Publications

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This report summarizes the work completed during FY2007 and FY2008 for the LDRD project ''Hybrid Plasma Modeling''. The goal of this project was to develop hybrid methods to model plasmas across the non-continuum-to-continuum collisionality spectrum. The primary methodology to span these regimes was to couple a kinetic method (e.g., Particle-In-Cell) in the non-continuum regions to a continuum PDE-based method (e.g., finite differences) in continuum regions. The interface between the two would be adjusted dynamically ased on statistical sampling of the kinetic results. Although originally a three-year project, it became clear during the second year (FY2008) that there were not sufficient resources to complete the project and it was terminated mid-year.

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Class 8 tractor-trailers are responsible for 11-12% of the total US consumption of petroleum. Overcoming aero drag represents 65% of energy expenditure at highway speeds. Most of the drag results from pressure differences and reducing highway speeds is very effective. The goal is to reduce aerodynamic drag by 25% which would translate to 12% improved fuel economy or 4,200 million gal/year. Objectives are: (1) In support of DOE's mission, provide guidance to industry in the reduction of aerodynamic drag; (2) To shorten and improve design process, establish a database of experimental, computational, and conceptual design information; (3) Demonstrate new drag-reduction techniques; and (4) Get devices on the road. Some accomplishments are: (1) Concepts developed/tested that exceeded 25% drag reduction goal; (2) Insight and guidelines for drag reduction provided to industry through computations and experiments; (3) Joined with industry in getting devices on the road and providing design concepts through virtual modeling and testing; and (4) International recognition achieved through open documentation and database.

Here the analytical plate penetration relationship derived by De Chant [An explanation for the minimal effect of body curvature on hypervelocity penetration hole formation. Sandia National Laboratories, Albuquerque, NM, no. SAND2003-2696J. To appear in: International Journal of Solids and Structures] using an analogy between hydrodynamics and penetration processes is combined with the Grady-Kipp dynamic fragmentation model [International Journal of Impact Engineering 14 (1993) 427; Fragmentation of solids under dynamic loading. In: Wierzbicki, T., Jones, N. (Eds.), Structural Failure. Wiley, New York, 1989; Mechanisms of dynamic fragmentation: factors governing growth size, SAND84-2304C, Sandia National Laboratories, Albuquerque, NM, 1984] to estimate the size and distribution of fragments generated due to impact and penetration of thin plates by spherical projectiles. Fragment statistics estimated using this analytical model are then compared to fragmentation as predicted by CTH, a shock-physics hydrocode [International Journal of Impact Engineering 10 (1990) 351]. Agreement between analytically based model and CTH simulations shows good agreement. Extending the hydrodynamic based analogy used to develop the analytical plate penetration model to include fragmentation processes, we derive expressions relation fragmentation to plate impact and penetration. The functional form of these equations is also in good agreement with CTH simulation. Finally, a deterministic (as opposed to statistical) cumulative particle size distribution relationship is derived using mass conservation concepts and gives a power law distribution. The power law distribution requires the presence of a non-zero minimum fragment size rather than a continuous distribution that includes particles approaching zero size. The presence of a non-zero fragment size is in agreement with computational models and theoretically based cumulative lognormal fragment distributions. © 2004 Elsevier Ltd. All rights reserved.

In this study, we extend self-similar, far-field, turbulent wake concepts to estimate the 2-d drag coefficient for a range of bluff body problems. The self-similar wake velocity defect that is normally independent of the near field wake (and hence body geometry) is modified using a combined approximate Green's function/Gram-Charlier series approach to retain the body geometry information. Formally a near field velocity defect profile is created using small disturbance theory and the inviscid flow field associated with the body of interest. The defect solution is then used as an initial condition in the approximate Green's function solution. Finally, the Green's function solution is matched to the Gram-Charlier series yielding profiles that are integrated to yield the net form drag on the bluff body. Preliminary results indicate that drag estimates computed using this method are within approximately 15% as compared to published values for flows with large separation. This methodology may be of use as a supplement to CFD and experimental solutions in reducing the heavy computational and experimental burden of estimating drag coefficients for blunt body flows for preliminary design type studies. © 2004 by the American Institute of Aeronautics and Astronautics, Inc.

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At 70 miles per hour, overcoming aerodynamic drag represents about 65% of the total energy expenditure for a typical heavy truck vehicle. The goal of this US Department of Energy supported consortium is to establish a clear understanding of the drag producing flow phenomena. This is being accomplished through joint experiments and computations, leading to the smart design of drag reducing devices. This paper will describe our objective and approach, provide an overview of our efforts and accomplishments, and discuss our future direction.

The Detached Eddy Simulation (DES) and steadystate Reynolds-Averaged Navier-Stokes (RANS) turbulence modeling approaches are examined for the incompressible flow over a square cross-section cylinder at a Reynolds number of 21,400. A compressible flow code is used which employes a second-order Roe upwind spatial discretization. Efforts are made to assess the numerical accuracy of the DES predictions with regards to statistical convergence, iterative convergence, and temporal and spatial discretization error. Three-dimensional DES simulations compared well with two-dimensional DES simulations, suggesting that the dominant vortex shedding mechanism is effectively two-dimensional. The two-dimensional simulations are validated via comparison to experimental data for mean and RMS velocities as well as Reynolds stress in the cylinder wake. The steady-state RANS models significantly overpredict the size of the recirculation zone, thus underpredicting the drag coefficient relative to the experimental value. The DES model is found to give good agreement with the experimental velocity data in the wake, drag coefficient, and recirculation zone length.

Though not discussed extensively in the literature, it is known among workers in impact and penetration dynamics, e.g. the CTH analysis and development team at Sandia National Laboratories, that curvature of thin plates has a minimal effect on the penetration hole diameter due to a hypervelocity impact. To understand why curvature introduces a minimal effect on penetration hole size we extend a flat plate penetration hole diameter relationship to include the effect of body curvature. The effect of the body curvature on the hole diameter is shown to scale according to the dimensionless plate thickness to radius of curvature of the body i.e. h/R, which is typically small. Indeed for most problems where a single layer shell (plate) can be meaningfully defined, the effect of curvature upon hole diameter is on the order of other uncertainties in the problem, e.g. doubts concerning the appropriate equation of state and strength model, and is often, therefore, negligible.