Tom Zickuhr

Summary of Data from the First AIAA CFD Drag Prediction Workshop

The results from the first AIAA CFD Drag Prediction Workshop are summarized. The workshop was designed specifically to assess the state-of-the-art of computational fluid dynamics methods for force and moment prediction. An impartial forum was provided to evaluate the effectiveness of existing computer codes and modeling techniques, and to identify areas needing additional research and development. The subject of the study was the DLR-F4 wing-body configuration, which is representative of transport aircraft designed for transonic flight. Specific test cases were required so that valid comparisons could be made. Optional test cases included constant-C(sub L) drag-rise predictions typically used in airplane design by industry. Results are compared to experimental data from three wind tunnel tests. A total of 18 international participants using 14 different codes submitted data to the workshop. No particular grid type or turbulence model was more accurate, when compared to each other, or to wind tunnel data. Most of the results overpredicted C(sub Lo) and C(sub Do), but induced drag (dC(sub D)/dC(sub L)(exp 2)) agreed fairly well. Drag rise at high Mach number was underpredicted, however, especially at high C(sub L). On average, the drag data were fairly accurate, but the scatter was greater than desired. The results show that well-validated Reynolds-Averaged Navier-Stokes CFD methods are sufficiently accurate to make design decisions based on predicted drag.

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.

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 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...

Comparison of NTF Experimental Data with CFD Predictions from the Third AIAA CFD Drag Prediction Workshop

Recently acquired experimental data for the DLR-F6 wing-body transonic transport configuration from the National Transonic Facility (NTF) are compared with the database of computational fluid dynamics (CFD) predictions generated for the Third AIAA CFD Drag Prediction Workshop (DPW-III). The NTF data were collected after the DPW-III, which was conducted with blind test cases. These data include both absolute drag levels and increments associated with this wing-body geometry. The baseline DLR-F6 wing-body geometry is also augmented with a side-of-body fairing which eliminates the flow separation in this juncture region. A comparison between computed and experimentally observed sizes of the side-ofbody flow-separation bubble is included. The CFD results for the drag polars and separation bubble sizes are computed on grids which represent current engineering best practices for drag predictions. In addition to these data, a more rigorous attempt to predict absolute drag at the design point is provided. Here, a series of three grid densities are utilized to establish an asymptotic trend of computed drag with respect to grid convergence. This trend is then extrapolated to estimate a grid-converged absolute drag level.

Writing the rules: An inside look at the AIAA student design/build/fly competition

The Cessna/ONR Student Design/Build/Fly contest has recently completed its 10 year. The competition was founded as a “hands on” student experience to enhance their knowledge and ability to work in an industry environment after graduation. The vehicle for this experience is to design, build, and fly an electric R/C airplane to achieve a specified objective (range, payload, speed, etc). The winner is determined by the best combination of written report and flight performance, the latter determined at the competition flyoff. The initial DBF proposal was a “grass roots” effort engineer driven by members of the AIAA Technical Committees and it has kept this flavor throughout its history. This nature has been one of the keys to the success of the contest. Also key was the desire to keep the competition fresh, by continually challenging the students with new design objectives every year. This also requires a fresh look by the organizing committee at writing the rules, with continued attempts to learn from the previous years’ experience. As always, however, the law of unintended consequences prevails, and new lessons await the students and the organizers together each year. As the Design/Build/Fly competition moves forward into its next decade, we anticipate many surprises, hopefully most of them good ones.