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Siegfried Raasch - One of the best experts on this subject based on the ideXlab platform.
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An Improved Surface Boundary Condition for Large-Eddy Simulations Based on Monin–Obukhov Similarity Theory: Evaluation and Consequences for Grid Convergence in Neutral and Stable Conditions
Boundary-Layer Meteorology, 2020Co-Authors: Björn Maronga, Christoph Knigge, Siegfried RaaschAbstract:Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first Grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing Grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves Grid Convergence of these quantities. The LES data indicate improved Grid Convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved Grid Convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the Grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
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an improved surface boundary condition for large eddy simulations based on monin obukhov similarity theory evaluation and consequences for Grid Convergence in neutral and stable conditions
Boundary-Layer Meteorology, 2020Co-Authors: Björn Maronga, Christoph Knigge, Siegfried RaaschAbstract:Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first Grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing Grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves Grid Convergence of these quantities. The LES data indicate improved Grid Convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved Grid Convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the Grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
Björn Maronga - One of the best experts on this subject based on the ideXlab platform.
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An Improved Surface Boundary Condition for Large-Eddy Simulations Based on Monin–Obukhov Similarity Theory: Evaluation and Consequences for Grid Convergence in Neutral and Stable Conditions
Boundary-Layer Meteorology, 2020Co-Authors: Björn Maronga, Christoph Knigge, Siegfried RaaschAbstract:Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first Grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing Grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves Grid Convergence of these quantities. The LES data indicate improved Grid Convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved Grid Convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the Grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
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an improved surface boundary condition for large eddy simulations based on monin obukhov similarity theory evaluation and consequences for Grid Convergence in neutral and stable conditions
Boundary-Layer Meteorology, 2020Co-Authors: Björn Maronga, Christoph Knigge, Siegfried RaaschAbstract:Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first Grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing Grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves Grid Convergence of these quantities. The LES data indicate improved Grid Convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved Grid Convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the Grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
Christoph Knigge - One of the best experts on this subject based on the ideXlab platform.
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An Improved Surface Boundary Condition for Large-Eddy Simulations Based on Monin–Obukhov Similarity Theory: Evaluation and Consequences for Grid Convergence in Neutral and Stable Conditions
Boundary-Layer Meteorology, 2020Co-Authors: Björn Maronga, Christoph Knigge, Siegfried RaaschAbstract:Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first Grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing Grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves Grid Convergence of these quantities. The LES data indicate improved Grid Convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved Grid Convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the Grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
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an improved surface boundary condition for large eddy simulations based on monin obukhov similarity theory evaluation and consequences for Grid Convergence in neutral and stable conditions
Boundary-Layer Meteorology, 2020Co-Authors: Björn Maronga, Christoph Knigge, Siegfried RaaschAbstract:Monin–Obukhov similarity theory is used in large-eddy simulation (LES) models as a surface boundary condition to predict the surface shear stress and scalar fluxes based on the gradients between the surface and the first Grid level above the surface. We outline deficiencies of this methodology, such as the systematical underestimation of the surface shear stress, and propose a modified boundary condition to correct for this issue. The proposed boundary condition is applied to a set of LES for both neutral and stable boundary layers with successively decreasing Grid spacing. The results indicate that the proposed boundary condition reliably corrects the surface shear stress and the sensible heat flux, and improves Grid Convergence of these quantities. The LES data indicate improved Grid Convergence for the surface shear stress, more so than for the surface heat flux. This is either due to a limited performance of the Monin–Obukhov similarity functions or due to problems in the LES model in representing stable conditions. Furthermore, we find that the correction achieved using the proposed boundary condition does not lead to improved Grid Convergence of the wind-speed and temperature profiles. From this we conclude that the sensitivity of the wind-speed and temperature profiles in the LES model to the Grid spacing is more likely related to under-resolved near-surface gradients and turbulent mixing at the boundary-layer top, to the SGS model formulation, and/or to numerical issues, and not to deficiencies due to the use of improper surface boundary conditions.
Christopher L. Rumsey - One of the best experts on this subject based on the ideXlab platform.
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Grid Convergence of reynolds averaged navier stokes solutions for benchmark flows in two dimensions
AIAA Journal, 2016Co-Authors: Boris Diskin, Christopher L. Rumsey, James L Thomas, Axel SchwoppeAbstract:A detailed Grid-Convergence study has been conducted to establish reference solutions corresponding to the one-equation linear eddy-viscosity Spalart–Allmaras turbulence model for two-dimensional turbulent flows around the NACA 0012 airfoil and a flat plate. The study involved the three widely used codes CFL3D (NASA), FUN3D (NASA), and TAU (DLR, The German Aerospace Center), as well as families of uniformly refined structured Grids that differed in the Grid density patterns. Solutions computed by different codes on different Grid families appeared to converge to the same continuous limit but exhibited strikingly different Convergence characteristics. The Grid resolution in the vicinity of geometric singularities, such as a sharp trailing edge, was found to be the major factor affecting accuracy and Convergence of discrete solutions; the effects of this local Grid resolution were more prominent than differences in discretization schemes and/or Grid elements. The results reported for these relatively simple...
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Grid Convergence for turbulent flows invited
53rd AIAA Aerospace Sciences Meeting, 2015Co-Authors: Boris Diskin, Christopher L. Rumsey, James L Thomas, Axel SchwoppeAbstract:A detailed Grid Convergence study has been conducted to establish accurate reference solutions corresponding to the one-equation linear eddy-viscosity Spalart-Allmaras turbulence model for two dimensional turbulent flows around the NACA 0012 airfoil and a flat plate. The study involved three widely used codes, CFL3D (NASA), FUN3D (NASA), and TAU (DLR), and families of uniformly refined structured Grids that differ in the Grid density patterns. Solutions computed by different codes on different Grid families appear to converge to the same continuous limit, but exhibit different Convergence characteristics. The Grid resolution in the vicinity of geometric singularities, such as a sharp trailing edge, is found to be the major factor affecting accuracy and Convergence of discrete solutions; the effects of this local Grid resolution are more prominent than differences in discretization schemes and/or Grid elements. The results reported for these relatively simple turbulent flows demonstrate that CFL3D, FUN3D, and TAU solutions are very accurate on the finest Grids used in the study, but even those Grids are not sufficient to conclusively establish an asymptotic Convergence order.
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Application of the FUN3D Solver to the 4th AIAA Drag Prediction Workshop
Journal of Aircraft, 2014Co-Authors: Elizabeth M. Lee-rausch, Dana P. Hammond, Eric J. Nielsen, S. Z. Pirzadeh, Christopher L. RumseyAbstract:FUN3D Navier–Stokes solutions were computed for the 4th AIAA Drag Prediction Workshop Grid-Convergence study, downwash study, and Reynolds-number study on a set of node-based mixed-element Grids. All of the baseline tetrahedral Grids were generated with the VGrid (developmental) advancing-layer and advancing-front Grid-generation software package following the Gridding guidelines developed for the workshop. With maximum Grid sizes exceeding 100 million nodes, the Grid-Convergence study was particularly challenging for the node-based unstructured Grid generators and flow solvers. At the time of the workshop, the super-fine Grid with 105 million nodes and 600 million tetrahedral elements was the largest Grid known to have been generated using VGrid. FUN3D Version 11.0 has a completely new pre- and postprocessing paradigm that has been incorporated directly into the solver and functions entirely in a parallel, distributed-memory environment. This feature allowed for practical preprocessing and solution times...
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Application of the FUN3D Unstructured-Grid Navier-Stokes Solver to the 4th AIAA Drag Prediction Workshop Cases
28th AIAA Applied Aerodynamics Conference, 2010Co-Authors: Elizabeth M. Lee-rausch, Dana P. Hammond, Eric J. Nielsen, S. Z. Pirzadeh, Christopher L. RumseyAbstract:FUN3D Navier-Stokes solutions were computed for the 4th AIAA Drag Prediction Workshop Grid Convergence study, downwash study, and Reynolds number study on a set of node-based mixed-element Grids. All of the baseline tetrahedral Grids were generated with the VGrid (developmental) advancing-layer and advancing-front Grid generation software package following the Gridding guidelines developed for the workshop. With maximum Grid sizes exceeding 100 million nodes, the Grid Convergence study was particularly challenging for the node-based unstructured Grid generators and flow solvers. At the time of the workshop, the super-fine Grid with 105 million nodes and 600 million elements was the largest Grid known to have been generated using VGrid. FUN3D Version 11.0 has a completely new pre- and post-processing paradigm that has been incorporated directly into the solver and functions entirely in a parallel, distributed memory environment. This feature allowed for practical pre-processing and solution times on the largest unstructured-Grid size requested for the workshop. For the constant-lift Grid Convergence case, the Convergence of total drag is approximately second-order on the finest three Grids. The variation in total drag between the finest two Grids is only 2 counts. At the finest Grid levels, only small variations in wing and tail pressure distributions are seen with Grid refinement. Similarly, a small wing side-of-body separation also shows little variation at the finest Grid levels. Overall, the FUN3D results compare well with the structured-Grid code CFL3D. The FUN3D downwash study and Reynolds number study results compare well with the range of results shown in the workshop presentations.
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CFL3D/OVERFLOW Results for DLR-F6 Wing/Body and Drag Prediction Workshop Wing
Journal of Aircraft, 2008Co-Authors: Anthony J. Sclafani, Christopher L. Rumsey, John C Vassberg, Neal A. Harrison, S. Melissa Rivers, Joseph H. MorrisonAbstract:A series of overset Grids was generated in response to the Third AIAA CFD Drag Prediction Workshop (DPW-III) which preceded the 25th Applied Aerodynamics Conference in June 2006. DPW-III focused on accurate drag prediction for wing/body and wing-alone configurations. The Grid series built for each configuration consists of a coarse, medium, fine, and extra-fine mesh. The medium mesh is first constructed using the current state of best practices for overset Grid generation. The medium mesh is then coarsened and enhanced by applying a factor of 1.5 to each (I, J, K) dimension. The resulting set of parametrically equivalent Grids increase in size by a factor of roughly 3.5 from one level to the next denser level. Computational fluid dynamics simulations were performed on the overset Grids using two different Reynolds-averaged Navier-Stokes flow solvers: CFL3D and OVERFLOW. The results were postprocessed using Richardson extrapolation to approximate Grid-converged values of lift, drag, pitching moment, and angle of attack at the design condition. This technique appears to work well if the solution does not contain large regions of separated flow (similar to that seen in the DLR-F6 results) and appropriate Grid densities are selected. The extra-fine Grid data helped to establish asymptotic Grid Convergence for both the OVERFLOW FX2B wing/body results and the OVERFLOW DPW-W1/W2 wing-alone results. More CFL3D data are needed to establish Grid Convergence trends. The medium Grid was used beyond the Grid Convergence study by running each configuration at several angles of attack so drag polars and lift/pitching moment curves could be evaluated. The alpha sweep results are used to compare data across configurations as well as across flow solvers. With the exception of the wing/body drag polar, the two codes compare well qualitatively showing consistent incremental trends and similar wing pressure comparisons.
Christopher D Cooper - One of the best experts on this subject based on the ideXlab platform.
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poisson boltzmann model for protein surface electrostatic interactions and Grid Convergence study using the pygbe code
Computer Physics Communications, 2016Co-Authors: Christopher D Cooper, Lorena A BarbaAbstract:Abstract Interactions between surfaces and proteins occur in many vital processes and are crucial in biotechnology: the ability to control specific interactions is essential in fields like biomaterials, biomedical implants and biosensors. In the latter case, biosensor sensitivity hinges on ligand proteins adsorbing on bioactive surfaces with a favorable orientation, exposing reaction sites to target molecules. Protein adsorption, being a free-energy-driven process, is difficult to study experimentally. This paper develops and evaluates a computational model to study electrostatic interactions of proteins and charged nanosurfaces, via the Poisson–Boltzmann equation. We extended the implicit-solvent model used in the open-source code PyGBe to include surfaces of imposed charge or potential. This code solves the boundary integral formulation of the Poisson–Boltzmann equation, discretized with surface elements. PyGBe has at its core a treecode-accelerated Krylov iterative solver, resulting in O ( N log N ) scaling, with further acceleration on hardware via multi-threaded execution on gpu s. It computes solvation and surface free energies, providing a framework for studying the effect of electrostatics on adsorption. We derived an analytical solution for a spherical charged surface interacting with a spherical dielectric cavity, and used it in a Grid-Convergence study to build evidence on the correctness of our approach. The study showed the error decaying with the average area of the boundary elements, i.e., the method is O ( 1 / N ) , which is consistent with our previous verification studies using PyGBe. We also studied Grid-Convergence using a real molecular geometry (protein G B1 D4 ′ ), in this case using Richardson extrapolation (in the absence of an analytical solution) and confirmed the O ( 1 / N ) scaling. With this work, we can now access a completely new family of problems, which no other major bioelectrostatics solver, e.g. APBS, is capable of dealing with. PyGBe is open-source under an MIT license and is hosted under version control at https://github.com/barbagroup/pygbe . To supplement this paper, we prepared “reproducibility packages” consisting of running and post-processing scripts in Python for replicating the Grid-Convergence studies, all the way to generating the final plots, with a single command.
