New special fluid-structure interaction (FSI) techniques, supplementing the ones developed earlier, are employed with the Stabilized Space-Time FSI (SSTFSI) technique. The new special techniques include improved ways of calculating the equivalent fabric porosity in Homogenized Modeling of Geometric Porosity (HMGP), improved ways of building a starting point in FSI computations, ways of accounting for fluid forces acting on structural components that are not expected to influence the flow, adaptive HMGP, and multiscale sequentially coupled FSI techniques. While FSI modeling of complex parachutes was the motivation behind developing some of these techniques, they are also applicable to other classes of complex FSI problems. We also present new ideas to increase the scope of our FSI and CFD techniques. Copyright © 2009 John Wiley & Sons, Ltd.
martes, 29 de diciembre de 2009
Space-time finite element computation of complex fluid-structure interactions
New special fluid-structure interaction (FSI) techniques, supplementing the ones developed earlier, are employed with the Stabilized Space-Time FSI (SSTFSI) technique. The new special techniques include improved ways of calculating the equivalent fabric porosity in Homogenized Modeling of Geometric Porosity (HMGP), improved ways of building a starting point in FSI computations, ways of accounting for fluid forces acting on structural components that are not expected to influence the flow, adaptive HMGP, and multiscale sequentially coupled FSI techniques. While FSI modeling of complex parachutes was the motivation behind developing some of these techniques, they are also applicable to other classes of complex FSI problems. We also present new ideas to increase the scope of our FSI and CFD techniques. Copyright © 2009 John Wiley & Sons, Ltd.
A new finite volume method on junction coupling and boundary treatment for flow network system analyses
To adequately analyze the flow in a pipe or duct network system, traditional node-based junction coupling methods require junction losses, which are specified by empirical or analytic correlations. In this paper, a new finite volume junction coupling method using a ghost junction cell is developed by considering the interchange of linear momentum as well as the important wall effect at the junction without requiring any correlation on the junction loss. Also, boundary treatment is modified to preserve the stagnation enthalpy across boundaries, such as the pipe end and the interface between the junction and the branch.The computational accuracy and efficiency of Godunov-type finite volume schemes are investigated by tracing the total mechanical energy of rapid transients due to sudden closure of a valve at the downstream end. Among the approximate Riemann solvers, the proposed RoeM scheme turns out to be more suitable for finite volume junction treatment than the original Roe's approximate Riemann solver because of conservation of the stagnation enthalpy across the geometric discontinuity. From the viewpoint of computational cost, the implicit LU-SGS time integration is appropriate for steady and slow transients, while the explicit third-order TVD Runge-Kutta scheme is advantageous for rapid transients. Copyright © 2009 John Wiley & Sons, Ltd.
A fast universal solver for 1D continuous and discontinuous steady flows in rivers and pipes
Simulation of 1D steady flow covers a wide range of practical applications, such as rivers, pipes and hydraulic structures. Various flow patterns coexist in such situations: free surface flows (supercritical, subcritical and hydraulic jump), pressurized flows as well as mixed flows. As a result, development of a unified 1D model for all the situations of interest in civil engineering remains challenging. In this paper, a fast universal solver for 1D continuous and discontinuous steady flows in rivers and pipes is set up and assessed. Developments are initiated from an original unified mathematical model using the Saint-Venant equations. Application of these equations, originally dedicated to free-surface flow, is extended to pressurized flow by means of the Preissmann slot model. In particular, an original negative slot is developed in order to handle sub-atmospheric pressurized flow. Next, the full unsteady model is simplified under the assumption of steadiness and reformulated into a single pseudo-unsteady differential equation. The derived pseudo-unsteady formulation aims at keeping the hyperbolic feature of the equation. Stability analysis of the differential equation suggests a unique splitting for the finite volume scheme whatever the flow conditions. The numerical scheme obtained is a universal Flux Vector Splitting which shows robustness and simplicity. Accuracy and performance of the new methodology is assessed by comparison with analytical and experimental results. Copyright © 2009 John Wiley & Sons, Ltd.
Residual-based variational multiscale methods for laminar, transitional and turbulent variable-density flow at low Mach number
In the present study, residual-based variational multiscale methods are developed for and applied to variable-density flow at low Mach number. In particular, two different formulations are considered in this study: a standard stabilized formulation featuring SUPG/PSG/grad-div terms and a complete residual-based variational multiscale formulation additionally containing cross- and Reynolds-stress terms as well as subgrid-scale velocity terms in the energy-conservation equation. The proposed methods are tested for various laminar flow test cases as well as a test case at laminar via transitional to turbulent flow stages. Stable and accurate results are obtained for all numerical examples. Substantial differences in the results between the two approaches do not become notable until a high temperature gradient is applied and the flow reaches a turbulent flow stage. The more pronounced influence of adding subgrid-scale velocity terms to the energy-conservation equation on the results than adding analogous terms to the momentum-conservation equation in this situation appears particularly noteworthy. Copyright © 2009 John Wiley & Sons, Ltd.
Equation-free model reduction for complex dynamical systems
This paper presents a reduced model strategy for simulation of complex physical systems. A classical reduced basis is first constructed relying on proper orthogonal decomposition of the system. Then, unlike the alternative approaches, such as Galerkin projection schemes for instance, an equation-free reduced model is constructed. It consists in the determination of an explicit transformation, or mapping, for the evolution over a coarse time-step of the projection coefficients of the system state on the reduced basis. The mapping is expressed as an explicit polynomial transformation of the projection coefficients and is computed once and for all in a pre-processing stage using the detailed model equation of the system. The reduced system can then be advanced in time by successive applications of the mapping. The CPU cost of the method lies essentially in the mapping approximation which is performed offline, in a parallel fashion, and only once. Subsequent application of the mapping to perform a time-integration is carried out at a low cost thanks to its explicit character.Application of the method is considered for the 2-D flow around a circular cylinder. We investigate the effectiveness of the reduced model in rendering the dynamics for both asymptotic state and transient stages. It is shown that the method leads to a stable and accurate time-integration for only a fraction of the cost of a detailed simulation, provided that the mapping is properly approximated and the reduced basis remains relevant for the dynamics investigated. Copyright © 2009 John Wiley & Sons, Ltd.
Atomistic analysis of B clustering and mobility degradation in highly B-doped junctions
In this paper we discuss from an atomistic point of view some of the issues involved in the modeling of electrical characteristics evolution in silicon devices as a result of ion implantation and annealing processes in silicon. In particular, evolution of electrically active dose, sheet resistance and hole mobility has been investigated for high B concentration profiles in pre-amorphized Si. For this purpose, Hall measurements combined with atomistic kinetic Monte Carlo atomistic simulations have been performed. An apparent anomalous behavior has been observed for the evolution of the active dose and the sheet resistance, in contrast to opposite trend evolutions reported previously. Our results indicate that this anomalous behavior is due to large variations in hole mobility with active dopant concentration, much larger than that associated to the classical dependence of hole mobility with carrier concentration. Simulations suggest that hole mobility is significantly degraded by the presence of a large concentration of boron-interstitial clusters, indicating the existence of an additional scattering mechanism. Copyright © 2009 John Wiley & Sons, Ltd.
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