Special Division/Lefschetz Center for Dynamical Systems Colloquium
Massachusetts Institute of Technology (MIT) | |
Abstract: Flow separation is a major cause of performance loss in engineering devices such as diffusers, airfoils and jet engines. In a landmark 1904 paper on boundary layers, L. Prandtl derived a criterion for flow separation in steady two-dimensional incompressible flows. Despite widespread effort, however, no unsteady or three-dimensional extension of Prandtl's criterion has emerged in the fluid dynamics literature.
In this talk, I discuss a recent extension of Prandtl's criterion using invariant manifold concepts from dynamical systems. I show numerical and experimental results confirming the extended separation criterion for unsteady and three-dimensional flows. I also discuss applications of the new criterion to flow control.
Brown University
Joint Materials/Solid Mechanics Seminar Series
Department of Materials Science and Engineering, Northwestern University | |
Abstract: Two examples involving the adhesion of thin, elastomeric films will be described. In the first example, we study the processes that control the transfer of continuous polymer film from an elastomeric substrate to a rigid indenter. The process is governed by cavitation at the film substrate interface at regions where the local hydrostatic stress exceeds the modulus of the elastomeric substrate. In the second example, we describe the development of a sensitive adhesion test based on the contact of a free standing membrane with a rigid surface. The test is designed to quantify the adhesion energy between surfaces that are interacting across water. Additional information about the nature of interfacial contact in model aqueous systems is obtained by using the quartz crystal microbalance as a contact sensor.
Center for Fluid Mechanics
And The Fluids, Thermal And Chemical Processes Group
Of
The Division of Engineering
Seminar Series
Team for Advanced Flow Simulation and Modeling (T*AFSM) Rice University, Houston, TX | |
Abstract: We describe the space-time finite element techniques we developed for computation of fluid-structure interaction (FSI) problems. Among these techniques are the Deforming-Spatial-Domain/Stabilized Space-Time (DSD/SST) formulation and the Enhanced-Discretization Space-Time Technique (EDSTT). Also among these techniques are the block-iterative, quasi-direct and direct coupling methods for the solution of the fully-discretized, coupled fluid and structural mechanics equations. The EDSTT was developed for the purpose of being able to, in the context of a space-time formulation, enhance the time-discretization in regions of the fluid domain requiring smaller time steps. Such requirements are often encountered in time-accurate computations of fluid-structure interactions, where the time-step size required by the structural dynamics part is smaller, and carrying out the entire computation with that time-step size would be too inefficient for the fluid dynamics part. A good coupling method for the solution of the fully-discretized equations becomes essential in computation of FSI problems where the structure is light. We present numerical examples where the fluid is governed by the Navier-Stokes equations of incompressible flows and the structure is governed by the membrane and cable equations. We also present some FSI test computations to show how the EDSTT works. Overall, we demonstrate that the techniques we have developed have increased the scope and accuracy of the methods used in FSI computations.
**Center for Computational Molecular Biology Seminar Series**
Abstract: Chromatin immunoprecipitation coupled with DNA microarray analysis (ChIP-chip) has evolved as a popular technique to study the genome level in vivo binding of transcription factors and chromatin remodeling and modifying proteins. Recently genome tiled microarrays have been developed that allow biologists to conduct unbiased genome-wide ChIP-chip experiments in mammalian genomes. However, they also generate massive amounts of data, and pose challenges for the development of effective analysis algorithms.
I will present an approach to analyze ChIP-chip on Affymetrix tiled arrays which are the cheapest, yet the most difficult to analyze. The low-level analysis pools data from multiple samples to estimate probe behavior, then uses a hidden Markov model to detect genomic regions bound by the transcription factor. The high-level analysis finds common sequence patterns from regions enriched by the transcription factor ChIP-chip, thus characterizes the binding of the transcription factor and its cooperative binding partners. I will present our analysis of p53 and estrogen receptor ChIP-chip on chr21/22 tiled arrays.
Graduate Student Pizza Seminar
Code Group Seminar
Applied Mathematics and Neuroscience | |
Synfire Compositional Systems: Complexity and Cognition | |
Abstract: This thesis attempts a synthesis of a number of ideas distributed across disciplines, including experimental neuroscience, modeling at the cell level, and modeling at the systems level. The picture that emerges is original and inspiring. It also is a bold endeavor, considering that it rests on a theoretical concept Abeles's synfire chain for which experimental evidence is still scarce. The author proposes that the regulation of activity in a synfire-superposition system (using conductance-based model neurons) may be achieved dynamically through the effect of background input on the membrane potential. High (balanced excitatory/inhibitory) input causes the opening of large numbers of synaptic channels, resulting in a much leakier membrane. The author's key insight is that this sharp reduction of the membrane time constant results in a more stringent synchrony requirement for synfire activity to propagate, and for synfire waves to cross-synchronize and cross-activate; this makes synfire connectivity at the system level more or less /effective/, depending on the level of background input. This effect is then exploited in a highly creative manner in a number of complex synfire-based systems. In spite of being highly speculative, this story to use the author's term has something compelling to it.
For future meeting schedule visit:
http://charlotte.neuro.brown.edu/~britt/popcode.html
PDE Seminar
Abstract: The shallow water or hydraulic limit for wave propagation and breaking is well understood. At shocks, conservation of mass and momentum completely define the hydraulic jump, and the energy equation gives, a posteriori, the rate at which "internal" energy (mostly in the form of small scale turbulence) is generated. For the case of two-layer shallow water, even the evolution of smooth solutions was not completely understood since there is the possibility for the system to be elliptic, a remnant of the Kelvin-Helmholtz instability. We first show that these flows are nonlinearly stable: smooth solutions never cross into the elliptic domain. Then, we consider shocks at the interface of miscible fluids - a problem of geophysical importance in the atmosphere and ocean. We require an additional postulate that would yield the mixing rate at the shock: one can imagine that the energy dissipated at the shock can now flow both into small scale turbulence and into mixing the fluid, but the partition between these sinks of macroscopic energy is unknown. We discuss two possibilities for deriving the additional constraint on the problem: kinematics and entropy maximization. We show that these are in fact equivalent and yield upper bounds on the mixing rates.
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