Colloquium to be attented

工学院系列报告
发布时间:2007-5-21 13:55:17　　点击次数:14
演 讲 者：Mohamed Gad-el-Hak, Virginia Commonwealth University
Richmond, Virginia, U.S.A.
主办单位：北京大学工学院
地 点：北京大学力学大楼434会议室

第一讲：2007年5月24日(周四)下午3：00—4：00
题 目： FLOW PHYSICS IN MICRO AND NANODEVICES
主持人：工学院吴介之教授

第二讲：2007年5月25日(周五)下午3：00—4：00
题 目： THE TAMING OF THE SHREW: WHY IS IT SO DIFFICULT TO CONTROL TURBULENCE?
主持人：工学院院长陈十一教授

第三讲：2007年5月25日(周五)下午4：00—5：00
题 目： THE ART AND SCIENCE OF LARGE-SCALE DISASTERS
主持人：工学院院长陈十一教授

BIOGRAPHICAL SKETCHMohamed Gad-el-Hak received his B.Sc. (summa cum laude) in mechanical engineering from Ain Shams University in 1966 and his Ph.D. in fluid mechanics from the Johns Hopkins University in 1973. Gad-el-Hak has since taught and conducted research at the University of Southern California, University of Virginia, University of Notre Dame, Institut National Polytechnique de Grenoble, Université de Poitiers, Friedrich-Alexander-Universität Erlangen-Nürnberg, Technische Universität München and Technische Universität Berlin, and has lectured extensively at seminars in the United States and overseas. Dr. Gad-el-Hak is currently the Inez Caudill Eminent Professor of Biomedical Engineering and Chair of Mechanical Engineering at Virginia Commonwealth University.Dr. Gad-el-Hak has published over 460 articles, authored/edited 18 books and conference proceedings, and presented 255 invited lectures. He is the author of the book "Flow Control: Passive, Active, and Reactive Flow Management," and editor of the books "Frontiers in Experimental Fluid Mechanics," "Advances in Fluid Mechanics Measurements," "Flow Control: Fundamentals and Practices," "The MEMS Handbook" (first and second editions), "Transition and Turbulence Control," and "Large-Scale Disasters: Prediction, Control and Mitigation."Professor Gad-el-Hak is a fellow of the American Physical Society, the American Society of Mechanical Engineers, and the American Academy of Mechanics.In 1998, Professor Gad-el-Hak was named the Fourteenth ASME Freeman Scholar. In 1999, Gad-el-Hak was awarded the prestigious Alexander von Humboldt Prize, Germany's highest research award for senior U.S. scientists and scholars in all disciplines. In 2002, Gad-el-Hak was named ASME Distinguished Lecturer, as well as inducted into the Johns Hopkins University Society of Scholars.

1. FLOW PHYSICS IN MICRO AND NANODEVICES

Interest in microelectromechanical systems (MEMS) has experienced explosive growth during the past few years. Such small devices typically have characteristic size ranging from 1 mm down to 1 micron, and may include sensors, actuators, motors, pumps, turbines, gears, ducts and valves. Microdevices often involve mass, momentum and energy transport. Modeling gas and liquid flows through MEMS may necessitate including slip, rarefaction, compressibility, intermolecular forces and other unconventional effects. In this presentation, I shall provide a methodical approach to flow modeling for a broad variety of microdevices. The continuum-based Navier-Stokes equations-with either the traditional no-slip or slip-flow boundary conditions-work only for a limited range of Knudsen numbers above which alternative models must be sought. These include molecular dynamics (MD), Boltzmann equation, Direct Simulation Monte Carlo (DSMC), and other deterministic/probabilistic molecular models. The present talk will broadly survey available methodologies to model and compute transport phenomena within microdevices.

2. THE TAMING OF THE SHREW: WHY IS IT SO DIFFICULT TO CONTROL TURBULENCE?

Considering the extreme complexity of the turbulence problem in general and the unattainability of first-principles analytical solutions in particular, it is not surprising that controlling a turbulent flow remains a challenging task, mired in empiricism and unfulfilled promises and aspirations. Brute force suppression, or taming, of turbulence via active control strategies is always possible, but the penalty for doing so often exceeds any potential savings. The artifice is to achieve a desired effect with minimum energy expenditure. Spurred by the recent developments in chaos control, microfabrication and neural networks, efficient reactive control of turbulent flows, where the control input is optimally adjusted based on feedforward or feedback measurements, is now in the realm of the possible for future practical devices. But regardless of how the problem is approached, combating turbulence is always as arduous as the taming of the shrew. The former task will be emphasized during the oral presentation, but for this abstract we reflect on a short verse from the latter.

From William Shakespeare's The Taming of the Shrew.
Curtis (Petruchio's servant, in charge of his country house): Is she so hot a shrew as she's reported?
Grumio (Petruchio's personal lackey): She was, good Curtis, before this frost. But thou know'st winter tames man, woman, and beast; for it hath tamed my old master, and my new mistress, and myself, fellow Curtis.


3. THE ART AND SCIENCE OF LARGE-SCALE DISASTERS

Large-scale disasters adversely affect considerable number of people, devastate sizable geographical area, and tax the resources of local communities and central governments. Disasters can naturally occur, but humans can also cause their share of devastation. There is also the possibility of anthropogenic calamity; human's actions causing a natural disaster to become more damaging than it would otherwise. The art and science of large-scale disasters aim at better prepare scientists, engineers, first responders, and above all politicians to deal with manmade and natural disasters. The last annus horribilis in particular has shown the importance of being prepared for large-scale catastrophes, and how the world can get together to help alleviate the resulting pain and suffering. In this talk, both the art and science of predicting, preventing and mitigating natural and manmade disasters are broadly discussed. The laws of nature govern the evolution of any disaster. In some cases, as for example weather-related disasters, those first-principles laws could be written in the form of field equations, but exact solutions of these often nonlinear differential equations are impossible to obtain particularly for turbulent flows, and heuristic models together with intensive use of supercomputers are necessary to proceed. In other cases, as for example earthquakes, the precise laws are not even known and prediction becomes more or less a black art. Management of any type of disaster is more art than science. Nevertheless, much can be done to alleviate the resulting pain and suffering.