| 1. |
- Boelens, O. J., et al.
(author)
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Description of the F-16XL geometry and computational grids used in CAWAPI
- 2007
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In: Collection of Technical Papers - 45th AIAA Aerospace Sciences Meeting. - 1563478900 - 9781563478901 ; , s. 5866-5878
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Conference paper (peer-reviewed)abstract
- 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 some 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. The baseline all-tetrahedral and hybrid unstructured grids were generated at NASA Langley Research Center and the U.S. Air Force Academy, respectively. To provide more geometrical resolution, additional unstructured grids were generated at EADS-MAS, the UTSimCenter, and Boeing Phantom Works. All the grids generated within the framework of CAWAPI will be discussed.
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| 2. |
- Boelens, O. J., et al.
(author)
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F-16XL Geometry and Computational Grids Used in Cranked-Arrow Wing Aerodynamics Project International
- 2009
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In: Journal of Aircraft. - : American Institute of Aeronautics and Astronautics (AIAA). - 0021-8669 .- 1533-3868. ; 46:2, s. 369-376
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Journal article (peer-reviewed)abstract
- 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.
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| 6. |
- Görtz, Stefan, et al.
(author)
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Evaluation of the Recursive Projection Method for Efficient Unsteady Turbulent CFD Simulations
- 2004
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In: 24th INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES. ; , s. 1-13
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Conference paper (peer-reviewed)abstract
- The Recursive Projection Method (RPM) hasbeen implemented into an unstructured CFD code to improve the efficiency of dual time steppingfor unsteady turbulent CFD simulations.RPM is a combined implicit-explicit method that enhances convergence. It can easily be implementedinto existing codes and the solver’s existing acceleration techniques can be used withoutchange. The method has been evaluated by computing the periodic self-induced shock oscillations over an 18% thick biconvex airfoil at0◦ angle of attack, a Mach number of 0.76 anda Reynolds number of 11 million. On average,RPM accelerated the convergence of the innerloop of dual time stepping to a predefined convergencecriterion by a factor of about 2.5.
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| 7. |
- Görtz, Stefan, et al.
(author)
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Numerical Solutions for the Cawapi Configuration on Unstructured Grids at KTH/FOI, Sweden : Part I
- 2009
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In: Understanding and Modeling Vortical Flows to Improve the Technology Readiness Level for Military Aircraft.
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Book chapter (other academic/artistic)abstract
- Steady and unsteady viscous simulations of a full-scale, semi-span and full-span model of the F-16XL-1 aircraft at seven different flight Reynolds/Mach number combinations have been performed with an unstructured CFD code. The steady-state simulations are with several turbulence models of different complexity. Detached-Eddy Simulation (DES) has been used to compute the unsteady flow. The computed results are compared with public domain flight-test data. Very good agreement is demonstrated for surface pressure distribution, local skin friction and boundary velocity profiles. The different turbulence models performed almost equally well, except the Spalart-Allmaras model, which failed to predict the flow qualitatively and quantitatively. The Differential Reynolds Stress Model (DRSM) outperformed all other models when it comes to local span-wise skin friction. DES was superior over RANS modeling at the highest angle of attack, where the flow over the outer wing is separated and partly unsteady.
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| 8. |
- Görtz, Stefan, 1975-
(author)
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Realistic simulations of delta wing aerodynamics using novel CFD methods
- 2005
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Doctoral thesis (other academic/artistic)abstract
- The overall goal of the research presented in this thesis is to extend the physical understanding of the unsteady external aerodynamics associated with highly maneuverable delta-wing aircraft by using and developing novel, more efficient computational fluid dynamics (CFD) tools. More specific, the main purpose is to simulate and better understand the basic fluid phenomena, such as vortex breakdown, that limit the performance of delta-wing aircraft. The problem is approached by going from the most simple aircraft configuration - a pure delta wing - to more complex configurations. As the flow computations of delta wings at high angle of attack have a variety of unusual aspects that make accurate predictions challenging, best practices for the CFD codes used are developed and documented so as to raise their technology readiness level when applied to this class of flows. Initially, emphasis is put on subsonic steady-state CFD simulations of stand-alone delta wings to keep the phenomenon of vortex breakdown as clean as possible. For half-span models it is established that the essential characteristics of vortex breakdown are captured by a structured CFD code. The influence of viscosity on vortex breakdown is studied and numerical results for the aerodynamic coefficients, the surface pressure distribution and breakdown locations are compared to experimental data where possible. In a second step, structured grid generation issues, numerical aspects of the simulation of this nonlinear type of flow and the interaction of a forebody with a delta wing are explored. Then, on an increasing level of complexity, time-accurate numerical studies are performed to resolve the unsteady flow field over half and full-span, stationary delta wings at high angle of attack. Both Euler and Detached Eddy Simulations (DES) are performed to predict the streamwise oscillations of the vortex breakdown location about some mean position, asymmetry in the breakdown location due to the interaction between the left and right vortices, as well as the rotation of the spiral structure downstream of breakdown in a time-accurate manner. The computed flow-field solutions are visualized and analyzed in a virtual-reality environment. Ultimately, steady-state and time-dependent simulations of a full-scale fighter-type aircraft configuration in steady flight are performed using the advanced turbulence models and the detached-eddy simulation capability of an edge-based, unstructured flow solver. The computed results are compared to flight-test data. The thesis also addresses algorithmic efficiency and presents a novel implicit-explicit algorithm, the Recursive Projection Method (RPM), for computations of both steady and unsteady flows. It is demonstrated that RPM can accelerate such computations by up to 2.5 times.
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| 9. |
- Görtz, Stefan, et al.
(author)
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Recursive Projection Method for efficient unsteady CFD simulations
- 2004
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Conference paper (peer-reviewed)abstract
- The Recursive Projection Method (RPM) has been implemented into the unstructured grid CFD code EDGE to accelerate the inner-loop convergence of dual time stepping. The method tries to identify the slowly converging subspace and applies Newton iterations in this subspace together with a fixed point scheme in the complement. The method has been employed to compute the steady and unsteady viscous flow around a circular cylinder for a Reynolds number of 100. When converging to machine accuracy, RPM accelerated the convergence of the steady-state solution by a factor of 2.5. The time-accurate simulations were accelerated by a factor of about two.
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| 10. |
- Görtz, Stefan, et al.
(author)
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Standard Unstructured Grid Solutions for Cranked Arrow Wing Aerodynamics Project International F-16XL
- 2009
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In: Journal of Aircraft. - : American Institute of Aeronautics and Astronautics (AIAA). - 0021-8669 .- 1533-3868. ; 46:2, s. 385-408
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Journal article (peer-reviewed)abstract
- 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.
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