Invited Tutorials

Friday, July 14, 2006 at 4:00PM
Building 200, Room 002

Dr. John I. Castor
Lawrence Livermore National Laboratory

Abstract:

Radiation hydrodynamics is defined to be the study of hydrodynamics coupled to radiation that is energetically important. This subject is reviewed with an emphasis on the computational approaches that might be used. First a few of the basic concepts of radiation transport are discussed. One of the most important of these is diffusion, which leads to a practical but approximate computational method. A complicating factor is the necessity of transforming between the stationary frame of the laboratory and the comoving frame of the material. After a comment about the applicability of the diffusion approximation, the more accurate methods of radiation transport are described. The numerical algorithms for computing transport are discussed in a little more detail, ending with a description of the implicit Monte Carlo method. A selection of general references is provided at the end.

Refreshments served at 5:30PM

Friday, July 21, 2006 at 4:00PM
Building 200, Room 002

Dr. Michael Wright
Senior Research Scientist
NASA Ames Research Center

Abstract:

Spacecraft which enter the atmosphere of solar system objects (Venus, Earth, Mars, Jupiter, Saturn, TItan, Uranus, Neptune) encounter high heating rates as they decelerate. Each of these destinations presents unique challenges in terms of protecting the vehicle from the high heating rates encountered during entry. These challenges arise due to large variations in the atmospheric composition and entry velocity at each destination. The current paper reviews the status of our ability to make conservative and accurate predictions of the aerothermal environment for such planetary entry missions. A range of techniques are discussed, ranging from engineering correlations to high fidelity Navier-Stokes CFD simulations. Key modeling issues are explored, and several common modeling uncertainties, including prediction of turbulent heating rates, heating from shock layer radiation, the effects of surface chemistry, and afterbody (base) heating, are discussed. Examples are taken from past, present, and future entry missions, including Pioneer Venus, Stardust, the Mars Science Laboratory, Galileo, and Huygens.

Refreshments served at 5:30PM

Friday, July 28, 2006 at 4:00PM
Bluiding 200, Room 002

Dr. Scott Ferson
Applied Biomathematics

Abstract:

An honest assessment of the uncertainty in calculations and model predictions may be the only difference between prudent analysis and mere wishful thinking. Although traditional methods of error analysis and uncertainty assessments are useful, they typically require untenable or unjustified assumptions. Methods are needed that can relax these assumptions to reflect what is actually known and what is not known about the underlying system. Analysts in many fields draw a careful distinction between epistemic uncertainty and aleatory uncertainty. The latter comes from variability across time or space, heterogeneity within a population, and other sources of stochasticity, and it is commonly modeled with the methods of probability theory. The former arises from measurement imprecision, residual scientific ignorance about model structure, and other forms of incertitude. Many analysts are coming to believe that alternative methods must be used to fully account for epistemic uncertainty and to properly distinguish it from aleatory uncertainty. Considerations of these two kinds of uncertainty have suggested new approaches to some of the fundamental tasks in model building, including uncertainty propagation and especially the treatment of model uncertainty, but also sensitivity analysis and validation exercises. There are some strategies that can be used even for extremely complex models that have high-dimensional inputs and require long calculation times. For example, Monte Carlo techniques and the Cauchy deviate method have errors determined only by the number of replications, rather than the dimensionality of the problem. The former can project probabilistic uncertainty and the latter projects interval-like incertitude. For models that are so complex that very few runs can be computed, Kolmogorov-Smirnov confidence procedures can assess the sampling uncertainty associated with having few replications. Neglect of model uncertainty, which is often the elephant in the living room, is especially egregious in modeling. Analysts usually construct a model and then act as though it correctly represents the world. This understates the uncertainty associated with the model’s predictions, because it fails to express that the model might be in error. Standard methods recommended to account for model uncertainty have serious deficiencies, and some tend to erase the uncertainty rather than truly propagate it through calculations. Alternative strategies will be discussed.

Wednesday, August 2, 2006 at 4:00PM
Building 370, Room 370

Dr. Marcus Herrmann
Stanford University

Abstract:

Two-phase flows play a key role in many natural phenomena and technical applications. Although a flow with any combination of two of the three common phases of matter, i.e. solid, liquid, and gas, falls under the category of two-phase flow, the focus of this tutorial will be on liquid/gas flows. Here, a deformable free surface separates the two phases while exerting a surface tension force on the liquid. Liquid/gas flows are typically characterized by a high contrast in density between the two fluids, a highly localized surface tension force, and the presence of a vast range of length and time scales, thus making it an extremely challenging problem to model and simulate. One technical application that encompasses all of these challenging conditions is that of atomization of a liquid jet in a turbulent environment. In this tutorial, different modeling strategies and numerical techniques will be discussed for simulating the initial phase of the liquid breakup, i.e. the primary atomization. Furthermore, modeling strategies for the subsequent phase of breakup, i.e. the secondary atomization, will be summarized and methods for coupling the different techniques used for primary and secondary atomization will be discussed.

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