COLLECTIONS

Documents published in Scipedia

• Simulation of the must field experiment using the FEFLO-URBAN CFD model

  F. Camelli, S. Hanna, R. Lohner

  Computer Science (2004). Vol. 13 (12), pp. 1-14

  
  

  Abstract

  The Setting Test (MUST) experiment, that was carried out at Dugway Proving Ground, was analyzed using Very Large Eddy Simulation (VLES). The CFD approaches used to simulate transport and dispersion in the atmosphere at the urban scale included Reynold Average Navier-Stokes (RANS) and LES. Improvements of the vertical turbulence production were introduced using the geometrical roughness applied to the ground surface. The results show that small changes in wind direction can produce large localized changes in concentration levels.

  Abstract
  The Setting Test (MUST) experiment, that was carried out at Dugway Proving Ground, was analyzed using Very Large Eddy Simulation (VLES). The CFD approaches used to simulate [...]

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• On the computation of multi-material flows using ALE formulation

  H. Luo, J. Baum, R. Lohner

  Journal of Computational Physics (2004). Vol. 194 (1), pp. 304-328

  
  

  Abstract

  Computation of compressible multi-fluid flows with a general equation of state using interface tracking and moving grid approach is discussed in this paper. The AUSM+, HLLC, and Godunov methods are presented and implemented in the context of arbitrary Lagrangian–Eulerian formulation for solving the unsteady compressible Euler equations. The developed methods are fully conservative, and used to compute a variety of multi-component flow problems, where the equations of state can be drastically different and stiff. Numerical results indicate that both ALE HLLC and Godunov schemes demonstrate their simplicity and robustness for solving such multi-phase flow problems, and yet ALE AUSM+ scheme exhibits strong oscillations around material interfaces even using a first order monotone scheme and therefore is not suitable for this class of problems.

  Abstract
  Computation of compressible multi-fluid flows with a general equation of state using interface tracking and moving grid approach is discussed in this paper. The AUSM+, [...]

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• Multistage explicit advective prediction for projection-type incompressible flow solvers

  R. Lohner

  Journal of Computational Physics (2004). Vol. 195 (1), pp. 143-152

  
  

  Abstract

  A multistep advective predictor has been developed within the context of projection schemes for incompressible flows. The key idea is to integrate with schemes of different order the different regions of the domain. In regions where advection dominates, multistepping yields a considerable benefit. In those regions where viscosity dominates, the scheme reverts naturally to the original one-step scheme. Several examples show savings of the order of 1:3–1:10 as compared with standard projection schemes, even for transient problems. Given that these benefits can be achieved with a very modest change in existing codes, the proposed multistage advective predictor should be widely applicable.

  Abstract
  A multistep advective predictor has been developed within the context of projection schemes for incompressible flows. The key idea is to integrate with schemes of different [...]

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• Projective prediction of pressure increments

  R. Lohner

  Commun. Numer. Meth. Engng (2004). Vol. 21 (4), pp. 201-207

  
  

  Abstract

  A simple projective predictor of pressure increments has been developed. The procedure requires the storage of previous pressure increments and right-hand sides, i.e. a modest amount of storage. Based on this information, the known right-hand sides are projected onto the right-hand side at the new timestep. The projection coefficients are then used to predict the pressure increment at the new timestep. Numerical tests indicate that the number of iterations required is reduced considerably. Furthermore, the main gains are achieved with a very modest number of basis vectors. Typically, no more than 2 previous results have to be stored. The procedure is easy to implement and should be applicable to a large number of codes. 

  Abstract
  A simple projective predictor of pressure increments has been developed. The procedure requires the storage of previous pressure increments and right-hand sides, i.e. a modest [...]

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• Large-scale fluid-structure interaction simulations

  R. Lohner, J. Cebral, C. Yang, J. Baum, E. Mestreau, C. Charman, D. Pelessone

  Computing in Science & Engineering (2004). Vol. 6 (3), pp. 27-37

  
  

  Abstract

  Combining computational-science disciplines, such as in fluid-structure interaction simulations, introduces a number of problems. The authors offer a convenient and cost-effective approach for coupling computational fluid dynamics (CFD) and computational structural dynamics (CSD) codes without rewriting them. With the advancement of numerical techniques and the advent, first, of affordable 3D graphics workstations and scalable compute servers, and, more recently, PCs with sufficiently large memory and 3D graphics cards, public-domain and commercial software for each of the computational core disciplines has matured rapidly and received wide acceptance in the design and analysis process. Most of these packages are now at the threshold mesh generation pre-processor. This has prompted the development of the next logical step: multidisciplinary links of codes, a trend that is clearly documented by the growing number of publications and software releases in this area. In this paper, we concentrate on fluid-structure and fluid-structure-thermal interaction, in which changes of geometry due to fluid pressure, shear, and heat loads considerably affect the flowfield, changing die loads in turn. Problems in this category include: steady-state aerodynamics of wings under cruise conditions; aeroelasticity of vibrating - that is, elastic - structures such as flutter and buzz (aeroplanes and turbines), galloping (cables and bridges), and maneuvering and control (missiles and drones); weak and nonlinear structures, such as wetted membranes (parachutes and tents) and biological tissues (hearts and blood vessels); and strong and nonlinear structures, such as shock-structure interaction (command and control centers, military vehicles) and hypersonic flight vehicles.

  Abstract
  Combining computational-science disciplines, such as in fluid-structure interaction simulations, introduces a number of problems. The authors offer a convenient and cost-effective [...]

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• A stabilized edge-based implicit incompressible flow formulation

  O. Soto, R. Lohner, J. Cebral, F. Camelli

  Comput. Methods Appl. Mech. Engrg., (2004). Vol. 193 (23-26), pp. 2139-2154

  
  

  Abstract

  An edge-based implementation of an implicit, monolithic, finite element (FE) scheme for the solution of the incompressible Navier–Stokes (NS) equations is presented. The original element formulation is based on the pressure stability properties of an implicit second-order in time fractional step (FS) method, which is conditionally stable. The final monolithic scheme preserves the second-order accuracy of the FS method, and is unconditionally stable. Furthermore, it can be demonstrated that the final pressure stabilizing term is practically the same fourth-order pressure term added by some authors (but following different arguments) to obtain high order accurate results, and that the final discretized convective terms are formally a second-order discretization of the respective continuous one. The development of the edge implementation is supported by two criteria: the properties of the element based one, which has already been extensively tested and for which convergence and stability analysis has already been presented, and on the enforcement of global conservation and symmetry at the discrete level. A monotonicity preserving term which decreases the discretization order in sharp gradient regions to avoid localized oscillations (overshoots and undershoots), is formulated and tested. Some numerical examples and experimental comparisons are presented

  Abstract
  An edge-based implementation of an implicit, monolithic, finite element (FE) scheme for the solution of the incompressible Navier–Stokes (NS) equations is presented. [...]

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• Dynamic deactivation for advection‐dominated contaminant transport

  R. Lohner, F. Camelli

  Commun. Numer. Meth. Engng (2004). Vol. 20 (8), pp. 639-646

  
  

  Abstract

  A simple dynamic deactivation procedure for advection‐dominated contaminant transport is presented. The key idea is to avoid any work in regions where the solution cannot change, i.e. where sources vanish and unknowns do not exhibit any spatial change. Saving factors in CPU of 1: 3–1: 10 are commonly achieved. The procedure is general and simple to implement, and should be applicable to a large number of codes and transport problems.

  Abstract
  A simple dynamic deactivation procedure for advection‐dominated contaminant transport is presented. The key idea is to avoid any work in regions where the solution cannot [...]

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• An Adjoint-Based Design Methodology for CFD Optimization Problems

  O. Soto, R. Lohner, C. Yang

  Int. J. Numer. Meth. Heat and Fluid Flows (2004). Vol. 14 (6), pp. 734-759

  
  

  Abstract

  A complete CFD design methodology is presented. The main components of this methodology are a general edge-based compressible/incompressible flow solver; a continuous adjoint formulation for the gradient computations; a steepest descent technique for the change of design variables; evaluation of the gradient of the discretized flow equations with respect to mesh by finite differences; a CAD-free pseudo-shell surface parametrization, allowing every point on the surface to be optimized to be used as a design parameter; and a level type scheme for the movement of the interior points. Several examples are included to demonstrate the methodology developed.

  Abstract
  A complete CFD design methodology is presented. The main components of this methodology are a general edge-based compressible/incompressible flow solver; a continuous adjoint [...]

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• Assessing maximum possible damage for contaminant release events

  F. Camelli, R. Lohner

  Engineering Computations (2004). Vol. 21 (7), pp. 748-760

  
  

  Abstract

  The combined use of damage criteria, genetic algorithms and advanced CFD solvers provides an effective strategy to identify locations of releases that produce maximum damage. The implementation is simple and does not require any change to flow solvers. A rather general criterion has been formulated to determine the damage inflicted by the intentional or unintentional release of contaminants. Results of two typical cases show that damage can vary considerably as a function of release location, implying that genetic algorithms are perhaps the only techniques suited for this type of optimization problem

  Abstract
  The combined use of damage criteria, genetic algorithms and advanced CFD solvers provides an effective strategy to identify locations of releases that produce maximum damage. [...]

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• Comparison of Classical And Simple Free-surface Green Functions

  C. Yang, R. Lohner, F. Noblesse

  International Journal of Offshore and Polar Engineering (2004). Vol. 14 (4), pp. 1-8

  
  

  Abstract

  The classical Green function and a related simpler Green function associated with the linearized free-surface boundary condition for 3D diffraction-radiation by a ship advancing in regular waves are considered for the special case of steady flows. Both the classical Green function and the simpler Green function satisfy the radiation condition and the linearized free-surface condition in the farfield (where the linear free-surface condition is valid). The classical Green function also satisfies the linear free-surface condition in the nearfield (where this linear condition is only an approximation, due to nearfield effects), whereas the simple Green function satisfies the linear free-surface condition only approximately in the nearfield. Numerical differences between these alternative free-surface Green functions are shown to vanish in the farfield, as expected, and to be relatively moderate in the nearfield. Copyright © 2004 by The International Society of Offshore and Polar Engineers.

  Abstract
  The classical Green function and a related simpler Green function associated with the linearized free-surface boundary condition for 3D diffraction-radiation by a ship advancing [...]

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