Cranked-Arrow Wing Aerodynamics Research Articles

Synthesis of Hybrid Computational Fluid Dynamics Results for F-16XL Aircraft Aerodynamics

A synthesis is presented of recent numerical predictions for the F-16XL aircraft flowfields and aerodynamics. The computational analyses were all performed with hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation formulations, with an emphasis on unsteady flows and associated aerodynamics, and results from five computational methods are included. The work focused on one particular low-speed high angle-of-attack flight-test condition, and comparisons against flight-test data are included. This work represents the third coordinated effort using the F-16XL aircraft, and a unique flight-test dataset, to advance the knowledge of slender airframe aerodynamics as well as the capability for predicting these aerodynamics with advanced computational fluid dynamics formulations. The prior efforts were identified as the Cranked-Arrow Wing Aerodynamics Project International.

Numerical Simulations of the F-16XL at Flight-Test Conditions Using Delayed Detached-Eddy Simulation

The F-16XL flight-test data have been used for many years in the Cranked-Arrow Wing Aerodynamics Program International series as a challenging test case for numerical prediction of military aircraft at full-scale flight conditions. The complexity of the configuration and the variety of flight-test conditions available challenge most computational fluid dynamics codes and turbulence models. This study documents the use of the Kestrel flow solver with its solution-based mesh refinement capability and delayed detached-eddy simulation turbulence models for F-16XL flight condition 25, which is a subsonic high-angle-of-attack flight condition that has proven particularly challenging for computational researchers. The spectral content of the pressure data and the integrated lift, drag, and pitching moment coefficients are also presented. The more advanced turbulence models, coupled with the more refined meshes, show very good agreement with flight-test data and reasonable resolution of the unsteady frequency content.

F-16XL Hybrid Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation on Unstructured Grids

The Cranked-Arrow Wing Aerodynamics Project International investigation is continued with the FUN3D and USM3D flow solvers to fuse flight test, wind-tunnel test, and simulation of swept-wing aerodynamic features. Simulations of a low-speed, high-angle-of-attack condition are compared: detached-eddy simulation, modified delayed detached-eddy simulation, and the Spalart–Allmaras Reynolds-averaged Navier–Stokes model. Isosurfaces of criterion show the development of coherent primary and secondary vortices on the upper surface of the wing that spiral, burst, and commingle. Mean detached-eddy simulation and modified delayed detached-eddy simulation pressures better predict the flight-test measurements than Spalart–Allmaras model predictions, especially on the outer-wing section. The USM3D simulations predicted many sharp tones in volume point pressure spectra with low broadband noise, and the FUN3D simulations predicted more broadband noise with weaker tones. Spectra of the volume points near the outer-wing leading edge were primarily broadband for both codes. Time-averaged forces are very similar between FUN3D simulations and between USM3D simulations, but FUN3D predicts slightly higher lift and lower drag than USM3D. There is more variation in the pitching moment predictions. Spectra of the unsteady forces and moment are mostly broadband for FUN3D and tonal for USM3D simulations.

USM3D Simulations for the F-16XL Aircraft Configuration

Numerical simulations of flow around the F-16XL are presented as a contribution to the Cranked Arrow Wing Aerodynamics Project International II. The NASA Tetrahedral Unstructured Software System with its USM3D solver is used to perform numerical simulations. This computational fluid dynamics suite is developed and maintained by the NASA Langley Research Center. The CRISP CFD package is used to provide error estimates and grid adaptation for validation of USM3D results. A subsonic high-angle-of-attack case flight condition 25 is computed and analyzed. Three turbulence models are used in the calculations: the one-equation Spalart–Allmaras, the two-equation shear-stress transport, and the turbulence models. Computational results and surface-static-pressure profiles are presented and compared with flight data. Solution validation is performed using formal grid-refinement studies, the solution of error-transport equations, and adaptive mesh refinement. The current study shows that the USM3D solver coupled with CRISP CFD can be used to provide valuable data in predicting vortex-flow physics on a complex configuration.

Flow Analysis of the F-16XL Aircraft at Transonic Flow Conditions

In the framework of the Cranked-Arrow Wing Aerodynamics Project International 2, Netherlands National Aerospace Laboratory performed an analysis of a set of transonic flight conditions available in the Cranked-Arrow Project database. Flight condition FC70 was used previously during the Cranked-Arrow Wing Aerodynamics Project International to investigate transonic flow on the F-16XL aircraft. During this project, it was discovered that flight condition FC70 was flown with a deflected leading-edge flap. To facilitate computational fluid analysis, a transonic flight condition without deflected control surfaces was judged desirable by the Cranked-Arrow Project members. Therefore, it was decided to search the Cranked-Arrow Project database for a transonic flight condition without any control surface deflections. Because no information on the control surface deflections of the other transonic flight conditions was available to the Cranked-Arrow Project partners, an alternative approach to scrutinize the available flight-test sectional surface pressure measurements for indications of control surface deflections was adopted. This analysis revealed that flight condition FC79 was the most likely transonic flight condition to be flown without any control surface deflections. Flight conditions FC70 and FC79 were analyzed using National Aerospace Laboratory’s in-house-developed flow simulation system ENFLOW. Comparison of the measured and simulated surface pressure coefficients confirmed that flight condition FC79 was flown without any control surface deflections, and that this flight condition is thus much better suited for further comparisons between flight-test data and computational fluid dynamics simulations.

Computational Fluid Dynamics Predictions of Control-Surface Effects for F-16XL Aircraft

Computational fluid dynamics methods play an increasingly important role in aircraft design and development. Some examples are conditions that cannot be tested before a flight test. To rely on these methods, it is essential that they are assessed and evaluated with a state-of-the-art wind tunnel test and/or in-flight data. In a previous project, Cranked-Arrow Wing Aerodynamics Project International, it was reported that all computational fluid dynamics methods failed to some degree in the transonic regime where shock–vortex interaction phenomena were present. Detailed analysis of surface pressure distribution showed that computational fluid dynamics was not able replicate the correct flowfield and produce acceptable results. This paper aims to restudy those transonic flight conditions for which computational fluid dynamics underperforms and to try to determine or shed light on the extent to which surface effects contribute to the computational fluid dynamics predictions.

What Was Learned in Predicting Slender Airframe Aerodynamics with the F-16XL Aircraft

The second Cranked-Arrow Wing Aerodynamics Project, International, coordinated project has been underway to improve high-fidelity computational-fluid-dynamics predictions of slender airframe aerodynamics. The work is focused on two flow conditions and leverages a unique flight data set obtained with the F-16XL aircraft for comparison and validation. These conditions, a low-speed high-angle-of-attack case and a transonic low-angle-of-attack case, were selected from a prior prediction campaign wherein the computational fluid dynamics failed to provide acceptable results. In revisiting these two cases, approaches for improved results include better, denser grids using more grid adaptation to local flow features as well as unsteady higher-fidelity physical modeling like hybrid Reynolds-averaged Navier–Stokes/unsteady Reynolds-averaged Navier–Stokes/large-eddy simulation methods. The work embodies predictions from multiple numerical formulations that are contributed from multiple organizations where some authors investigate other possible factors that could explain the discrepancies in agreement (e.g., effects due to deflected control surfaces during the flight tests as well as static aeroelastic deflection of the outer wing). This paper presents the synthesis of all the results and findings and draws some conclusions that lead to an improved understanding of the underlying flow physics, finally making the connections between the physics and aircraft features.

Sub- and Transonic Vortex Breakdown Flight Condition Simulations of the F-16XL Aircraft

Free-flight Navier–Stokes simulations of the Cranked Arrow Wing Aerodynamics Project International F-16XL aircraft were performed to probe for the complex vortex flowfield structures at high angle of attack close to and beyond vortex breakdown conditions. The scope of the paper probes for the quality and quantity of high-resolution numerical simulations possible with the current computer capacity at free-flight Reynolds numbers. It provides an insight into the effect of turbulence modeling of these complicated flows; the free-flight measured pressure distributions are compared with solutions achieved with the one-equation Spalart–Allmaras turbulence model, the one-equation Spalart–Allmaras turbulence model with rotation correct, the model, and the turbulence model with scale-adaptive simulation. The third topic of the investigation is the complexity of the vortex flow system of the configuration at sub- and transonic speeds at vortex breakdown conditions. For this purpose, the details of the different vortex systems and their behavior are prepared and discussed.

F16-XL Geometry and Computational Grids Used in Cranked-Arrow Wing Aerodynamics Project International

The objective of the Cranked-Arrow Wing Aerodynamics Project International was to allow a comprehensive validation of computational fluid dynamics methods against the Cranked-Arrow Wing Aerodynamic ...

Reynolds-Averaged Navier-Stokes Solutions for the CAWAPI F-16XL Using Different Hybrid Grids

Ten groups participating in the Cranked Arrow Wing Aerodynamics Project International (CAWAPI) within the NATO-RTO task group AVT 113 have contributed viscous simulations of a full scale model of the F-16XL aircraft at different flight conditions. Three different classes of grids have been used by the different participants: a common structured grid, a common unstructured grid, and so-called individually tailored grids, which have been used by four participants: Lockheed Martin Aeronautics Company, University of Tennessee, Boeing Phantom Works, and EADS Military Air Systems. This paper focuses on the use of these individually generated grids. The different grid generation techniques and strategies (nonadapted, manually adapted, and solution-based adapted) are discussed and the solutions obtained in these individually tuned grids are compared with flight test data for different test cases.

Standard Unstructured Grid Solutions for Cranked Arrow Wing Aerodynamics Project International F-16XL

Steady and unsteady viscous flow simulations of a full-scale, semispan, and full-span model of the F-16XL-1 aircraft are performed with three different computational fluid dynamics codes using a common unstructured grid. Six different flight conditions are considered. They represent Reynolds and Mach number combinations at subsonic speeds, with and without sideslip. The steady computations of the flow at these flight conditions are made with several Reynolds-averaged Navier-Stokes turbulence models of different complexity. Detached-eddy simulation, delayed detached-eddy simulation, and an algebraic hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation model are used to quantify unsteady effects at the same flight conditions. The computed results are compared with flight-test data in the form of surface pressures, skin friction, and boundary-layer velocity profiles. The focus of the comparison is on turbulence modeling effects and effects of unsteadiness. The overall agreement with flight data is good, with no clear trend as to which physical modeling approach is superior for this class of flow. The Reynolds-averaged Navier-Stokes turbulence models perform well in predicting the flow in an average sense. However, some of the flow conditions involve locally unsteady flow over the aircraft, which are held responsible for the scatter between the different turbulence modeling approaches. The detached-eddy simulations are able to quantify the unsteady effects, although they are not consistently better than the Reynolds-averaged Navier-Stokes turbulence models in predicting the flow in an average sense in these flow regions. Detached-eddy simulation fails to predict boundary-layer profiles consistently over a range of flow regimes, with delayed detached-eddy simulation and hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation models offering a remedy to recover some of the predictive capabilities of the underlying Reynolds-averaged Navier-Stokes turbulence model. Nonetheless, the confidence in the predictive capabilities of the computational fluid dynamics codes with regard to complex vortical flowfields around high-performance aircraft of this planform increased significantly during this study.

Lessons Learned from Numerical Simulations of the F-16XL Aircraft at Flight Conditions

Nine groups participating in the Cranked Arrow Wing Aerodynamics Project International (CAWAPI) project have contributed steady and unsteady viscous simulations of a full-scale, semi-span model of the F-16XL aircraft. Three different categories of flight Reynolds/Mach number combinations were computed and compared with flight-test measurements for the purpose of code validation and improved understanding of the flight physics. Steady-state simulations are done with several turbulence models of different complexity with no topology information required and which overcome Boussinesq-assumption problems in vortical flows. Detached-eddy simulation (DES) and its successor delayed detached-eddy simulation (DDES) have been used to compute the time accurate flow development. Common structured and unstructured grids as well as individually-adapted unstructured grids were used. Although discrepancies are observed in the comparisons, overall reasonable agreement is demonstrated for surface pressure distribution, local skin friction and boundary velocity profiles at subsonic speeds. The physical modeling, steady or unsteady, and the grid resolution both contribute to the discrepancies observed in the comparisons with flight data, but at this time it cannot be determined how much each part contributes to the whole. Overall it can be said that the technology readiness of CFD-simulation technology for the study of vehicle performance has matured since 2001 such that it can be used today with a reasonable level of confidence for complex configurations.

Prediction of F-16XL Flight-Flow Physics

T HIS special section is the result of fruitful endeavors by an international group of researchers in industry, government laboratories, and university-led efforts to improve the technology readiness level of their CFD solvers through comparisons with flight data collected on the F-16XL-1 aircraft at a variety of test conditions. These 1996flight datawere documented in [1] and detailed theflightflow physics of this aircraft through surface tufts, pressures, boundary-layer rakes, and skin-friction measurements. The flight project was called the Cranked-Arrow Wing Aerodynamics Project (CAWAP), due to its leading-edge sweep crank (70 deg inboard, 50 deg outboard), and served as a basis for the international comparisons to bemade, called CAWAPI. This highly focused effort was one of two vortical flow studies facilitated by the NATO Research and Technology Organization (RTO) through its Applied Vehicle Technology (AVT) Panel with a title of “Understanding and Modeling Vortical Flows to Improve the Technology Readiness Level for Military Aircraft.” It was given a task group number of AVT-113 and had an official start date of spring 2003. The companion part of this task group dealt with fundamentals of vortical flow from both an experimental and numerical perspective on an analytically describable delta-wing model, for which much surface pressure data had already beenmeasured at NASALangleyResearch Center at a variety of Mach and Reynolds numbers, and is called the “Vortex FlowExperiment–2” (VFE-2) as proposed in [2]. These two parts or facets helped one another in understanding the predictions and data that had been or were being collected. The CAWAPI facet had the distinction of using actual aircraft geometry, which is subject to the ITAR controls, and required much cooperation between NASA Langley Research Center, NASA Headquarters, and the leadership of the various NATO or Partnersfor-Peace participating organizations in establishing how the geometry and grids could be shared. This and other background information is contained in the first paper that follows. The second paper discusses how the supplied geometry was processed into acceptable computational grids for both the structured and unstructured solver communities, papers three to five detail the comparisons of three classes of grid solutionswith theflight data, and the last paper provides what has been learned from CAWAPI. The author is extremely proudof themany outstanding researchers and organizations that have had a part in the CAWAPI facet. These RTO task groups do not come with funded support, so each participating organization had to anticipate that the benefit it accrues would be greater than the expense encountered. The participating researchers accommodated the work often on their own time under the benevolent eye of their employers who saw this work to be of importance and provided an opportunity for their staff to make a novel contribution, as well as to test their own solvers. We have also benefited from significant graduate student involvement; in particular, CAWAPI has lasted long enough for some to complete their advanced degrees and to be part of the coauthorship of two of the following papers. The author alsowishes to thank theAIAA for providing ameans of rapid dissemination of results obtained duringCAWAPI through two special sessions at the 45th AIAA Aerospace Sciences Meeting and Exhibit (January 2007), and now through the Journal of Aircraft. In particular, Professor Frank Coton (University of Glasgow) and the Applied Aerodynamics Technical Committee are thanked for advocating and facilitating these special sessions at the general meeting in which results from nine organizations were reported in 13 papers. The themes of these papers are carried over as articles in this special section, either individually or in combination. The Journal of Aircraft editors, editorial staff, and reviewers are also thanked for the many helpful suggestions made during the publication process.

Overview of the Cranked-Arrow Wing Aerodynamics Project International

This paper provides a brief history of the F-16XL-1 aircraft, its role in the High-Speed Research Program, and how it was morphed into the Cranked-Arrow Wing Aerodynamics Project. Various flight, wind-tunnel, and computational fluid dynamics data sets were generated as part of the project. These unique and open flight data sets for surface pressures, boundary-layer profiles, and skin-friction distributions, along with surface flow data, are described and sample data comparisons are given. This is followed by a description of how the project became internationally known as Cranked-Arrow Wing Aerodynamics Project International and is concluded by an introduction to the results of a four-year computational predictive study of data collected at flight conditions by participating researchers.