Abstract: This presentation will introduce two new ideas for laser diagnostics applicable to combustion and propulsion and two new ideas for high-speed imaging of combustion phenomena in a shock tube.  The first laser diagnostic to be discussed is Spectrally-Resolved Fluorescence, in which a narrow-linewidth wavelength-tunable laser source is rapidly scanned over one or more absorption transitions, allowing collection of a spatially resolved laser-induced fluorescence signal that reflects the strength and shape of the absorption feature.  This diagnostic can potentially provide accurate determinations of temperature, pressure, flow velocity and the concentration of the absorbing species.  The example to be presented will be of OH excited and detected in the ultraviolet (UV).  The second new laser diagnostic utilizes a narrow-linewidth infrared laser source that can be scanned rapidly over a relatively wide wavelength range, thereby enabling acquisition of spectral absorption cross-sections over a full rovibrational absorption band of combustion-relevant species in a short time (a few milliseconds), compatible with the test times available in reflected shock wave experiments.   The goal of the experiments is to generate a unique data base for cross-sections over a wide range of temperature, not feasible in a heated static cell.  Two other experiments will be presented, both based on imaging of shock-heated gases. In the first experiment, high-speed imaging is used to visualize and characterize aspects of inhomogeneous ignition that can occur in reflected shock wave experiments of hydrocarbon fuel ignition; such inhomogeneous ignition is undesirable and can contaminate data sets aimed at providing high-quality information on ignition delay times.  As a second example of imaging in a shock tube, a new experiment will be introduced that measures the burning velocity of a flame produced by laser ignition of combustible gases behind a reflected shock wave.  The objective is to enable flame speed measurements at elevated temperatures not accessible with conventional flame speed techniques.  Such conventional methods are limited by the partial reaction of the mixture that occur during the lengthy period of preparing reactive mixtures, whereas with a shock tube experiment the time interval between shock wave heating and ignition can be adjusted to be quite small.  Results obtained in heptane-air flames provide clear evidence of cool flame effects not previously seen in flame speed experiments.  In current work, the observation of flame speed from the time-resolved position of chemiluminescent emission is also being augmented by various laser absorption diagnostics to additionally characterize the burned gases behind the flame.

Bio Sketch: Professor Hanson received his bachelor’s degree from Oregon State University in mechanical engineering and his doctoral degree from Stanford University where he currently holds the Woodard Chair in Mechanical Engineering.  He has been an international leader in the development of laser-based diagnostic methods for combustion and propulsion, and in the development of shock tube methods for accurate determination of chemical reaction rate parameters needed for modeling combustion and propulsion systems, and together with his students he has made several pioneering contributions that have advanced the pace of propulsion research and development worldwide.  He is a Fellow of AIAA, ASME and OSA, a member of the National Academy of Engineering, and a recipient of gold medal awards from the Combustion Institute, the Institute for Dynamics of Explosions and Reactive Systems, and multiple gold medals from the AIAA. He has published over one thousand papers and advised over 100 doctoral students, including 31 now holding faculty appointments around the world.

Abstract: Advanced computational modeling, high performance computing technology, and extensive knowledge of simulation form a strong and unique foundation of research, development and engineering at Sandia National Laboratories that enable the Lab to meet its commitment of ensuring the national security of the United States.  Computational models are utilized extensively to predict the complex behavior of materials in multiphysics environments across a wide range of length and time scales, and analysts run simulations routinely to evaluate the performance and reliability of complicated engineering systems designed for national security applications.  In the past three decades, capabilities of simulation tools and models were advanced significantly, and numerous scientific questions and engineering challenges were resolved successfully with the help from computational simulations.  Examples include providing insight of non-linear material response in complex loading environments, examining the integrity of engineering structure when test data are insufficient, modeling microstructure and its linkage to material properties, and predicting aging and material property changes during service.  Although the progress of developing computational predictive capabilities has been highly encouraging so far, it is well recognized in the computational mechanics community that many issues in theories and numerical algorithms yet to be addressed.  While modeling and simulation are being used increasingly as the information-generating and decision-making tools in the cycle of engineering product from design to retirement, how to create and demonstrate credibility of computational analyses, especially for applications in the solid mechanics discipline, is becoming an inevitable challenge for model developers and computational analysts simply because neither codes nor models are perfect.

At this seminar, years of efforts at Sandia to advance the capability of computational solid mechanics modeling for national security and industry applications will be presented, highlighting their challenges and successes.  Lessons learned from bridging physics at different length scales and coupling different simulation codes will be shared.  Most importantly, strategies, including effective and ineffective ones, of developing and presenting model credibility will be discussed. 

Biographical Sketch: Dr. Eliot Fang is the Manager of the Solid Mechanics Department at Sandia National Laboratories. He received his B.S. degree from the National Central University in Taiwan and M.S. and Ph.D. degrees from the University of California at Santa Barbara, all in mechanical engineering. Dr. Fang’s research interest is to apply modeling approaches and high performance computing to elucidate mechanisms of material behaviors and to predict material behaviors at various length scales in different environments.  He has over 60 publications and 70 invited presentations reporting his technical accomplishments and contributions to materials modeling and mechanical simulations.  Dr. Fang is a Fellow of the American Society of Mechanical Engineers and a recipient of the 2006 Asian American Engineer of the Year Award.