Bulletin of the American Physical Society
67th Annual Gaseous Electronics Conference
Volume 59, Number 16
Sunday–Friday, November 2–7, 2014; Raleigh, North Carolina
Session AM2: Workshop: Plasma Verification and Validation |
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Chair: John Verboncoeur, Michigan State University Room: State C |
Monday, November 3, 2014 8:00AM - 8:50AM |
AM2.00001: Validation and Verification with Applications to a Kinetic Global Model Invited Speaker: J.P. Verboncoeur As scientific software matures, verification, validation, benchmarking, and error estimation are becoming increasingly important to ensure predictable operation. Having well-described and consistent data is critical for consistent results. This presentation briefly addresses the motivation for V{\&}V, the history and goals of the workshop series. A roadmap of the current workshop is presented. Finally, examples of V{\&}V are applied to a novel kinetic global model for a series of low temperature plasma problems ranging from verification of specific rate equations to benchmarks and validation with other codes and experimental data for Penning breakdown and hydrocarbon plasmas. The results are included in the code release to ensure repeatability following code modifications.\\[4pt] In collaboration with G. Parsey, J. Kempf, and A. Christlieb, Michigan State University. [Preview Abstract] |
Monday, November 3, 2014 8:50AM - 9:40AM |
AM2.00002: Verification and Validation of Kinetic Codes Invited Speaker: Andrew Christlieb We review the last three workshops held on Validation and Verification of Kinetic Codes. The goal of the workshops was to highlight the need to develop benchmark test problems beyond traditional test problems such as Landau damping and the two-stream instability. These test problems provide a limited understanding how a code might perform and mask key issues in more complicated situations. Developing these test problems highlights the strengths and weaknesses of both mesh- and particle-based codes. One outcome is that designing test problems that clearly deliver a path forward for developing improved methods is complicated by the need to create a completely self-consistent model. For example, two test cases proposed by the authors as simple test cases turn out to be ill defined. The first case is the modeling of sheath formation in a 1D 1V collisionless plasma. We found that losses to the wall lead to discontinuous distribution functions, a challenge for high order mesh-based solvers. The semi-infinite case was problematic because the far field boundary condition poses difficulty in computing on a finite domain. Our second case was flow of a collisionless electron beam in a pipe. Here, numerical diffusion is a key problem we are testing; however, two-stream instability at the beam edges introduces other issues in terms of finding convergent solutions. For mesh-based codes, before particle trapping takes place, mesh-based methods find themselves outside of the asymptotic regime. Another conclusion we draw from this exercise is that including collisional models in benchmark test problems for mesh-based plasma simulation tools is an important step in providing robust test problems for mesh-based kinetic solvers.\\[4pt] In collaboration with Yaman Guclu, David Seal, and John Verboncoeur, Michigan State University. [Preview Abstract] |
Monday, November 3, 2014 9:40AM - 10:00AM |
AM2.00003: Break
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Monday, November 3, 2014 10:00AM - 10:50AM |
AM2.00004: Benchmark solutions for simulations of capacitively coupled discharges Invited Speaker: Miles Turner Benchmarks are an important element of Verification and Validation strategies. Such strategies define a process for increasing confidence in the fidelity of computer simulations, with the aim of making confident predictions of physical behaviour under conditions of practical interest. Such confidence can be increased by developing benchmark solutions for representative conditions. A benchmark solution is a high quality solution that is accepted to be correct. In this paper, we describe an attempt to develop such solutions for capacitive discharges, and we show that a number of independently developed particle-in-cell simulations can reproduce the benchmark solutions. These solutions are useful not only for particle-in-cell simulations, but also for other kinds of plasma simulations. We will show comparisons of fluid model solutions with the benchmarks. [Preview Abstract] |
Monday, November 3, 2014 10:50AM - 11:40AM |
AM2.00005: LXCat: A web-based, community-wide project on data for modeling low temperature plasmas Invited Speaker: L.C. Pitchford LXCat is an open-access website (www.lxcat.net) for exchanging data related to ion and electron transport and scattering cross sections in cold, neutral gases. At present 30 people from 12 countries have contributed to the LXCat project. This presentation will focus on the status of the data available for electrons on LXCat. These data are primarily in the form of ``complete'' sets of cross sections, compiled or calculated by different contributors, covering a range of energies from thermal up to about 1 keV. The cross section data can be used directly in Monte Carlo simulations and can also be used as input to Boltzmann equation solvers. Solution of the homogeneous, steady-state Boltzmann equation yields electron energy distribution functions (edf) as a function of reduced electric field strength, E/N, integrals over which yield electron transport and rate coefficients. The transport and rate coefficient data are required input for fluid models of low temperature plasmas. Evaluation of the cross section data sets available on LXCat is a key issue. To this end, the LXCat team has been making systematic intercomparisons of cross section data and comparisons of calculated and measured transport and rate coefficients. Our evaluations have been reported previously for noble gases and for common atmospheric gases. The LXCat team is now evaluating data for more complex molecules. [Preview Abstract] |
Monday, November 3, 2014 11:40AM - 1:10PM |
AM2.00006: Lunch
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Monday, November 3, 2014 1:10PM - 2:00PM |
AM2.00007: Richardson Extrapolation Based Error Estimation for Stochastic Kinetic Plasma Simulations Invited Speaker: Keith Cartwright To have a high degree of confidence in simulations one needs code verification, validation, solution verification and uncertainty qualification. This talk will focus on numerical error estimation for stochastic kinetic plasma simulations using the Particle-In-Cell (PIC) method and how it impacts the code verification and validation. A technique Is developed to determine the full converged solution with error bounds from the stochastic output of a Particle-In-Cell code with multiple convergence parameters (e.g. $\Delta$t, $\Delta$x, and macro particle weight). The core of this method is a multi parameter regression based on a second-order error convergence model with arbitrary convergence rates. Stochastic uncertainties in the data set are propagated through the model usin gstandard bootstrapping on a redundant data sets, while a suite of nine regression models introduces uncertainties in the fitting process. These techniques are demonstrated on Flasov-Poisson Child-Langmuir diode, relaxation of an electro distribution to a Maxwellian due to collisions and undriven sheaths and pre-sheaths. [Preview Abstract] |
Monday, November 3, 2014 2:00PM - 2:50PM |
AM2.00008: The Role of V{\&}V in Total Prediction Uncertainty Invited Speaker: Christopher Roy Computational Fluid Dynamics (CFD) simulations are frequently used for decision making in scientific and engineering systems. However, the accuracy and reliability of CFD simulations is often poorly understood. There are three sources of uncertainty in CFD predictions: uncertainty in model inputs, uncertainty due to numerical errors, and uncertainty due to modeling errors. When model input uncertainties are stochastic, they are appropriately described by precise probability distributions, and their effects on output quantities are often determined by standard techniques. In general, not all inputs have precisely specified probability distributions. In such cases, different techniques, such as segregated uncertainty propagation, are needed to propagate mixed aleatory and epistemic uncertainty. Verification and Validation (V{\&}V) address the processes used to estimate uncertainties due to numerical errors and modeling errors, respectively. During Verification, one estimates the numerical errors in a simulation. This estimation process leads one to treat these as uncertainties; however, they are not random (aleatory) uncertainties, but are instead lack of knowledge (epistemic) uncertainties. During Validation, one estimates the errors due to model form. This process usually involves comparison of nondeterministic outcomes from simulation and experiment the estimation process leads us to treat the modeling errors as uncertainties. Finally, estimating the total prediction uncertainty requires that all three sources be accounted for: input uncertainty (via uncertainty propagation), numerical uncertainty (via Verification), and model form uncertainty (via Validation). [Preview Abstract] |
Monday, November 3, 2014 2:50PM - 3:10PM |
AM2.00009: Break
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Monday, November 3, 2014 3:10PM - 4:00PM |
AM2.00010: What is Necessary To Succeed in V{\&}V? Experience From The DOE ASC V{\&}V Program Invited Speaker: William Rider Verification and validation is a route toward examining the credibility and confidence in computations. ~These are inter-dependent and complementary activities that are usually combined with uncertainty quantification. Validation is usually the emphasis for most scientific endeavors being predicted upon experimental science and physical theory. Verification is similarly based upon the mathematical basis of the numerical methods. Upon this point PIC methods in particular are disadvantaged as the mathematical basis is quite weak. Nonetheless useful empirical results can be examined. A great deal of experience has been gained in the application of a systematic V{\&}V process including uncertainty quantification. The lessons from these efforts can be applied profitably to PIC methods. [Preview Abstract] |
Monday, November 3, 2014 4:00PM - 5:00PM |
AM2.00011: Panel Discussion: Future of V\&V in Low Temperature Plasma Community |
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