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Papers by Paul Fischer

Research paper thumbnail of SHARP Assembly-Scale Multiphysics Demonstration Simulations

Research paper thumbnail of Effects of Mesh Density and Flow Conditioning in Simulating 7-Pin Wire Wrapped Fuel Pins

Volume 2: Fuel Cycle and High Level Waste Management; Computational Fluid Dynamics, Neutronics Methods and Coupled Codes; Student Paper Competition, 2008

In response to the goals outlined by the U.S. Department of Energy's Global Nucle... more In response to the goals outlined by the U.S. Department of Energy's Global Nuclear Energy Partnership program, Argonne National Laboratory has initiated an effort to create an integrated multi-physics multi-resolution thermal hydraulic simulation tool package for the evaluation of nuclear power plant design and safety. As part of this effort, the applicability of a variety of thermal hydraulic analysis methods

Research paper thumbnail of Parallel Simulation of High Reynolds Number Vascular Flows

Parallel Computational Fluid Dynamics 2005, 2006

Research paper thumbnail of Direct simulations of transitional flow in a patient-specific carotid bifurcation with stenosis

2008 Third International Forum on Strategic Technologies, 2008

Diseased vessels experience a significantly different biomechanical environment than healthy vess... more Diseased vessels experience a significantly different biomechanical environment than healthy vessels due to the presence of transitional and turbulent flow, which may be responsible for cell damage or plaque disruption. With an objective of investigating turbulent characteristics in a diseased vessel, direct numerical simulations were conducted based on a patient-specific carotid bifurcation with a severe stenosis under pulsatile flow conditions.

Research paper thumbnail of Intravascular Turbulence in a Diseased Carotid Bifurcation: Numerical Study

Research paper thumbnail of Surface Reconstruction of In Vivo Geometry Based on Medical Images Using Multilevel Radial Basis Functions

Advances in Bioengineering, 2004

Compactly supported radial basis functions (RBFs) were used for surface reconstruction of in vivo... more Compactly supported radial basis functions (RBFs) were used for surface reconstruction of in vivo geometry, translated from two dimensional (2D) medical images. RBFs provide a flexible approach to interpolation and approximation for problems featuring unstructured ...

Research paper thumbnail of Status report on high fidelity reactor simulation

This report presents the effort under way at Argonne National Laboratory toward a comprehensive, ... more This report presents the effort under way at Argonne National Laboratory toward a comprehensive, integrated computational tool intended mainly for the high-fidelity simulation of sodium-cooled fast reactors. The main activities carried out involved neutronics, thermal hydraulics, coupling strategies, software architecture, and high-performance computing. A new neutronics code, UNIC, is being developed. The first phase involves the application of a spherical

Research paper thumbnail of Velocity and Wall Shear Stress in an Arteriovenous Graft Under Steady Flow Conditions

Research paper thumbnail of Direct Numerical Simulations of Transition to Turbulent Flow in a Subject-Specific Arteriovenous Graft

Research paper thumbnail of Development of a General Geometry Large Eddy Simulation Code

We present a general geometry large eddy simulation (LES) code based upon three-dimensional spect... more We present a general geometry large eddy simulation (LES) code based upon three-dimensional spectral elements. We employ a two parameter PDE-motivated filter which can be tuned for both order and cut-off wave number, and which satisfies boundary conditions appropriate for complex geometry flows. The SGS terms are treated by the Lilly version of the dynamic subgrid scale model. The filter has been tested for complex geometry flows, including a horseshoe vortex flow in a flat-plate/appendage configuration. We have observed that the filter is capable of retaining the large structures, such as hairpin vortices, while removing the high energy ``noise'' in under-resolved regions. The SGS model and the filter described above have been combined into a full LES code. We are currently validating the code against 3-D channel flow benchmark results.

Research paper thumbnail of The effects of order and resolution on the accuracy and performance of LES

Research paper thumbnail of Numerical simulation of carotid bifurcation hemodynamics

Research paper thumbnail of Velocity Measurements Inside a Hemodialysis Graft In Vitro Model

... Illinois Francis Loth University of Illinois at Chicago Department of Mechanical and Industri... more ... Illinois Francis Loth University of Illinois at Chicago Department of Mechanical and Industrial Engineering Chicago, Illinois Hisham ... Illinois Jennifer K. Grogan University of Chicago Department of Surgery Chicago, Illinois Sang ...

Research paper thumbnail of SHARP Reactor Performance and Safety Analysis Simulation Suite, invited

Research paper thumbnail of Polynomial regression with derivative information in nuclear reactor uncertainty quantification

We introduce a novel technique of uncertainty quantification using polynomial regression with der... more We introduce a novel technique of uncertainty quantification using polynomial regression with derivative information and apply it to analyze the performance of a model of a sodium-cooled fast reactor. We construct a surrogate model as a goal-oriented projection onto an incomplete space of polynomials, find coordinates of projection by collocation, and use derivative information to reduce the number of sample points required by the collocation procedure. This surrogate model can be used to estimate range, sensitivities and the statistical distribution of the output. Numerical experiments show that the suggested approach is significantly more computationally efficient than random sampling, or approaches that do not use derivative information, and that it has greater precision than linear models.

Research paper thumbnail of Proposed Experiment for Validation of CFD Methods for Advanced SFR Design: Upper Plenum Thermal Striping and Stratification

Research paper thumbnail of Flow Destabilization and Heat Transfer Augmentation in an Array of Grooved Passages With Developing Flow

Research paper thumbnail of Turbulence and Dynamos in a Rotating Disks

Research paper thumbnail of Large-eddy simulations of stratification layer erosion by a jet

Research paper thumbnail of Accurate prediction of the wall shear stress in rod bundles with the spectral element method at high Reynolds numbers

ABSTRACT Resolving flow near walls is critical to reproducing the high rates of shear that genera... more ABSTRACT Resolving flow near walls is critical to reproducing the high rates of shear that generate turbulence in high Reynolds number, wall-bounded flows. In the present study, we examine the resolution requirements for correctly reproducing mean flow quantities and wall shear stress distribution in a large eddy simulation using the spectral element method. In this method, derivatives are only guaranteed in a weak sense, and the same is true of quantities composed of derivatives, such as the wall shear stress. We are interested in what is required to resolve the wall shear stress in problems that lack homogeneity in at least one direction. The problem of interest is that of parallel flow through a rod bundle configuration. Several meshes for this problem are systematically compared. In addition, we conduct a study of channel flow in order to examine the issues in a canonical flow that contains spanwise homogeneity missing in rod bundle flow. In the case of channel flow, we compare several meshes and subgrid scale models. We find that typical measures of accuracy, such as the law of the wall, are not sufficient for determining the resolution of quantities that vary along the wall. Spanwise variation of wall shear stress in underresolved flows is characterized by spikes—physical points without well-defined derivatives of the velocity—found at element boundaries. These spikes are not particular to any subgrid scale model and are the unavoidable consequence of underresolution. Accurately reproducing the wall shear stress distribution, while minimizing the computational costs, requires increasing the number of elements along the wall (local h -refinement) and using very high order (N=19N=19) basis functions (p-refinement). We suggest that while these requirements are not easily generalized to grid spacing guidelines, one can apply a general process: construct a mesh that progressively increases elements along any walls, and increase the order of basis functions until the distribution of wall shear stress or any other quantity of interest is smooth.

Research paper thumbnail of SHARP Assembly-Scale Multiphysics Demonstration Simulations

Research paper thumbnail of Effects of Mesh Density and Flow Conditioning in Simulating 7-Pin Wire Wrapped Fuel Pins

Volume 2: Fuel Cycle and High Level Waste Management; Computational Fluid Dynamics, Neutronics Methods and Coupled Codes; Student Paper Competition, 2008

In response to the goals outlined by the U.S. Department of Energy's Global Nucle... more In response to the goals outlined by the U.S. Department of Energy's Global Nuclear Energy Partnership program, Argonne National Laboratory has initiated an effort to create an integrated multi-physics multi-resolution thermal hydraulic simulation tool package for the evaluation of nuclear power plant design and safety. As part of this effort, the applicability of a variety of thermal hydraulic analysis methods

Research paper thumbnail of Parallel Simulation of High Reynolds Number Vascular Flows

Parallel Computational Fluid Dynamics 2005, 2006

Research paper thumbnail of Direct simulations of transitional flow in a patient-specific carotid bifurcation with stenosis

2008 Third International Forum on Strategic Technologies, 2008

Diseased vessels experience a significantly different biomechanical environment than healthy vess... more Diseased vessels experience a significantly different biomechanical environment than healthy vessels due to the presence of transitional and turbulent flow, which may be responsible for cell damage or plaque disruption. With an objective of investigating turbulent characteristics in a diseased vessel, direct numerical simulations were conducted based on a patient-specific carotid bifurcation with a severe stenosis under pulsatile flow conditions.

Research paper thumbnail of Intravascular Turbulence in a Diseased Carotid Bifurcation: Numerical Study

Research paper thumbnail of Surface Reconstruction of In Vivo Geometry Based on Medical Images Using Multilevel Radial Basis Functions

Advances in Bioengineering, 2004

Compactly supported radial basis functions (RBFs) were used for surface reconstruction of in vivo... more Compactly supported radial basis functions (RBFs) were used for surface reconstruction of in vivo geometry, translated from two dimensional (2D) medical images. RBFs provide a flexible approach to interpolation and approximation for problems featuring unstructured ...

Research paper thumbnail of Status report on high fidelity reactor simulation

This report presents the effort under way at Argonne National Laboratory toward a comprehensive, ... more This report presents the effort under way at Argonne National Laboratory toward a comprehensive, integrated computational tool intended mainly for the high-fidelity simulation of sodium-cooled fast reactors. The main activities carried out involved neutronics, thermal hydraulics, coupling strategies, software architecture, and high-performance computing. A new neutronics code, UNIC, is being developed. The first phase involves the application of a spherical

Research paper thumbnail of Velocity and Wall Shear Stress in an Arteriovenous Graft Under Steady Flow Conditions

Research paper thumbnail of Direct Numerical Simulations of Transition to Turbulent Flow in a Subject-Specific Arteriovenous Graft

Research paper thumbnail of Development of a General Geometry Large Eddy Simulation Code

We present a general geometry large eddy simulation (LES) code based upon three-dimensional spect... more We present a general geometry large eddy simulation (LES) code based upon three-dimensional spectral elements. We employ a two parameter PDE-motivated filter which can be tuned for both order and cut-off wave number, and which satisfies boundary conditions appropriate for complex geometry flows. The SGS terms are treated by the Lilly version of the dynamic subgrid scale model. The filter has been tested for complex geometry flows, including a horseshoe vortex flow in a flat-plate/appendage configuration. We have observed that the filter is capable of retaining the large structures, such as hairpin vortices, while removing the high energy ``noise'' in under-resolved regions. The SGS model and the filter described above have been combined into a full LES code. We are currently validating the code against 3-D channel flow benchmark results.

Research paper thumbnail of The effects of order and resolution on the accuracy and performance of LES

Research paper thumbnail of Numerical simulation of carotid bifurcation hemodynamics

Research paper thumbnail of Velocity Measurements Inside a Hemodialysis Graft In Vitro Model

... Illinois Francis Loth University of Illinois at Chicago Department of Mechanical and Industri... more ... Illinois Francis Loth University of Illinois at Chicago Department of Mechanical and Industrial Engineering Chicago, Illinois Hisham ... Illinois Jennifer K. Grogan University of Chicago Department of Surgery Chicago, Illinois Sang ...

Research paper thumbnail of SHARP Reactor Performance and Safety Analysis Simulation Suite, invited

Research paper thumbnail of Polynomial regression with derivative information in nuclear reactor uncertainty quantification

We introduce a novel technique of uncertainty quantification using polynomial regression with der... more We introduce a novel technique of uncertainty quantification using polynomial regression with derivative information and apply it to analyze the performance of a model of a sodium-cooled fast reactor. We construct a surrogate model as a goal-oriented projection onto an incomplete space of polynomials, find coordinates of projection by collocation, and use derivative information to reduce the number of sample points required by the collocation procedure. This surrogate model can be used to estimate range, sensitivities and the statistical distribution of the output. Numerical experiments show that the suggested approach is significantly more computationally efficient than random sampling, or approaches that do not use derivative information, and that it has greater precision than linear models.

Research paper thumbnail of Proposed Experiment for Validation of CFD Methods for Advanced SFR Design: Upper Plenum Thermal Striping and Stratification

Research paper thumbnail of Flow Destabilization and Heat Transfer Augmentation in an Array of Grooved Passages With Developing Flow

Research paper thumbnail of Turbulence and Dynamos in a Rotating Disks

Research paper thumbnail of Large-eddy simulations of stratification layer erosion by a jet

Research paper thumbnail of Accurate prediction of the wall shear stress in rod bundles with the spectral element method at high Reynolds numbers

ABSTRACT Resolving flow near walls is critical to reproducing the high rates of shear that genera... more ABSTRACT Resolving flow near walls is critical to reproducing the high rates of shear that generate turbulence in high Reynolds number, wall-bounded flows. In the present study, we examine the resolution requirements for correctly reproducing mean flow quantities and wall shear stress distribution in a large eddy simulation using the spectral element method. In this method, derivatives are only guaranteed in a weak sense, and the same is true of quantities composed of derivatives, such as the wall shear stress. We are interested in what is required to resolve the wall shear stress in problems that lack homogeneity in at least one direction. The problem of interest is that of parallel flow through a rod bundle configuration. Several meshes for this problem are systematically compared. In addition, we conduct a study of channel flow in order to examine the issues in a canonical flow that contains spanwise homogeneity missing in rod bundle flow. In the case of channel flow, we compare several meshes and subgrid scale models. We find that typical measures of accuracy, such as the law of the wall, are not sufficient for determining the resolution of quantities that vary along the wall. Spanwise variation of wall shear stress in underresolved flows is characterized by spikes—physical points without well-defined derivatives of the velocity—found at element boundaries. These spikes are not particular to any subgrid scale model and are the unavoidable consequence of underresolution. Accurately reproducing the wall shear stress distribution, while minimizing the computational costs, requires increasing the number of elements along the wall (local h -refinement) and using very high order (N=19N=19) basis functions (p-refinement). We suggest that while these requirements are not easily generalized to grid spacing guidelines, one can apply a general process: construct a mesh that progressively increases elements along any walls, and increase the order of basis functions until the distribution of wall shear stress or any other quantity of interest is smooth.