Joseph H. Morrison

Data summary from Second AIAA computational fluid dynamics Drag Prediction Workshop

Results from the Second AIAA Drag PredictionWorkshop are summarized. The workshop focused on absolute and configuration delta drag prediction of the DLR, German Aerospace Research Center F6 geometry, which is representative of transport aircraft designed for transonic flight. Both wing–body and wing–body–nacelle–pylon configurations are considered. Comparisons are made using industry relevant test cases that include single-point conditions, drag polars, and drag-rise curves. Drag, lift, and pitching moment predictions from several different Reynolds averagedNavier–Stokes computational fluid dynamics codes are presented and compared to experimental data. Solutions on multiblock structured, unstructured, and overset structured grids using a variety of turbulence models are considered. Results of a grid-refinement study and a comparison of tripped transition vs fully turbulent boundary-layer computations are reported.

Turbulence Model Predictions of Strongly Curved Flow in a U-Duct

The ability of three types of turbulence models to accurately predict the effects of curvature on the flow in a U-duct is studied. An explicit algebraic stress model performs slightly better than one- or two-equation linear eddy viscosity models, although it is necessary to fully account for the variation of the production-to-dissipation-rate ratio in the algebraic stress model formulation. In their original formulations, none of these turbulence models fully captures the suppressed turbulence near the convex wall, whereas a full Reynolds stress model does. Some of the underlying assumptions used in the development of algebraic stress models are investigated and compared with the computed flowfield from the full Reynolds stress model. Through this analysis, the assumption of Reynolds stress anisotropy equilibrium used in the algebraic stress model formulation is found to be incorrect in regions of strong curvature. By the accounting for the local variation of the principal axes of the strain rate tensor, the explicit algebraic stress model correctly predicts the suppressed turbulence in the outer part of the boundary layer near the convex wall.

Abridged Summary of the Third AIAA Computational Fluid Dynamics Drag Prediction Workshop

Results from the Third AIAA Drag Prediction Workshop (DPW-III) are summarized. The workshop focused on the prediction of both absolute and differential drag levels for wing-body and wing-alone configurations that are representative of transonic transport aircraft The baseline DLR-F6 wing-body geometry, previously used in DPW-II, is also augmented with a side-of-body fairing to help reduce the complexity of the flow physics in the wing-body juncture region. In addition, two new wing-alone geometries have been developed for DPW-III. Numerical calculations are performed using industry-relevant test cases that include lift-specific and fixed-alpha flight conditions, as well as full drag polars. Drag, lift, and pitching-moment predictions from numerous Reynolds-averaged Navier-Stokes computational fluid dynamics methods are presented, focused on fully turbulent flows. Solutions are performed on structured, unstructured, and hybrid grid systems. The structured grid sets include point-matched multiblock meshes and overset grid systems. The unstructured and hybrid grid sets are composed of tetrahedral, pyramid, and prismatic elements. Effort was made to provide a high-quality and parametrically consistent family of grids for each grid type about each configuration under study. The wing-body families are composed of a coarse, medium, and fine grid, whereas the wing-alone families also include an extra-fine mesh. These mesh sequences are used to help determine how the provided flow solutions fare with respect to asymptotic grid convergence, and are used to estimate an absolute drag for each configuration.

Summary of the Fourth AIAA CFD Drag Prediction Workshop

Results of the Thrid AIAA Drag Prediction Workshop are summarized. The workshop is focused on the prediction of both absolute and differential drag levels for wing-body and wing-alone configuarations that are representative of transonic transport aircraft.

Summary of Data from the Fifth AIAA CFD Drag Prediction Workshop

Results from the 5, AIAA CFD Drag Prediction Workshop are presented. This workshop is focused on force/moment predictions for the NASA Common research wing-body configuration, including a grid refinement study and an optional buffet study. The article presents the summary of data of all participants.

Grid Quality and Resolution Issues from the Drag Prediction Workshop Series

The drag prediction workshop series (DPW), held over the last six years, and sponsored by the AIAA Applied Aerodynamics Committee, has been extremely useful in providing an assessment of the state-of-the-art in computationally based aerodynamic drag prediction. An emerging consensus from the three workshop series has been the identification of spatial discretization errors as a dominant error source in absolute as well as incremental drag prediction. This paper provides an overview of the collective experience from the worksho series regarding the effect of grid-related issues on overall drag prediction accuracy. Examples based on workshop results are used to illustrate the effect of grid resolution and grid quality on drag prediction, and grid convergence behavior is examined in detail. For fully attached flows, various accurate and successful workshop results are demonstrated, while anomalous behavior is identified for a number of cases involving substantial regions of separated flow. Based on collective workshop experiences, recommendations for improvements in mesh generation technology which have the potential to impact the state-of-the-art of aerodynamic drag prediction are given.

Summary of Data from the Fifth Computational Fluid Dynamics Drag Prediction Workshop

Results from the Fifth AIAA Computational Fluid Dynamics Drag Prediction Workshop are presented. As with past workshops, numerical calculations are performed using industry-relevant geometry, methodology, and test cases. This workshop focused on force/moment predictions for the NASA Common Research Model wing-body configuration, including a grid refinement study and an optional buffet study. The grid refinement study used a common grid sequence derived from a multiblock topology structured grid. Six levels of refinement were created, resulting in grids ranging from 0.64×106 to 138×106 hexahedra, a much larger range than is typically seen. The grids were then transformed into structured overset and hexahedral, prismatic, tetrahedral, and hybrid unstructured formats all using the same basic cloud of points. This unique collection of grids was designed to isolate the effects of grid type and solution algorithm by using identical point distributions. This study showed reduced scatter and standard deviation fr...

CFD Sensitivity Analysis of a Drag Prediction Workshop Wing/Body Transport Configuration

The current work re-visits calculations for the First AIAA Drag Prediction Workshop (DPW-I) configuration and uses a grid convergence study to evaluate the quantitative effects of discretization error on the code-tocode variation of forces and moments. Four CFD codes commonly used at NASA Langley Research Center are used in the study: CFL3D and OVERFLOW are structured grid codes, and NSU3D and FUN3D are unstructured grid codes. Although the drag variation reported in the summary of DPW-I results was for the constantlift cruise condition, the focus of the current grid convergence study is a constant angle-of-attack condition (α = 0◦) near the same cruise lift in order to maintain identical boundary conditions for all of the CFD codes. Forces and moments were computed on the standard DPW-I structured overset and node-based unstructured grids, and the results were compared for the required transonic drag polar case. The range in total drag predicted using the workshop standard grids at α = 0◦ was 14 counts. The variation of drag in terms of standard deviation was 6 counts. Additional calculations at α = 0◦ were performed on the two families of structured and unstructured grids to evaluate the variation in forces and moments with grid refinement. The structured grid refinement study was inconclusive because of difficulties computing on the fine grid. The grid refinement study for the unstructured grid codes showed an increase in variation of forces and moments with grid refinement. However, all of the unstructured grid results were not definitively in the range of asymptotic grid convergence. The study indicated that certain numerical schemes (central vs. upwind, thin-layer vs. full viscous) or other code-to-code differences may have a larger effect than previously thought on grid sizes considered to be “medium” or “fine” by current standards. ∗Member AIAA, Research Engineer NASA Langley Research Center(LaRC), Hampton, Virginia. †Associate Fellow AIAA, Senior Research Scientist NASA LaRC. ‡Associate Fellow AIAA, Research Fellow National Institute of Aerospace, Hampton, Virginia. §Senior Member AIAA, Research Scientist NASA LaRC. ¶Member AIAA, Research Engineer NASA LaRC. ‖Member AIAA, Research Scientist NASA LaRC. ∗∗Associate Fellow AIAA, Senior Research Scientist NASA LaRC. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Introduction The AIAA Applied Aerodynamics Technical Committee conducted a Drag Prediction Workshop (DPW-I) in the summer of 2001 to evaluate CFD transonic cruise drag predictions for subsonic transports. Workshop participants were required to calculate the lift, drag and pitching moment for the DLR-F4 wing-body configuration at the cruise condition (Mach = 0.75, CL = 0.5), as well as the Mach = 0.75 drag polar. The participants were given a required grid to run and were encouraged to develop their own grid. The DLR-F4 wing-body was chosen since it had been tested in multiple wind tunnels. 1 A total of 35 solutions were computed with 14 different CFD codes; multiple turbulence models were used; structured and unstructured grids were used; 21 solutions were submitted on the required grids and an additional 14 solutions were provided on grids developed by the participants. In Ref. 2, Levy et al. provided a description of the workshop requirements and summary of the data submitted by the workshop participants. Hemsch 3 analyzed all of the solutions using a statistical framework. The variation in the drag from all 35 solutions at the cruise condition as measured by an estimate of the population standard deviation was 0.0021. The variation in the drag from the experiment was 0.0004. Thus, the computational drag variation was over 5 times the variation between wind tunnels. Designers typically state that they require drag prediction within one count (one count = 0.0001). Thus, the wind tunnel variation was 4 times the designer’s requirement, and the CFD variation was 21 times the designer’s requirement. Roache4 stated that multiple grids must always be used in order to verify a CFD solution. The design of the first DPW-I did not require that the participants provide solutions on multiple grids. Hence, the solutions were evaluated in the original study without the benefit of a quantitative measure of grid convergence. Each participant was free to choose whichever turbulence model and numerical scheme that they preferred for their calculations. Additionally, in order to accommodate the maximum number of CFD codes possible, the transition was specified at the leading edge of the vehicle, i.e. fully turbulent, rather than matching the experimentally determined transition pattern. Also, although aeroelastic deformations were incorporated into the geometry, they

Summary of the Fourth AIAA Computational Fluid Dynamics Drag Prediction Workshop

Results from the Fourth AIAA Drag Prediction Workshop are summarized. The workshop focused on the prediction of both absolute and differential drag levels for wing–body and wing–body/horizontal-tail configurations of the NASA Common Research Model, which is representative of transonic transport aircraft. Numerical calculations are performed using industry-relevant test cases that include lift-specific flight conditions, trimmed drag polars, downwash variations, drag rises, and Reynolds-number effects. Drag, lift, and pitching moment predictions from numerous Reynolds-averaged Navier–Stokes computational fluid dynamics methods are presented. Solutions are performed on structured, unstructured, and hybrid grid systems. The structured-grid sets include point-matched multiblock meshes and overset grid systems. The unstructured and hybrid grid sets comprise tetrahedral, pyramid, prismatic, and hexahedral elements. Effort is made to provide a high-quality and parametrically consistent family of grids for each g...

Uncertainty in Computational Aerodynamics

J. M. Luckring, M. J. Hemsch , J. H. MorrisonAerodynamics, Aerothermodynamics, and Acoustics CompetencyNASA Langley Research CenterHampton, VirginiaABSTRACTAn approach is presented to treat computationalaerodynamics as a process, subject to the fundamentalquality assurance principles of process control andprocess improvement. We consider several aspectsaffecting uncertainty for the computationalaerodynamic process and present a set of stages todetermine the level of management required to meetrisk assumptions desired by the customer of thepredictions•CA