John C. Vassberg

Development of a Common Research Model for Applied CFD Validation Studies

The development of a wing/body/nacelle/pylon/horizontal-tail configuration for a common research model is presented, with focus on the aerodynamic design of the wing. Here, a contemporary transonic supercritical wing design is developed with aerodynamic characteristics that are well behaved and of high performance for configurations with and without the nacelle/pylon group. The horizontal tail is robustly designed for dive Mach number conditions and is suitably sized for typical stability and control requirements. The fuselage is representative of a wide/body commercial transport aircraft; it includes a wing-body fairing, as well as a scrubbing seal for the horizontal tail. The nacelle is a single-cowl, high by-pass-ratio, flow-through design with an exit area sized to achieve a natural unforced mass-flow-ratio typical of commercial aircraft engines at cruise. The simplicity of this un-bifurcated nacelle geometry will facilitate grid generation efforts of subsequent CFD validation exercises. Detailed aerodynamic performance data has been generated for this model; however, this information is presented in such a manner as to not bias CFD predictions planned for the fourth AIAA CFD Drag Prediction Workshop, which incorporates this common research model into its blind test cases. The CFD results presented include wing pressure distributions with and without the nacelle/pylon, ML/D trend lines, and drag-divergence curves; the design point for the wing/body configuration is within 1% of its max-ML/D. Plans to test the common research model in the National Transonic Facility and the Ames 11-ft wind tunnels are also discussed.

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.

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.

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.

Computational Fluid Dynamics for Aerodynamic Design: Its Current and Future Impact

This paper discusses the role that computational fluid dynamics plays in the design of aircraft. An overview of the design process is provided, covering some of the typical decisions that a design team addresses within a multi-disciplinary environment. On a very regular basis trade-offs between disciplines have to be made where a set of conflicting requirements exist. Within an aircraft development project, we focus on the aerodynamic design problem and review how this process has been advanced, first with the improving capabilities of traditional computational fluid dynamics analyses, and then with aerodynamic optimizations based on these increasingly accurate methods. The optimization method of the present work is based on the use of the adjoint of the flow equations to compute the gradient of the cost function. Then, we use this gradient to navigate the design space in an efficient manner to find a local minimum. The computational costs of the present method are compared with that of other approaches to aerodynamic optimization. A brief discussion regarding the formulation of a continuous adjoint, as opposed to a discrete one, is also included. Two case studies are provided which highlight the benefits of utilizing automatic aerodynamic shape optimization. The first case chronicles the application of such software during a complete aircraft design effort, while the second case shows to what extent the aerodynamic design cycle can be compressed. In both efforts, the optimizations were performed in design spaces with dimensions greater than 4, 000. Furthermore, they were conducted on very affordable computer equipment and turned around within a few hours of wall-clock time. ∗AIAA Fellow, T. V. Jones Prof. of Engineering †AIAA Associate Fellow, Boeing Technical Fellow Copyright c ©2001 by Jameson & Vassberg. Published by the AIAA with permission. The paper finishes with some visions for the future. Extrapolating the trends of computer weight and cost, it is interesting to speculate on how the aircraft design environment may evolve in the years to come.

In Pursuit of Grid Convergence for Two-Dimensional Euler Solutions

Grid-convergence trends of two-dimensional Euler solutions are investigated. The airfoil geometry under study is based on the NACA0012 equation. However, unlike the NACA0012 airfoil, which has a blunt base at the trailing edge, the study geometry is extended in chord so that its trailing edge is sharp. The flow solutions use extremely- high-quality grids that are developed with the aid of the Karman-Trefftz conformal transformation. The topology of each grid is that of a standard O-mesh. The grids naturally extend to a far-field boundary approximately 150 chord lengths away from the airfoil. Each quadrilateral cell of the resulting mesh has an aspect ratio of one. The intersecting lines of the grid are essentially orthogonal at each vertex within the mesh. A family of grids is recursively derived starting with the finest mesh. Here, each successively coarser grid in the sequence is constructed by eliminating every other node of the current grid, in both computational directions. In all, a total of eight grids comprise the family, with the coarsest-to-finest meshes having dimensions of 32 x 32-4096 x 4096 cells, respectively. Note that the finest grid in this family is composed of over 16 million cells, and is suitable for 13 levels of multigrid. The geometry and grids are all numerically defined such that they are exactly symmetrical about the horizontal axis to ensure that a nonlifting solution is possible at zero degrees angle-of-attack attitude. Characteristics of three well-known flow solvers (FLO82, OVERFLOW, and CFL3D) are studied using a matrix of four flow conditions: (subcritical and transonic) by (nonlifting and lifting). The matrix allows the ability to investigate grid-convergence trends of 1) drag with and without lifting effects, 2) drag with and without shocks, and 3) lift and moment at constant angles-of-attack. Results presented herein use 64-bit computations and are converged to machine-level-zero residuals. All three of the flow solvers have difficulty meeting this requirement on the finest meshes, especially at the transonic flow conditions. Some unexpected results are also discussed. Take for example the subcritical cases. FLO82 solutions do not reach asymptotic grid convergence of second-order accuracy until the mesh approaches one quarter of a million cells. OVERFLOW exhibits at best a first-order accuracy for a central-difference stencil. CFL3D shows second-order accuracy for drag, but only first-order trends for lift and pitching moment. For the transonic cases, the order of accuracy deteriorates for all of the methods. A comparison of the limiting values of the aerodynamic coefficients is provided. Drag for the subcritical cases nearly approach zero for all of the computational fluid dynamics methods reviewed. These and other results are discussed.

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...