COLLECTIONS

Documents published in Scipedia

• An ALE method for compressible multi-material flows on unstructured grids

  H. Luo, J. Baum, R. Lohner

  (2005). Shock Waves. Springer, Berlin, Heidelberg

  
  

  Abstract

  Multi-material flows, where a moving interface exists between two immiscible fluids, can be found in a variety of engineering problems. Development of numerically accurate and computationally efficient algorithms for multi-material flow simulations remains one of the unresolved issues in computational fluid dynamics. These flow problems are characterized by the existence of material interfaces. The modeling of these complicated free boundaries poses a difficult numerical challenge, as they are either time dependent or unknown a priori and determined as part of the solution. A number of numerical methods exist for solving the interface problems: interface capturing methods (mixed cell methods), level set methods, volume of fluid and interface reconstruction methods, interface tracking methods, free-Lagrange methods. Unfortunately, there are still limitations and shortcomings attached to each of them. In general, for all these methods that allow mixed cells, the computation of thermodynamical variables such as pressure, speed of sound, and temperature on mixed cells is difficult to achieve correctly. In particular, when the equations of state for different materials are drastically different, a small error on the thermodynamical variables can lead to collapse or meaningless of the computation. For all these methods where the interface is represented and tracked explicitly either by marking it with special marker points, or by treating it like a boundary, it is difficulty to handle very complex free surface problems, especially those involving interface topological changes such as merging or breakup of the interface.

  Abstract
  Multi-material flows, where a moving interface exists between two immiscible fluids, can be found in a variety of engineering problems. Development of numerically accurate [...]

  • 0
  •  read

• Recent development of a coupled CFD/CSD methodology using an embedded approach

  J. Baum, E. Mestreau, H. Luo, R. Lohner, D. Pelessone, C. Charman

  (2005). Shock Waves. Springer, Berlin, Heidelberg

  
  

  Abstract

  A new algorithm for modeling the response of structures to severe airblast and fragment loading, including the modeling of large plastic deformations, structural failure and break-up, is described in this paper. The coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies required to describe these phenomena include the FEFL098 flow solver and DYNA3D structural solver. The original coupling between the two domains was based on the so-called “glued-mesh” approach, where the CFD and CSD interfaces match. Recent failure of this approach to model severe structural deformation, as well as crack propagation in steel and concrete, led us to the development and use of the “embedded-mesh” approach. Here, the CSD objects float through the CFD domain. While each approach has its own advantages, limitations and deficiencies, the embedded approach was proven to be superior for the problems modeled here. Critical applications of both approaches are described, including weapon detonation and fragmentation, airblast interaction with a reinforced concrete wall, and fragment/airblast interaction with a steel wall. The final applications model the interaction of an external airblast with a generic steel ship hull and a generic multi-chamber steel tower.

  Abstract
  A new algorithm for modeling the response of structures to severe airblast and fragment loading, including the modeling of large plastic deformations, structural failure and [...]

  • 0
  •  read

• On the coupling of CFD and CSD methodologies for modeling blast-structure interactions

  J. Baum, E. Mestreau, R. Lohner, D. Pelessone, M. Giltrud, J. Gran

  WIT Transactions on The Built Environment (2005). Vol. 84, pp. 361-371

  
  

  Abstract

  This paper describes applications of a coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodology to the simulation of blast waves generated by bare explosive charges in a test facility with rigid and deformable walls. The coupled algorithm combines FEFLO98 (CFD) and MARS3D (CSD) via an embedded approach, where the CSD objects float through the CFD domain. This combination enables an easier and more accurate prediction of structural deformation, cracking and failure under blast loading. Several experiments were conducted to characterize blast load and structural response as a function of charge size, weapon ignition point (nose or tail) and orientation (horizontal or vertical). The numerical simulations helped in understanding the experimental results, some of which were not intuitively understood. Good agreement between the experimental results and the numerical predictions were demonstrated for pressure data, blast loading and the corresponding structural response. Keywords: blast-structure interaction, coupled CFD and CSD, blast wave evolution, structural response to blast loading.

  Abstract
  This paper describes applications of a coupled Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodology to the simulation of blast waves [...]

  • 0
  •  read

• High-Reynolds Number Viscous Flow Computations Using Unstructured-Grid Method

  H. Luo, J. Baum, R. Lohner

  Journal of Aircraft (2005). Vol. 42 (2), pp. 483-462

  
  

  Abstract

  An unstructured grid method is presented to compute three-dimensional compressible turbulent flows for complex geometries. The Navier-Stokes equations along with the one equation turbulence model of Spalart-Allmaras are solved using a parallel, matrix-free implicit method on unstructured tetrahe- dral grids. The developed method has been used to predict drags in the transonic regime for both DLR- F4 and DLR-F6 configurations to assess the accuracy and efficiency of the method. The results obtained are in good agreement with experimental data, indicating that the present method provides an accurate, efficient, and robust algorithm for computing turbulent flows for complex geometries on unstructured grids.

  Abstract
  An unstructured grid method is presented to compute three-dimensional compressible turbulent flows for complex geometries. The Navier-Stokes equations along with the one equation [...]

  • 1
  •  read

• Optimal placement of sensors for contaminant detection based on detailed 3D CFD simulations

  R. Lohner, F. Camelli

  Engineering Computations (2005). Vol. 22 (3), pp. 260-273

  
  

  Abstract

  Purpose - Develop a method for the optimal placement of sensors in order to detect the largest number of contaminant release scenarios with the minimum amount of sensors. Design/methodology/approach - The method considers the general sensor placement problem. Assuming a given number of sensors, every release scenario leads to a sensor input. The data recorded from all the possible release scenarios at all possible sensor locations allow the identification of the best or optimal sensor locations. Clearly, if only one sensor is to be placed, it should be at the location that recorded the highest number of releases. This argument can be used recursively by removing from further consideration all releases already recorded by sensors previously placed. Findings - The method developed works well. Examples showing the effect of different wind conditions and release locations demonstrate the effectiveness of the procedure. Practical implications - The method can be used to design sensor systems for cities, subway stations, stadiums, concert halls, high value residential areas, etc. Originality/value - The method is general, and can be used with other physics-based models (puff, mass-conservation, RANS, etc.). The investigation also shows that first-principles CFD models have matured sufficiently to be run in a timely manner on PCs, opening the way to optimization based on detailed physics.

  Abstract
  Purpose - Develop a method for the optimal placement of sensors in order to detect the largest number of contaminant release scenarios with the minimum amount of sensors. [...]

  • 5
  •  read

• Efficient simulation of blood flow past complex endovascular devices using an adaptive embedding technique

  J. Cebral, R. Lohner

  IEEE Transactions on Intelligent Transportation Systems (2005). Vol. 24 (4), pp. 468-476

  
  

  Abstract

  The simulation of blood flow past endovascular devices such as coils and stents is a challenging problem due to the complex geometry of the devices. Traditional unstructured grid computational fluid dynamics relies on the generation of finite element grids that conform to the boundary of the computational domain. However, the generation of such grids for patient-specific modeling of cerebral aneurysm treatment with coils or stents is extremely difficult and time consuming. This paper describes the application of an adaptive grid embedding technique previously developed for complex fluid structure interaction problems to the simulation of endovascular devices. A hybrid approach is used: the vessel walls are treated with body conforming grids and the endovascular devices with an adaptive mesh embedding technique. This methodology fits naturally in the framework of image-based computational fluid dynamics and opens the door for exploration of different therapeutic options and personalization of endovascular procedures.

  Abstract
  The simulation of blood flow past endovascular devices such as coils and stents is a challenging problem due to the complex geometry of the devices. Traditional unstructured [...]

  • 2
  •  read

• Extension of Harten-Lax-van Leer Scheme for Flows at All Speeds

  H. Luo, J. Baum, R. Lohner

  AIAA Journal (2005). Vol. 43 (6), pp. 1160-1166

  
  

  Abstract

  The Harten, Lax, and van Leer with contact restoration (HLLC) scheme has been modified and extended in conjunction with time-derivative preconditioning to compute How problems at all speeds. It is found that a simple modification of signal velocities in the HLLC scheme based on the eigenvalues of the preconditioned system is only needed to reduce excessive numerical diffusion at the low Mach number. The modified scheme has been implemented and used to compute a variety of flow problems in both two and three dimensions on unstructured grids. Numerical results obtained indicate that the modified HLLC scheme is accurate, robust, and efficient for flow calculations across the Mach-number range.

  Abstract
  The Harten, Lax, and van Leer with contact restoration (HLLC) scheme has been modified and extended in conjunction with time-derivative preconditioning to compute How problems [...]

  • 8
  •  read

• Comparison of Classical And Weakly-singular Representations of Freesurface Potential Flows

  C. Yang, R. Lohner, F. Noblesse

  International Journal of Offshore and Polar Engineering (2005). Vol. 15 (4), pp. 241-248

  
  

  Abstract

  Within the potential-flow framework, a flow in a domain is determined in terms of the flow at the boundary surface of the flow domain by means of a classical boundary-integral representation, which defines the flow potential фin terms of a Green function G and its gradient ▼G. An alternative flow representation is the weakly-singular representation (which defines фin terms of G and a vector Green function G associated to G via the relation ▼×G = ▼G) given by the authors for diffraction radiation by a ship advancing in regular waves. The alternative mathematical representations of far-field waves associated with the classical and weakly-singular potential representations are compared here in the special case of steady flows.

  Abstract
  Within the potential-flow framework, a flow in a domain is determined in terms of the flow at the boundary surface of the flow domain by means of a classical boundary-integral [...]

  • 2
  •  read

• Adaptividad En El Método Sin Malla De Puntos Finitos

  F. Perazzo, R. Lohner, N. Ipinza

  Mecánica Computacional (2006). Vol. XXV (11), pp. 1039-1050

  
  

  Abstract

  En este trabajo se presentan un estimador del error a posteriori y un proceso de refinamiento adaptivo para el método sin malla de puntos finitos (MPF). El indicador del error se formula a partir de la evaluación del funcional de mínimos cuadrados, utilizado en el cálculo de la función de forma. Nuevos grados de libertad o nodos adicionales pueden ser incorporados sin dificultad en las regiones donde el estimador del error presenta un valor elevado, mediante las técnicas de refinamiento h y p. La validez del estimador del error propuesto se demuestra, mediante el desarrollo de problemas de la mecánica de sólidos y fluidos tanto 2D como 3D, utilizando un proceso de refinamiento adaptivo de la solución

  Abstract
  En este trabajo se presentan un estimador del error a posteriori y un proceso de refinamiento adaptivo para el método sin malla de puntos finitos (MPF). El indicador [...]

  • 0
  •  read

• 30 Years of FCT: Status and Directions

  R. Lohner, J. Baum

  (2005). Flux-Corrected Transport. Scientific Computation. Springer, Berlin, Heidelberg

  
  

  Abstract

  A somewhat historical perspective of the use of FCT for fluid dynamics is given. The particular emphasis is on large-scale blast problems. A comparison with other high-resolution CFD solvers is included to highlight the differences between them, as well as the relative cost. Results from test runs, as well as several relevant production runs are shown. Outstanding issues that deserve further investigation are identified.

  Abstract
  A somewhat historical perspective of the use of FCT for fluid dynamics is given. The particular emphasis is on large-scale blast problems. A comparison with other high-resolution [...]

  • 0
  •  read