David R. McDaniel

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.

Aerodynamic Analysis of a Generic Fighter Using Delayed Detached-Eddy Simulation

The modular transonic vortex interaction configuration was developed at the NASA Langley Research Center to investigate the aerodynamic characteristics of a generic fighter incorporating a chined fuselage and delta wing. Previous experiments showed that the fuselage and leading-edge vortex interactions are detrimental to the vehicles aerodynamic characteristics for angles of attack greater than 23 deg at low angles of sideslip. This is largely due to abrupt asymmetric vortex breakdown, which leads to pronounced pitch-up and significant nonlinearities in lateral stability that could result in roll departure. An improved understanding of the exact origins of this nonlinear behavior would improve future fighter design, and predictive capabilities of such nonlinearities could drastically reduce the cost associated with flight testing new or modified aircraft. The nonlinearities experienced by the modular transonic vortex interaction configuration at a 30 deg angle of attack, Reynolds number of 2.68 x 10 6 , and Mach number of 0.4 are computed using delayed detached-eddy simulation. Computational predictions of rolling moment compare very well with previous wind-tunnel experiments at the same conditions, including the abrupt nonlinear increase in rolling moment as a function of sideslip angle at small sideslip angles. A detailed investigation of the computational fluid dynamic data confirms that this nonlinearity is due to a rapid change in the flowfield structures from symmetric to asymmetric vortex breakdown.

F- 16XL Unsteady Simulations for the CAWAPI Facet of RTO Task Group AVT- 113

This work represents the USAF Academy portion of a culmination of three years of cooperative research in the Cranked Arrow Wing Aerodynamics International (CAWAPI) RTO Task Group, AVT-113. The objective of the group was to compute high resolution CFD simulations of a subset of the conditions created in the CAWAP flight test program managed by NASA Langley researchers and others. Seven flight conditions were chosen with four of them at symmetric conditions of medium to high angle of attack and subsonic Mach numbers, one symmetric condition at a transonic low angle of attack condition, and two conditions at medium angle of attack and subsonic Mach number but with positive and negative sideslips. The emphasis of the USAF Academy team was to explore unsteady effects and the ability of current methods to predict them. Very good agreement with flight test was found in almost all cases and the unsteadiness was documented with flowfield visualization and unsteady surface pressure coefficient data.

Towards an Efficient Aircraft Stability and Control Analysis Capability Using High-Fidelity CFD

This paper documents recent advances towards an efficient computational method for accurately determining the static and dynamic stability and control (S&C) characteristics of high-performance aircraft. In contrast to the “brute force” approach to filling an entire S&C database for an aircraft, the present approach is to reduce the number of simulations required to generate a complete aerodynamic model of a particular configuration at selected flight conditions by using one or a few complex dynamic motions and nonlinear system identification (SID) techniques. The approach is demonstrated by gathering high-fidelity computational fluid dynamics (CFD) data for a rigid F-16 in prescribed motion that approximates dynamic wind-tunnel testing techniques and SID input signals. The motions are optimized to minimize the computational expense and to take full advantage of the tighter control of the CFD environment. They are specified interactively using a newly developed, GUI-based maneuver file generation tool. Global nonlinear parameter modeling and other SID techniques are then used to identify parametric models from the computed aerodynamic force and moment data. These compact models are used to predict the aerodynamic response to maneuvers that were computed for validation purposes and that were not used to derive the models. Partial derivatives of the analytical models can be used to determine the corresponding static and dynamic stability derivatives. The models can also be used to perform real time 6-DOF/aeroelastic simulations of the vehicle in conditions susceptible to spin, tumble, and lateral/longitudinal instabilities. The main benefits of this effort are: 1) early discovery of complex aerodynamic phenomena that are typically only present in dynamic flight maneuvers and therefore not discovered until flight test, and 2) rapid generation of an accurate aerodynamic model to support aircraft and weapon certification by reducing required flight test hours and complementing current stability and control testing.

Multiple Bodies, Motion, and Mash-Ups: Handling Complex Use-Cases with Kestrel

ed, object-based approach to the input definitions, it was fairly easy to remove this explicit behavior and automatically determine the components needed in a simulation based on the job characteristics, motion types, etc. Finally, it is worth noting that Kestrel performs the same validation checks on the input data at run time that were accomplished during the job setup, protecting the user against incorrect manual modifications to the inputs. This has proven to be helpful to users since the complexity of the input file increases rapidly with more involved cases. B. Input Management and Consolidation One of the more important behind-the-scenes pieces of the Kestrel code base is the shared utility code that collects and “normalizes” all of the disparate user inputs collected into the hierarchical XML input file into a format that each of the execution components can easily utilize – termed the “body hierarchy” (inspired by Ref. 6). The body hierarchy has two main purposes. First, it collects all of the inputs and presents them to each of the components in a manner such that the component can only extract the inputs it cares about (e.g., the control surface motion component on a particular compute rank only cares about the control surface definitions on the body being computed on the local rank). The implementation includes a number of helper methods to assist components with extracting the desired component-specific inputs from the rich body hierarchy data structure. Second, the body hierarchy converts all input data into a user-specified unit system, a user-specified scale, and an internal coordinate system so that individual components do not have to worry about those types of conversions. The XML input file represents each section of the user inputs in its own specific coordinate and unit system as requested by the user. At run time, the body hierarchy reconciles various input contexts and resolves inputs into the common unit system, scale, and coordinate system. C. Setup Validation Even with the measures taken to try to reduce errors associated with the simulation inputs, it is likely that problems will still exist due to a misunderstanding of a particular input definition, user mistakes, etc. Therefore, Kestrel includes a utility called “jobview” that allows users to visualize their complex simulation setups and see how Kestrel will interpret the job inputs. Jobview displays the surface meshes of each of the bodies in their assembled state. The user is then free to interact with the view by panning, rotating, zooming, and selecting additional view options that decorate the scene with icons that illustrate how the inputs are interpreted. Examples of inputs that can be shown are boundary condition on each patch, center of mass for each body, arrows indicating the location, direction, and relative magnitude of external forces, bounds of the Cartesian mesh and size of the coarsest cell (Cartesian off-body paradigm), structural nodes, and tap points. Jobview performs the same operations that Kestrel does when it prepares a job for simulation. This includes converting to the internal coordinate system, scaling to match the user-specified unit system and simulation scale, and rotating/translating into the assembled position according to the body relationships. When setting up a simulation that entails multiple bodies and/or motions of any kind, jobview is indispensible and makes mistakes with the various interconnected inputs very clear. Figure 4 shows a screenshot of the jobview display for an assembled mesh system and some selected inputs.

Rigid, Maneuvering, and Aeroelastic Results for Kestrel - A CREATE Simulation Tool

This paper documents results from the first version of a new integrating product that allows cross-over between simulation of aerodynamics, dynamic stability and control, and structures in the first version, as well as propulsion and store separation in later versions. The Kestrel software product is an integrating product written in modular form with a Python infrastructure to allow growth to additional capabilities as needed. The Kestrel software product has been designed from the ground up to address fixed-wing aircraft in flight regimes ranging from subsonic through supersonic flight, including maneuvers, multi-aircraft configurations, and operational conditions. Kestrel v1.0 has three simulation capabilities, static rigid body aircraft, rigid body maneuvering aircraft, and aeroelastic wings. Results for two F-16C configurations at two Mach numbers and the Joint Stand Off Weapon (JSOW) are provided for static rigid body aircraft. Results are presented for an F-16C in a dynamic pitch maneuver and an F-22 in a wind up turn both compared to Lockheed performance data. Results are also presented for an AGARD 445.6 aeroelastic wing and compared to experiment and other researcher’s data.

Comparisons of computational fluid dynamics solutions of static and manoeuvring fighter aircraft with flight test data

Abstract As the capabilities of computational fluid dynamics (CFD) to model full aircraft configurations improve, and the speeds of massively parallel machines increase, it is expected that CFD simulations will be used more and more to steer or in some cases even replace traditional flight test analyses. The mission of the US Air Force SEEK EAGLE office is to clear any new weapon configurations and loadings for operational use. As more complex weapons are developed and highly asymmetric loadings are requested, the SEEK EAGLE office is tasked with providing operational clearances for literally thousands of different flight configurations. High-fidelity CFD simulations employing the turbulent Navier—Stokes equations are in a prime position to help reduce some of the required wind-tunnel and/or flight test workload. However, these types of CFD simulations are still too time consuming to populate a full stability and control parameter database in a brute-force manner. This article reviews results previously published by the authors, which validate the ability of high-fidelity CFD techniques to compute static force and moment characteristics of aircraft configurations. A methodology to generate efficient but non-linear reduced-order aerodynamic loads models from dynamic CFD solutions, which in-turn may be used to quickly analyse various stability and control characteristics at a particular flight condition, is introduced, and the results based on the US Air Force F-16C fighter aircraft that exemplify the process are discussed.

Efficient Mesh Deformation for Computational Stability and Control Analyses on Unstructured Viscous Meshes

This paper documents the initial development results of an efficient mesh deformation technique to support the modeling of elastic aircraft with traditional aircraft control surfaces in high-fidelity computational fluid dynamics (CFD) simulations on unstructured meshes. Even though unsteady Navier-Stokes simulations of full-scale aircraft are routinely computed on meshes with tens of millions cells to support current stability and control analyses, these simulations still take on the order of days to complete on state of the art computing clusters. Thus, as multi-disciplinary simulations incorporating CFD solutions are applied to full aircraft configurations, it is imperative that the supporting mesh deformation schemes are extremely efficient and robust. The techniques developed here are based on localized, algebraic-deformation schemes. These types of schemes are known to be computationally cheap but commonly suffer from robustness issues, especially when applied to flight Reynold’s number viscous meshes where some element aspect ratios are on the order of one million. To alleviate these issues, a two-phased approach is used where a “viscous layer” of grid nodes is moved rigidly with the deforming boundary surface, and a separate scheme is used for the remaining nodes in the outer region. A modified surface influence scheme is investigated for the outer region deformation as well as a modification of a scheme based on mapping nodes in a Delaunay triangulation between the deforming boundary and the farfield. The mesh deformation schemes are applied to a flapped 2D NACA airfoil and a conventional aileron on a NACA 0015 wing. The computational efficiency of the technique is discussed, and qualitative comments are made regarding the mesh cell quality throughout the full range of boundary motions. The associated benefits regarding a-priori exercising of the computational grid are also discussed.

Determining the Applicability and Effectiveness of Current CFD Methods in Store Certification Activities

This paper documents results of a ShadowOps project conducted by t he Air Force SEEK EAGLE office at Eglin AFB from August to Sept 2008 in support of the DoD CREATE-AV program. The objective of the project was to determine the applicability and effectiveness of current computational fluid dynamics (CFD) methods in ongoing store certification activities from a stability and control (S&C) perspective. Confirmation that CFD calculations can accurately reproduce experimental data from an S&C perspective is critical to the integration of CFD methods into the S&C flight test, clearance, and certification process of new weapons systems and the broader acquisition process. Current F -16 store certification activities were shadowed by comparing results of a commercial cell-centered, finite volume CFD code, Cobalt, with recently acquired wind tunnel data and data available in the aerodynamic database of Lockheed Martin’s F-16 6-DOF ATLAS. Unstructured grids were created corresponding to critical wind tunnel test configurations. Full and 1/20th scale static steady and unsteady (tim e accurate) simulations at both atmospheric and wind tunnel conditions were performed. Also, CFD rigid body prescribed motion maneuvers were created using flight test data to simulate a select number of actual flight test maneuvers. The main benefits of this effort are: 1) early discovery of complex aerodynamic phenomena that are typically only present in dynamic flight maneuvers and therefore not discovered until flight test, and 2) rapid generation of an accurate aerodynamic model to support aircraft an d weapon certification by reducing required flight test hours and complementing current stability and control testing. The necessity of DoD HPC resources to gather the necessary data in a timely manner in order to support the warfighter is reaffirmed.

Aerodynamic Control Surface Implementation in Kestrel v2.0

Kestrel is an integrating software product in the CREATE program that allows crossover between simulation of aerodynamics, dynamic stability and control, structures, propulsion, and store separation. It is written in a modular form with a Python infrastructure to allow growth to additional capabilities as needed. The second version of the Kestrel product includes the addition of two new components that enable both static and dynamic movement of aerodynamic control surfaces to be included in a fixed-wing simulation. This paper documents the design and implementation of these new components as well as their integration into the overall Kestrel environment. Both components are fully parallel and capable of handling any number of conventional control surfaces in a single unstructured mesh. Results from the Kestrel v2.0 testing involving a NACA 0015 wing with a single trailing edge control surface are presented.