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• A stabilized pseudo‐shell approach for surface parametrization in CFD design problems

  O. Soto, R. Lohner, C. Yang

  Commun. Numer. Meth. Engng (2002). Vol. 18 (4), pp. 251-258

  
  

  Abstract

  A surface representation for computational fluid dynamics (CFD) shape design problems is presented. The surface representation is based on the solution of a simplified pseudo‐shell problem on the surface to be optimized. A stabilized finite element formulation is used to perform this step. The methodology has the advantage of being completely independent of the CAD representation. Moreover, the user does not have to predefine any set of shape functions to parameterize the surface. The scheme uses a reasonable discretization of the surface to automatically build the shape deformation modes, by using the pseudo‐shell approach and the design parameter positions. Almost every point of the surface grid can be chosen as design parameter, which leads to a very rich design space. Most of the design variables are chosen in an automatic way, which makes the scheme easy to use. Furthermore, the surface grid is not distorted through the design cycles which avoids remeshing procedures. An example is presented to demonstrate the proposed methodology.

  Abstract
  A surface representation for computational fluid dynamics (CFD) shape design problems is presented. The surface representation is based on the solution of a simplified pseudo‐shell [...]

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• Minimization of indirect addressing for edge‐based field solvers

  R. Lohner, M. Galle

  Commun. Numer. Meth. Engng (2002). Vol. 18 (5), pp. 335-343

  
  

  Abstract

  A point renumbering scheme that halves the number of indirect addressing operations required for edge‐based field solvers has been developed. At the same time, an alternative (and entirely serendipitous) assembly of right‐hand side vectors was found to enhance performance considerably. The new loop structure is especially attractive for vector‐parallel machines with direct memory access. Timings on the CRAY‐SV1 and NEC‐SX5 show that for Laplacian loops, which appear in many CFD and CEM codes, CPU can be reduced considerably. The new renumbering scheme does not require any new data structures or arrays, allowing for progressive porting of loops in large‐scale production codes.

  Abstract
  A point renumbering scheme that halves the number of indirect addressing operations required for edge‐based field solvers has been developed. At the same time, an alternative [...]

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• Calculation of Ship Sinkage and Trim Using a Finite Element Method and Unstructured Grids

  C. Yang, R. Lohner

  Int. J. Computational Fluid Dynamics (2002). Vol. 16 (3), pp. 217-227

  
  

  Abstract

  An unstructured grid-based, parallel free-surface flow solver has been extended to account for sinkage and trim effects in the calculation of steady ship waves. The overall scheme of the solver combines a finite-element, equal-order, projection-type three-dimensional incompressible flow solver with a finite element, two-dimensional advection equation solver for the free surface equation. The sinkage and trim, wave profiles, and wave drag computed using the present approach are in good agreement with experimental measurements for two hull forms at a wide range of Froude numbers. Numerical predictions indicate significant differences between the wave drag for a ship fixed in at-rest position and free to sink and trim, in agreement with experimental observations.

  Abstract
  An unstructured grid-based, parallel free-surface flow solver has been extended to account for sinkage and trim effects in the calculation of steady ship waves. The overall [...]

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• A feature‐preserving volumetric technique to merge surface triangulations

  J. Cebral, F. Camelli, R. Lohner

  Int. J. Numer. Meth. Engng. (2002). Vol. 55 (2), pp. 177-190

  
  

  Abstract

  Several extensions and improvements to surface merging procedures based on the extraction of iso-surfaces from a distance map defined on an adaptive background grid are presented. The main objective is to extend the application of these algorithms to surfaces with sharp edges and corners. In order to deal with objects of different length scales, the initial background grids are created using a Delaunay triangulation method and local voxelizations. A point enrichment technique that introduces points into the background grid along detected surface features such as ridges is used to ensure that these features are preserved in the final merged surface. The surface merging methodology is extended to include other Boolean operations between surface triangulations. The iso-surface extraction algorithms are modified to obtain the correct iso-surface for multi-component objects. The procedures are demonstrated with various examples, ranging from simple geometrical entities to complex engineering applications. The present algorithms allow realistic modelling of a large number of complex engineering geometries using overlapping components defined discretely, i.e. via surface triangulations. This capability is very useful for grid generation starting from data originated in measurements or images.

  Abstract
  Several extensions and improvements to surface merging procedures based on the extraction of iso-surfaces from a distance map defined on an adaptive background grid are presented. [...]

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• Comparisons of model simulations with observations of mean flow and turbulence within simple obstacle arrays

  S. Hanna, S. Tehranian, B. Carissimo, R. Macdonald, R. Lohner

  Atmospheric Environment (2002). Vol. 36 (32), pp. 5067-5079

  
  

  Abstract

  A three-dimensional numerical code with unstructured tetrahedral grids, the finite element flow solver (FEFLO), was used to simulate the mean flow and the turbulence within obstacle array configurations consisting of simple cubical elements. Model simulations were compared with observations from a hydraulic water flume at the University of Waterloo. FEFLO was run in large eddy simulation mode, using the Smagorinsky closure model, to resolve the larger scales of the flow field. There were four experiment test cases consisting of square and staggered arrays of cubical obstacles with separations of 1.5 and 0.5 obstacle heights. The mean velocity profile for the incoming neutral boundary layer was approximated by a power law, and the turbulent fluctuations in the approach flow were generated using a Monte Carlo model. The numerical simulations were able to capture, within 40% on average, the general characteristics of the mean flow and the turbulence, such as the strong mean wind shears and the maximum turbulence at the elevation of the obstacles and the nearly constant mean wind and the 50% reduction in the turbulent velocity within the obstacle canopy. As expected, the mean wind speeds were significantly decreased (by about a factor of two or three) in the array with closer obstacle packing. It was found that, a “street canyon” effect was more obvious for the square arrays, with higher flow speeds in between the obstacles, than for the staggered arrays.

  Abstract
  A three-dimensional numerical code with unstructured tetrahedral grids, the finite element flow solver (FEFLO), was used to simulate the mean flow and the turbulence within [...]

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• Fluid Dynamics of Flapping Aquatic Flight in the Bird Wrasse: Three-Dimensional Unsteady Computations With Fin Deformation

  R. Ramamurti, W. Sandberg, R. Lohner, J. Walker, M. Westneat

  Journal of Experimental Biology (2002). Vol. 205 (9), pp. 299-3008

  
  

  Abstract

  Many fishes that swim with the paired pectoral fins use fin-stroke parameters that produce thrust force from lift in a mechanism of underwater flight. These locomotor mechanisms are of interest to behavioral biologists, biomechanics researchers and engineers. In the present study, we performed the first three-dimensional unsteady computations of fish swimming with oscillating and deforming fins. The objective of these computations was to investigate the fluid dynamics of force production associated with the flapping aquatic flight of the bird wrasse Gomphosus varius. For this computational work, we used the geometry of the wrasse and its pectoral fin, and previously measured fin kinematics, as the starting points for computational investigation of three-dimensional (3-D) unsteady fluid dynamics. We performed a 3-D steady computation and a complete set of 3-D quasisteady computations for a range of pectoral fin positions and surface velocities. An unstructured, grid-based, unsteady Navier-Stokes solver with automatic adaptive remeshing was then used to compute the unsteady flow about the wrasse through several complete cycles of pectoral fin oscillation. The shape deformation of the pectoral fin throughout the oscillation was taken from the experimental kinematics. The pressure distribution on the body of the bird wrasse and its pectoral fins was computed and integrated to give body and fin forces which were decomposed into lift and thrust. The velocity field variation on the surface of the wrasse body, on the pectoral fins and in the near-wake was computed throughout the swimming cycle. We compared our computational results for the steady, quasi-steady and unsteady cases with the experimental data on axial and vertical acceleration obtained from the pectoral fin kinematics experiments. These comparisons show that steady state computations are incapable of describing the fluid dynamics of flapping fins. Quasi-steady state computations, with correct incorporation of the experimental kinematics, are useful when determining trends in force production, but do not provide accurate estimates of the magnitudes of the forces produced. By contrast, unsteady computations about the deforming pectoral fins using experimentally measured fin kinematics were found to give excellent agreement, both in the time history of force production throughout the flapping strokes and in the magnitudes of the generated forces.

  Abstract
  Many fishes that swim with the paired pectoral fins use fin-stroke parameters that produce thrust force from lift in a mechanism of underwater flight. These locomotor mechanisms [...]

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• Blood Flow Modeling in Carotid Arteries with Computational Fluid Dynamics and MR Imaging

  J. Cebral, P. Yim, R. Lohner, O. Soto, P. Choyke

  Academic Radiology (2002). Vol. 9 (11), pp. 1286-1299

  
  

  Abstract

  The authors' goal was to develop a noninvasive method for detailed assessment of blood flow patterns from direct in vivo measurements of vessel anatomy and flow rates. The authors developed a method to construct realistic patient-specific finite element models of blood flow in carotid arteries. Anatomic models are reconstructed from contrast material-enhanced magnetic resonance (MR) angiographic images with a tubular deformable model along each arterial branch. A surface-merging algorithm is used to create a watertight model of the carotid bifurcation for subsequent finite element grid generation, and a fully implicit scheme is used to solve the incompressible Navier-Stokes equations on unstructured grids. Physiologic boundary conditions are derived from cine phase-contrast MR flow velocity measurements at two locations below and above the bifurcation. Vessel wall compliance is incorporated by means of fluid-solid interaction algorithms. The method was tested on imaging data from a healthy subject and a patient with mild stenosis. Finite element grids were successfully generated, and pulsatile blood flow calculations were performed. Computed and measured velocity profiles show good agreement. Flow patterns and wall shear stress distributions were visualized. Patient-specific computational fluid dynamics modeling based on MR images can be performed robustly and efficiently. Preliminary validation studies in a physical flow-through model suggest that the model is accurate. This method can be used to characterize blood flow patterns in healthy and diseased arteries and may eventually help physicians to supplement imaging-based diagnosis and predict and evaluate the outcome of interventional procedures.

  Abstract
  The authors' goal was to develop a noninvasive method for detailed assessment of blood flow patterns from direct in vivo measurements of vessel anatomy and flow rates. [...]

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• On Incompressible Flow Solvers

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

  (2003). Numerical Simulations of incompressible flows, pp. 50-71

  
  

  Abstract

  A family of low-order finite element solvers for incompressible flows is described. Both the advection and divergence terms are treated using consistent numerical fluxes along edges. Several techniques to accelerate convergence to steady state are explored and compared. The techniques are then used in a fully implicit time-marching scheme that solves a steady problem at every timestep. Several examples demonstrate the usefulness of the developed scehmes.

  Abstract
  A family of low-order finite element solvers for incompressible flows is described. Both the advection and divergence terms are treated using consistent numerical fluxes along [...]

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• Development and Applications of a Coupled CFD/CSD Methodology Using an Embedded CSD Approach

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

  (2012). Computational Fluid Dynamics. Springer, Berlin, Heidelberg

  
  

  Abstract

  This paper describes recent algorithm developments and select applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies. FEFL098 is the CFD code used while DYNA3D handles the CSD portion. FEFL098 solves the time-dependent, compressible Euler and Reynolds-Averaged Navier-Stokes equations on an unstructured mesh of tetrahedral elements. DYNA3D solves explicitly the large deformation, large strain formulation equations on an unstructured grid composed of bricks and hexahedral elements. While the initial coupled algorithm used the so-called ”glued-mesh” approach, where the CFD and CSD faces match identically, failure of this approach to model severe structural deformation, as well as crack propagation in steel and concrete, led us to the development of the so-called ”embedded-mesh” approach. here, the CSD objects float through the CFD domain. While each approach has it’s own advantages, limitations and deficiencies, the embedded approach was proven to be superior for the class of problems modeled here. Critical applications of both approaches are described, including weapon detonation and fragmentation, fragment/airblast interaction with a steel wall, and an external airblast interaction with a generic steel ship hull.

  Abstract
  This paper describes recent algorithm developments and select applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural [...]

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• Development And Applications Of An Embedded CSD Approach For Coupled CFD/CSD Modeling Of Blastlstructure Interactions

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

  WIT Transactions on the Built Environment (2003). Vol. 71 (10), pp. 263-272

  
  

  Abstract

  This paper describes recent developments and select applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural Dynamics (CSD) methodologies. FEFL098 is the CFD code used while DYNA3D handles the CSD portion. FEFL098 solves the time-dependent, compressible Euler and Reynolds-Averaged Navier-Stokes equations on an unstructured mesh of tetrahedral elements. DYNA3D solves explicitly the large deformation, large strain formulation equations on an unstructured grid composed of bricks and hexahedral elements. The initial algorithm constructed to model the coupled processes used the so-called "glued-mesh" approach, where the CFD and CSD faces match identically. Failure of this approach to model severe structural deformations in steel plates, as well as crack growth and propagation in steel and concrete, led us to the development of the so-called "embedded-mesh" approach, where the CSD mesh 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 class of 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 interactions with steel wall structures including a generic steel ship hull and a steel tower.

  Abstract
  This paper describes recent developments and select applications of a program that couples parallel Computational Fluid Dynamics (CFD) and Computational Structural Dynamics [...]

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