The RTO Task Group AVT-113 Understanding and Modeling Vortical Flows to Improve the Technology Readiness Level for Military Aircraft was established in April 2003. Two facets of the group, Cranked Arrow Wing Aerodynamic Project International (CAWAPI) and Vortex Flow Experiment-2 (VFE-2), worked closely together. However, because of the different requirements of each part, the CAWAPI facet concluded its work earlier (December 2006) than the VFE-2 facet (December 2007). In this first chapter of the Final Report of the Task Group an overview on its work is given, and the objectives for the Task Group are described.

This paper highlights the three aerodynamic pillars of aeronautics; namely, theory/CFD, wind-tunnel experiments and flight tests, and notes that at any given time these three are not necessarily at the same level of maturity. After an initial history of these three pillars, the focus narrows to a brief history of some vortical-flow flight experiments on slender aircraft that have impacted the advancement of aeronautics in recent decades. They include the F-106, Concorde, SR-71, light-weight fighters (F-16, F/A-18), and F-16XL. These aircraft share in common the utilization of vortical flow and have flown at transonic speeds during a part of the flight envelope. Due to the vast amount of information from flight and CFD that has recently become available for the F-16XL, this aircraft is highlighted and its results detailed. Lastly, it is interesting to note that, though complicated, vortical flows over the F-16XL aircraft at subsonic speeds can be reliably and generally well-predicted with the current CFD flow solvers. However, these solvers still have some problems in matching flight pressure data at transonic speeds. That this problem has been highlighted is both an advancement in aeronautics and a tempting prize to those who would seek its solution.

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 Aerodynamics Project flight database. A major part of this work involved the generation of high-quality computational grids. Before the grid generation, an airtight geometry of the F-16XL, aircraft was generated by a cooperation of the Cranked-Arrow Wing Aerodynamics Project International partners. Based on this geometry description, both structured and unstructured grids have been generated. The baseline structured (multiblock) grid (and a family of derived grids) has been generated by the National Aerospace Laboratory. Although the algorithms used by the National Aerospace Laboratory had become available just before the Cranked-Arrow Wing Aerodynamics Project International and thus only a limited experience with their application to such a complex configuration had been gained, a grid of good quality was generated well within four weeks. This time compared favorably with that required to produce the unstructured grids in the Cranked-Arrow Wing Aerodynamics Project International. The baseline all-tetrahedral and hybrid unstructured grids have been generated at NASA Langley Research Center and the U.S. Air Force Academy, respectively. To provide more geometrical resolution, trimmed unstructured grids have been generated at the European Aeronautic Defence and Space Company's Military Air Systems, University of Tennessee at Chattanooga SimCenter, Boeing Phantom Works, Royal Institute of Technology, and the Swedish Defence Research Agency. All grids generated within the framework of the Cranked-Arrow Wing Aerodynamics Project International will be discussed in the paper. Both results obtained on the structured grids and the unstructured grids showed a significant improvement in agreement with flight-test data in comparison with those obtained on the structured multiblock grid used during the Cranked-Arrow Wing Aerodynamics Project.

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.

Nine organizations participated in the Cranked-Arrow Wing Aerodynamics Project International study and have contributed steady and unsteady viscous simulations of a full-scale semispan model of the F-16XL aircraft. Three different categories of flight Reynolds/Mach number combinations are 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) that overcome Boussinesq-assumption problems in vortical flows. Detached-eddy simulation and its successor, delayed detached-eddy simulation, are 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, be it steady or unsteady flow, and the grid resolution both contribute to the discrepancies observed in the comparisons with flight data, but at this time, how much each part contributes to the whole cannot be determined. Overall, it can be said that the technology readiness of computational fluid dynamics 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.

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.

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.

Ships produce vortices and air-wakes while either underway or stationary in a wind. These flow fields can be detrimental to the conduction of air operations in that they can adversely impact the air vehicles and flight crews. There are potential solutions to these problems for both frigates/destroyers and carriers through the use of novel vortex flow or flow control devices. This appendix highlights several devices which may have application and points out that traditional wind-tunnel testing using smoke, laser-vapor screen, and Particle Image Velocimetry can be useful in sorting out the effectiveness of different devices.

In support the Cranked Arrow Wing Aerodynamic Project International (CAWAPI) to improve the Technology Readiness Level of flow solvers by comparing results with measured F-16XL-1 flight data, NASA Langley employed the TetrUSS unstructured grid solver, USM3D, to obtain solutions for all seven flight conditions of interest. A newly available solver version that incorporates a number of turbulence models, including the two-equation linear and non-linear k-epsilon, was used in this study. As a first test, a choice was made to utilize only a single grid resolution with the solver for the simulation of the different flight conditions. Comparisons are presented with three turbulence models in USM3D, flight data for surface pressure, boundary-layer profiles, and skin-friction results, as well as limited predictions from other solvers. A result of these comparisons is that the USM3D solver can be used in an engineering environment to predict flow physics on a complex configuration at flight Reynolds numbers with a two-equation linear k-epsilon turbulence model.

This paper provides a brief history of the F-16XL-1 aircraft, its role in the High Speed Research (HSR) program and how it was morphed into the Cranked Arrow Wing Aerodynamics Project (CAWAP). Various flight, wind-tunnel and Computational Fluid Dynamics (CFD) data sets were generated during the CAWAP. These unique and open flight datasets for surface pressures, boundary-layer profiles and skinfriction distributions, along with surface flow data, are described and sample data comparisons given. This is followed by a description of how the project became internationalized to be known as Cranked Arrow Wing Aerodynamics Project International (CAWAPI) and is concluded by an introduction to the results of a 4 year CFD predictive study of data collected at flight conditions by participating researchers.

The objective of the Cranked-Arrow Wing Aerodynamics Project International (CAWAPI) was to allow a comprehensive validation of Computational Fluid Dynamics methods against the CAWAP flight database. A major part of this work involved the generation of high-quality computational grids. Prior to the grid generation an IGES file containing the air-tight geometry of the F-16XL aircraft was generated by a cooperation of the CAWAPI partners. Based on this geometry description both structured and unstructured grids have been generated. The baseline structured (multi-block) grid (and a family of derived grids) has been generated by the National Aerospace Laboratory NLR. Although the algorithms used by NLR had become available just before CAWAPI and thus only a limited experience with their application to such a complex configuration had been gained, a grid of good quality was generated well within four weeks. This time compared favourably with that required to produce the unstructured grids in CAWAPI. The baseline all-tetrahedral and hybrid unstructured grids has been generated at NASA Langley Research Center and the USAFA, respectively. To provide more geometrical resolution, trimmed unstructured grids have been generated at EADS-MAS, the UTSimCenter, Boeing Phantom Works and KTH/FOI. All grids generated within the framework of CAWAPI will be discussed in the article. Both results obtained on the structured grids and the unstructured grids showed a significant improvement in agreement with flight test data in comparison with those obtained on the structured multi-block grid used during CAWAP.

A transonic wind tunnel test of an 8% F/A-18E model was conducted in the NASA Langley Research Center (LaRC) 16 ft Transonic Tunnel (16-ft TT) to investigate on-surface flow physics during stall. The technical approach employed focused on correlating static (or time-averaged) and unsteady wind-tunnel test data to the unsteady wing-stall events using force, moment, pressure, and pressure-sensitive-paint measurements. This paper focuses on data obtained on the pre-production configuration of the F/A-18E aircraft at Mach number of 0.90. The flow unsteadiness occurring on the wing as the wing went through the stall process was captured using the time history of balance and pressure measurements and by calculating the root mean square (RMS) for a number of instrument signals. The second step was to gather global perspectives on the pressures influencing the wing stall process. The abrupt wing stall experienced by the 8% F/A-18E Model was observed to be an unsteady event triggered by the rapid advancement of separation, which had migrated forward from the trailing edge, to the leading-edge flap hingeline over a very small increment in angle of attack. The angle of attack at which this stall occurred varied, from run to run, over an 1 degree increment. The abrupt wing stall was observed, using pressure-sensitive-paint, to occur simultaneously on both wing panels or asymmetrically. The pressure-sensitive paint data and wing-root bending moment data were essential in providing insight to the flow structures occurring over the wing and the possible asymmetry of those flow structures. A repeatability analysis conducted on eight runs of static data provided a quick and inexpensive examination of the unsteady aerodynamic characteristics of abrupt wing stall. The results of the repeatability analysis agreed extremely well with data obtained using unsteady measurement techniques. This approach could be used to identify test conditions for more complex unsteady data measurements using special instrumentation.