a postdoctoral researcher at Center for Turbulence Research, Stanford. Yang received his Ph.D. in mechanical engineering in 2016 from the Johns Hopkins University under the mentorship of Dr. Meneveau and Dr. Mittal. His doctoral work focuses on LES wall modeling, rough wall modeling and theoretical modeling of turbulent boundary layers. His current work at CTR is on shock-wave/transitional-boundary-layer interaction.
Nathan Ip, Ph.D. Candidate
Advisor: Kevin Turner
"Experimental Investigation of Polymer Adhesion Mechanics Using a Blister Contact Test"
3:00 p.m., Room 216, Moore Building
The adhesion of thin layers of soft polymers is important in many
applications, such as tapes, microtransfer printing, and bioinspired
adhesives. Traditional adhesion tests based on probe contacts are not
suitable for characterizing thin layers and common separation-based
specimens, such as the peel test, have well-known limitations. The
blister contact test (BCT) was developed in this dissertation to
overcome the limitations of current methods and was used to investigate
the adhesion and separation of several technologically relevant adhesive
systems. In the BCT, a thin sheet was elastically deformed into
adhesive contact with a reference substrate and the contact area was
optically imaged. Modulated pressure was applied to generate both
advancing and receding adhesive contact. Digital image correlation was
used to measure the displacements of the specimen. The strain energy
release rate at the interface was determined from the measured contact
radius, applied pressure, system geometry, and elastic properties of the
specimen using a mechanics model. An analytical mechanics model based
on von Kármán plate theory was developed and used for analysis of the
BCT data. Finite element analysis was used to validate and identify the
range of applicability of the analytical model.
The BCT was used to investigate the adhesion and separation behaviors of three different polymer adhesive systems. First, experiments between a silicone elastomer (polydimethylsiloxane – PDMS) and a stiff substrate were performed to investigate rate effects in adhesion and separation. For the first time, the rate dependence during advancing contact was characterized. Second, the effect of acid-base interactions on performance of pressure sensitive adhesives (PSAs) was examined via a series of BCTs in which adhesion between different formulations of adhesives and multiple substrates was investigated. Viscoelastic contributions to PSA adhesion were also studied. Finally, the effect of layer thickness on rate dependence was investigated through experiments between polyethylene terephthalate (PET) sheets and PDMS films of different thicknesses. The work in this dissertation demonstrates the flexibility and capability of the BCT as a method to characterize adhesion of flat polymer sheets and provides new understanding of several types of polymer adhesive contacts.
Celia Reina, William K. Gemmill Term Assistant Professor, Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania
"Multiscale Modeling and Simulation: Some Challenges and New Perspectives"
The design and optimization of the next generation of materials and applications strongly hinge on our understanding of the processing-microstructure-performance relations; and these, in turn, result from the collective behavior of materials’ features at multiple length and time scales. Although the modeling and simulation techniques are now well-developed at each individual scale (quantum, atomistic, mesoscale and continuum), there remain long-recognized grand challenges that limit the quantitative and predictive capability of multiscale modeling and simulation tools. In this talk we will discuss three of these challenges and provide solution strategies in the context of specific applications. These comprise (i) the homogenization of the mechanical response of materials in the absence of a complete separation of length and/or time scales, for the simulation of metamaterials with exotic dynamic properties; (ii) the collective behavior of materials’ defects, for the understanding of the kinematics of large elasto-plastic deformations; and (iii) the upscaling of non-equilibrium material behavior for the modeling of phase change materials.
Celia Reina is the William K. Gemmill Term Assistant Professor in Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. She joined in 2014 after holding the Lawrence Postdoctoral Fellowship at Lawrence Livermore National Laboratory and the HCM Postdoctoral Fellowship at the Hausdorff Center of Mathematics in Bonn, Germany. Dr. Reina received her PhD from the California Institute of Technology in Aeronautics in 2011, with Prof. Michael Ortiz, following a B.S. in Mechanical Engineering from the University of Seville in Spain, and a Master in Structural Dynamics from Ecole Centrale Paris in France.
Jay D. Humphrey, John C. Malone Professor and Chair, Department of Biomedical Engineering, Yale University
“Mechanics of Arteries in Health and Disease”
Arteries are complex structures, consisting of diverse load bearing extracellular matrix constituents and cells. Understanding the mechanics of these vessels is doubly important. Stress analyses can aid in the understanding of catastrophic failure (e.g., dissection and rupture) and similarly mechano-biological responses by cells (i.e., changes in gene expression in response to changes in stress). In this talk, we will survey different applications of mechanics (based on nonlinear continuum mechanics) to understand better the biomechanics of arteries in health and disease, including the progression of aneurysms and development of dissections.
Jay Humphrey is the John C. Malone Professor and Chair of Biomedical Engineering at Yale University. He received his Ph.D. in Applied Mechanics and Bioengineering from the Georgia Institute of Technology and served as a post-doctoral researcher in cardiovascular biomechanics at the Johns Hopkins University. His research focuses on the design and construction of novel computer-controlled multiaxial test systems, testing and analysis of tissue engineered constructs, measurement of vascular mechanical properties, non-linear constitutive formulations, measurement of in vivo hemodynamics, and computational biomechanics (mainly finite elements). His lab has formulated a unique “Constrained Mixture Theory” for soft tissue growth and remodeling (G&R) that has provided significant insight into the biomechanics of arterial adaptations to altered hemodynamics as well as aneurysmal enlargement, vein graft maladaptation, and tissue engineered vascular graft development. Other work involved developing both a finite element model of the effects of pooled glycosaminoglycans within the aortic wall, a histopathological characteristic unique to thoracic aortic aneurysms and dissections, and a fluid-solid-interaction model of the aortic tree that enables hypothesis generation and testing as well as experimental design. Given that intraluminal and intramural thrombosis play important roles in many vascular conditions, he has also developed growth and remodeling models of thrombus initiation, progression, and remodeling. Finally, he has recently published a first of its kind comparative biomechanical phenotyping of common carotid arteries from seven different mouse models that suggested that mural cells attempt to maintain material stiffness constant.
Dr. Humphrey has won numerous grants from the National Heart, Lung, and Blood Institute of the National Institutes of Health, among other granting authorities. He is author of over 245 published articles, and two leading textbooks in the field (one aimed at the graduate level and one undergraduate). He has served on several journal editorial boards and was the founding co-Editor in Chief of the journal Biomechanics and Modeling in Mechanobiology. Among many other distinguished awards, he is the recipient of the ASME H.R. Lissner Medal, recognizing outstanding achievements in bioengineering.
April 18: ELSEVIER DISTINGUISHED LECTURE IN MECHANICS
Howard A. Stone, Donald R. Dixon '69 and Elizabeth W. Dixon Professor, Department of Mechanical and Aerospace Engineering, Princeton University
"Seeking Simplicity in the Flows of Complex Fluids"
Fluid mechanics is a discipline with rich phenomena, spanning a wide range of laminar and turbulent flows, instabilities, and applications in industry, nature, and biology and medicine. I will provide examples of our work highlighting (i) new features of classical instabilities triggered by changes in geometry, (ii) multiphase flows relevant to the design of liquid-infused substrates exhibiting effective slip, and, if there is a time, (iii) unexpected dynamics in flow at a T-junction.
Howard A. Stone is the Donald R. Dixon ’69 and Elizabeth W. Dixon Professor in Mechanical and Aerospace Engineering at Princeton University. Stone is a fluid dynamicist who uses experiments, theory and numerical simulations to study transport problems at the intersections of engineering, biology, physics and applied mathematics. Stone received the Bachelor of Science degree in Chemical Engineering from the UC Davis in 1982 and the PhD in Chemical Engineering from Caltech in 1988. In 1989 Stone joined the faculty of the School of Engineering and Applied Sciences at Harvard University, where he eventually became the Vicky Joseph Professor of Engineering and Applied Mathematics. In 2000 he was named a Harvard College Professor for his contributions to undergraduate education. In July 2009 Stone moved to Princeton University. He is a Fellow of the APS and is past Chair of the Division of Fluid Dynamics. In 2008 he was the first recipient of the G.K. Batchelor Prize in Fluid Dynamics and in 2016 he received the APS Fluid Dynamics Prize. He was elected to the National Academy of Engineering in 2009, the American Academy of Arts and Sciences in 2011, and the National Academy of Sciences in 2014.