Conferences, Lectures, & Seminars
Events for March
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Formulation of the k-omega Turbulence Model Revisited
Wed, Mar 07, 2007 @ 03:30 PM - 04:30 AM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
David C. Wilcox President DCW Industries, Inc. La Cañada, CA With the rapidly developing field of Detached Eddy Simulation (DES) has come renewed interest in classical Reynolds-averaged (RANS) models of turbulence. DES solves the exact Navier-Stokes equation for the largest eddies and uses a conventional turbulence model to determine Reynolds stresses in thin shear layers. The quality of the DES, of course, depends critically upon how accurate the RANS model is. This seminar presents a new version of the author's k-omega model of turbulence, which is the most widely used turbulence model of its type for Computational Fluid Dynamics applications. The revisions include addition of just one new closure coefficient and a minor adjustment to the dependence of eddy viscosity on turbulence properties. The result is a model that applies to both boundary layers and free shear flows for all speed ranges from incompressible to hypersonic. The modifications to the new k-omega model have been made using the methodology developed by Wilcox in his popular textbook entitled "Turbulence Modeling for CFD." In this methodology, boundary layers and free shear flows are first dissected and analyzed using perturbation methods and similarity solutions. All aspects of the model, including boundary conditions for rough surfaces and surfaces with mass injection, are then developed and validated. Finally, a series of computations is performed for approximately 100 different applications including free shear flows, attached boundary layers, backward-facing steps and separated flows. The test cases include flows from incompressible speeds to Mach numbers in excess of 10. All computations have been done with state-of-the-art numerical flow solvers. The improvements to the k-omega model represent a significant expansion of its range of applicability. The new model, like preceding versions, provides accurate solutions for mildly-separated flows and simple geometries such as that of a backward-facing step. The model's improvement over earlier versions lies in its accuracy for even more complicated separated flows. This seminar demonstrates the enhanced capability for supersonic flow into compression corners and hypersonic shock/boundary-layer interactions. The excellent agreement is achieved without introducing any compressibility modifications to the turbulence model.
Location: Seaver Science Library (SSL) Room 150
Audiences: Everyone Is Invited
Contact: April Mundy
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Anatomy of Complex Reaction Systems. Combustion Reaction Mechanisms from Ignition Delay Times
Wed, Mar 21, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Assa Lifshitz Emeritus Faculty Member Department of Physical Chemistry The Hebrew University of Jerusalem Jerusalem, Israel One of the very useful approaches to the understanding of the kinetics and mechanism of complex combustion systems is the measurement and modeling of the induction period that precedes the ignition of a fuel in a shock tube. When a mixture of a fuel and oxidant is subjected to shock heating, it ignites, following an induction period known as the ignition delay time. This delay is the outcome of the exponential character of the overall reaction rate resulting from various chain branching reactions and adiabatic temperature rise during the course of the reaction. The delay time which is a readily measurable quantity is a function of the initial temperature, pressure and composition of the reaction mixture. The measurement of its dependence on the reactant concentrations and temperature provides a very powerful basis for modeling and understanding the oxidation mechanism. The high potential of this methods was recognized by many combustion kineticists and a very large volume of experimental results and kinetics modeling have been published. The following picture is a typical pressure record showing the reflected shock heating and the ignition process. It is useful to summarize the dependence of the ignition delay times on the composition of the system and on the temperature in a simple parametric relation that can later serve as a basis for computer modeling. It has been shown in the past in numerous ignition studies behind shock waves that the general relation between the induction times and the concentrations is very similar to the relation between a rate of a chemical reaction and the concentrations, that is, tignition = A Ði Ci âi where tignition is the ignition delay time, Ci is a concentration of a component i, and âi is a parameter somewhat similar to an empirical reaction order. It has also been shown that the concentration independent parameter can be presented as, A = 10á exp [E/RT] an expression very similar to a rate constant (except that A decreases with temperature). The parameters E and âi are determined by a complex kinetic scheme. They are experimentally determined quantities and provide a very useful means to summarize the experimental results in a quantitative manner. After establishing an empirical relation as above and determining the parameters, one can perform computer experiments under conditions similar to the laboratory experiments and try, for a given reaction scheme to reproduce these parameters. One then arbitrarily varies the magnitude of the various rate constants in the kinetic scheme and examines the influence of such variations on the magnitude of the ignition delay time and its dependence on the concentrations and on the temperature. From the results of this type of experiments, the role of each reaction in the overall mechanism can be elucidated. By employing such methods, many interesting combustion schemes were analyzed in the past and details of the kinetics and an understanding of the oxidation mechanisms were achieved. We will present and discuss several such systems.
Location: Seaver Science Library, (SSL) Rm 150
Audiences: Everyone Is Invited
Contact: April Mundy
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High-Angle Grain Boundaries and the Evolution of Texture during Severe Plastic Deformation (SPD) Pro
Wed, Mar 28, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Terry R McNelleyCenter for Materials Science and EngineeringDepartment of Mechanical and Astronautical EngineeringNaval Postgraduate SchoolMonterey, CaliforniaAbstractThe production of highly refined microstructures in engineering alloys by application of novel SPD technologies may lead to dramatic property improvements, but realization of this potential will require improved understanding of microstructure control and microstructure processing property relationships. This presentation will examine high-angle boundary formation in microstructures after conventional and SPD processing of aluminum and its alloys, and the relationship between these boundaries and components of the texture. Recent orientation imaging microscopy investigations in this laboratory have revealed distinct, meso-scale band - or block-like features in processed materials. The lattice orientations within these features alternate between prominent texture orientations in a manner reminiscent of deformation banding in fcc metals. Analytical transmission electron microscopy has shown that the interfaces between these features are dislocation boundaries that may be precursors to disordered high-angle grain boundaries. Recent results on materials processed by large-strain extrusion machining will be included.About the PresenterTerry McNelley is a native of Fort Wayne, Indiana. He received his BS in Metallurgical Engineering in 1967 from Purdue and his PhD in Materials Science and Engineering in 1973 from Stanford. For the period 1972-76 he was a faculty member in the Department of Mechanical Engineering at the University of Wyoming, and from 1976 present he has been in the Mechanical Engineering Department at the Naval Postgraduate School, serving as Department Chair from 1996 2002. He has held visiting appointments at institutions in England (1980-81), Japan (1993) and Spain (1999). Professor McNelley's interests include microstructure processing property relationships in metallic materials; deformation processing, microstructures, recrystallization and superplasticity; and metal matrix composites. He was elected Fellow of ASMI in 2001.
Location: Seaver Science Library (SSL) Room 150
Audiences: Everyone Is Invited
Contact: April Mundy