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The laminar flow around a NACA 4402 airfoil is investigated at a Reynolds number of Re = 6000 using the RANS solver FLOWer. Significant flow separation occurs over almost the whole regime of angles of attack for steady onset flow conditions. Therefore, the calculated propulsive efficiencies of pure plunge motions are rather poor. This leads to the need of a combined plunge and pitch motion, which is investigated subsequently. For combined motions, cases with high propulsive efficiencies are found over a wide range of produced thrust. [ABSTRACT FROM AUTHOR]

The MEGAFLOW projects — I and II — are key drivers for CFD (Computational Fluid Dynamics) technology research within Germany, possibly in Europe as a whole. MEGAFLOW combines the development of most modern CFD tools and suites with the daily application of these tools for aircraft design and data processes. Due to aircraft industries being forced to optimize their designs and to give performance predictions in highest accuracy terms, while the time for design cycles are reduced, high level CFD becomes more and more a basic design tool. However, in many cases CFD reaches its limits due to the ongoing increase of complexity with respect to geometry and flow conditions. Further development of physical flow modeling and further validation of new application cases need to be continued. This paper describes AIRBUS-D requirements on methods and tools development in MEGAFLOW, validated by AIRBUS-D test cases. It outlines which requirements are fulfilled, which are still open, and which have had to be added over the years. [ABSTRACT FROM AUTHOR]

The present contribution outlines several selected applications of the MEGAFLOW software at DLR, roughly according to the time-frame of the MEGAFLOW II project from 1998 - 2002. The majority of the applications is based on 3-dimensional viscous computations featuring the solution of the compressible Reynolds-averaged Navier-Stokes equation in combination with one or two transport equation turbulence models. The examples highlight a quite broad range of applications. This refers to onflow speed as well as to the mode of application, covering analysis as well as design and optimization tasks. In general a clear trend in the use of the MEGAFLOW system becomes visible. On the one hand the analyses of very specific flow and/or aircraft details on overall configurations of increasing complexity is carried out. On the other hand more design and optimization applications are requested. For both types of applications the MEGAFLOW software has become an indispensable tool for the aerodynamic and also multi-disciplinary tasks of DLR. [ABSTRACT FROM AUTHOR]

The 3D Navier-Stokes solver TAU is coupled with linear stability analysis methods in order to predict flows including transition due to Tollmien-Schlichting (TS) and crossflow (CF) instabilities. The new simulation capability is investigated for an airfoil and compared with data of 2D boundary layer methods that include transition prediction based on a well-known envelope method and with experiments. The results indicate the levels of grid and residual convergence needed for accurate transition prediction. First applications of transition prediction in 3D for a 1:6 prolate spheroid are discussed. It is shown that transition calculations for fully 3D flows are numerical feasible and yield physically reasonable results for moderate angles of attack. [ABSTRACT FROM AUTHOR]

A Large-eddy simulation version of the FLOWer code is introduced to compute attached flow around a quasi three-dimensional airfoil at a Mach number Ma = 0.088, Reynolds number Re = 8 × 105, and an angle of attack of 3.3°. For the treatment of the subgrid-scale stresses the MILES approach is chosen. The visualization of instantaneous flow fields shows the typical flow features such as the streaky structures in the near-wall region to be well resolved and the overall agreement of the computational results with experimental data to be satisfactory. [ABSTRACT FROM AUTHOR]

In the aerodynamic industrial design process, the use of numerical simulation is of ever increasing importance. In order to adequately capture flow features such as pressure-induced separation or shock-boundary-layer interaction, an appropriate representation of turbulence is needed. This contribution summarizes the efforts undertaken at TU Berlin to develop, implement and validate advanced linear and non-linear models in the aerodynamic flow solvers FLOWer and TAU in the framework of MEGAFLOW and related projects. The accuracy of the approaches is discussed on various cases and statements with respect to their computational performance are given. The results indicate that improved predictive accuracy can be obtained from advanced Eddy-Viscosity Models at a moderate computational surplus. [ABSTRACT FROM AUTHOR]

Shape parametrization has been identified as an import issue in aerodynamic design optimisation based on high-fidelity CFD-methods. For given shapes, which are available as CAD-models, post-parameterization method, based on freeform deformation, has been established to simplify and to automate the generation of geometrical variants to be used for CFD analyses. To create the necessary deformation lattices, structured grid generation techniques of a grid generation system, developed at DLR, are utilized. As this grid generation system has the salient feature to store and to replay a sequence of processes with different parameter settings, modifications of shapes, given by polygonal curves and surfaces, can be performed instantly. The present freeform deformation method has reached a state, where it can be integrated into design loops to handle a variety of shape optimisation tasks. In two examples the applicability of the method for aerodynamic wing design and detailed design of a wing tip is demonstrated. [ABSTRACT FROM AUTHOR]

The present paper aims at describing the potential of the adjoint technique for aerodynamic shape optimization. After a brief description of the aerodynamic optimization process developed at DLR, specific requirements for an optimization framework combined with the adjoint technique are introduced. The drag reduction by constant lift and pitching moment for the RAE 2822 airfoil in transonic flow is then presented as validation case. An extension to multi-point optimization demonstrates the capability of the methodology to solve more complex problems. At the end, the body optimization and the wing optimization of a supersonic commercial aircraft confirm the flexibility of the framework and the efficiency of the adjoint technique. [ABSTRACT FROM AUTHOR]

Detailed numerical shape optimization will play a strategic role for future aircraft design. It offers the possibility of designing or improving aircraft components with respect to a pre-specified figure of merit subject to geometrical and physical constraints. However, the extremely high computational expense of straightforward methodologies currently in use prohibits the application of numerical optimization for industry relevant problems. Optimization methods based on the calculation of the derivatives of the cost function with respect to the design variables suffer from the high computational costs if many design variables are used. However, these gradients can be efficiently obtained by solution of the continuous adjoint flow equations. [ABSTRACT FROM AUTHOR]

This part of the MEGAFLOW project is concerned with the numerical simulation of the viscous compressible flow around transport aircraft high lift configurations and its validation against wind tunnel experiments. The investigations are based on the solution of the Reynolds-averaged Navier-Stokes equations (RANS) using the MEGAFLOW code system with a finite volume parallel solution algorithm. Both the block-structured (FLOWer) and the unstructured (TAU) code modules are used for this task. Based on the high lift configurations DLR-ALVAST and DLR-F11 lift polars are computed with both codes and compared against each other and against measurements. Further on the structured Chimera technique implemented in FLOWer is applied to a high lift configuration. [ABSTRACT FROM AUTHOR]

It is demonstrated that the unstructured, hybrid method of the MEGAFLOW project is capable to compute the engine-airframe installation drag for several engine types although drag differences for varying engine positions can be very small. Results are presented for the DLR-F6 and ALVAST configurations with through-flow nacelles and powered engines. Verification by grid refinement as well as validation with wind tunnel data completes this investigation. [ABSTRACT FROM AUTHOR]

The accuracy of the DLR structured and unstructured computational fluid dynamics (CFD) codes in predicting aircraft forces and moments on several configurations at low and high Mach and Reynolds numbers is investigated. Using a combination of a high quality grid, i.e. a grid with sufficient resolution of important flow features, low levels of artificial dissipation and advanced turbulence models, the structured code (FLOWer) is able to both qualitatively and quantitatively predict the experimentally measured drag, lift, pitching moment and pressure distribution. Compared to the structured methods the total time for grid generation is significantly reduced with the unstructured approach (TAU). The quality of the flow solution is comparable to the structured method at significantly higher computational costs. [ABSTRACT FROM AUTHOR]

A possible solution for the growing demand for graphical user interfaces for simulation codes is shown. The situation at the German Aerospace Center is described and a set of resulting requirements for a flexible, configurable software system is collected. The main idea of separating the description of parameters and the editor is briefly summarized. Finally the current development status is presented, including some screenshots. [ABSTRACT FROM AUTHOR]

At the present time, aerodynamic analysis and numerical methods are inexorably linked. Thus, a quality control system for the aerodynamic simulation of a complete aircraft in varying configurations is very useful. For this purpose it is critical that suitable turbulence models for the various tasks in hand are available to a prospective user. These models should be robust, and yet sufficiently accurate to enable a proper evaluation of the flow in question. This paper provides an overview of the progress in turbulence model implementation in the TAU code during the duration of the MEGAFLOW project. [ABSTRACT FROM AUTHOR]

Local grid refinement for unstructured meshes is a common approach to improve the accuracy of a solution for a given CFD problem or to reduce the amount of needed points for a numerical calculation. As part of the DLR-TAU code the Adaptation provides a hierarchical refinement and de-refinement tool for 2D and 3D hybrid grids. The object of this tool is to adapt the grid automatically to a given solution. This article described the major components of the tool, the grid refinement with surface approximation and the y+ adjustment along wall-normal lines. Furthermore an overview of the used sensor functions is given, which are available for the detection of the local refinement and de-refinement. [ABSTRACT FROM AUTHOR]

The paper describes a selection of algorithmic developments which have been implemented in the hybrid Navier-Stokes solver TAU during the MEGAFLOW II project. The paper concentrates on algorithms that help to improve the performance, the accuracy as well as the functionality. The algorithms presented are implicit MAPS-smoothing, low Mach number preconditioning, least square reconstruction in combination with a cell centered approach, the actuator disk boundary condition and a formulation for moving coordinate systems enabling steady solutions in a rotating frame. Results are presented in comparison to earlier versions of the TAU code, highlighting the improvements with respect to performance and/or accuracy. Comparisons with experimental data and results obtained with the FLOWer code are used to validate the new functionalities. [ABSTRACT FROM AUTHOR]

A brief introduction is given which first describes the history of frame in which the TAU code was developed before explaining the main advantages which were the drivers for the selection of the approach. In the following an algorithmic overview describes shortly the code functionality before a section about the code design gives some more insight about the implementation and its scripting capability. [ABSTRACT FROM AUTHOR]

The turbulence models implemented into the FLOWer code are briefly characterized, considering their basic equations and emphasizing their differences with respect to their aimed at field of application. The influence of the models on the flow solution is demonstrated for two simple test cases, the flow around the RAE 2822 airfoil, representing transonic conditions, and the flow around the Aérospatiale A airfoil, representing high-lift conditions. Results for industrially more relevant test cases of the flow around two different wing-body configurations and a three-element airfoil are presented, confirming the findings for the simple geometries at least under transonic conditions. From this, recommendations for the choice of suitable models are derived. [ABSTRACT FROM AUTHOR]

This paper summarizes the developments of transition prescription and transition prediction techniques which were implemented into the DLR Reynolds-averaged Navier-Stokes (RANS) solver FLOWer in the framework of the DLR projects MEGAFLOW and MEGAFLOW II and the German research project MEGAFLOW. The very basic transition handling functionalities which FLOWer provided before the projects started were generalized in order to prescribe arbitrary transition lines on very complex aircraft geometries with different components, such as wings, fuselages or nacelles. A number of transition prediction methods were incorporated into the code and an infrastructure was built up in order to handle the underlying transition prediction strategy which results in an iteration process within the solution process of the RANS equations. Finally, physical models for the modeling of transitional flow were implemented and tested. [ABSTRACT FROM AUTHOR]

This paper comprises the developments of the block structured Navier-Stokes solver FLOWer during the DLR project MEGAFLOW II. At first the status of the FLOWer code including its numerical and physical capabilities at the end of MEGAFLOW II is presented. After introducing the objectives of the FLOWer development for MEGAFLOW II the major developments and corresponding results are discussed. [ABSTRACT FROM AUTHOR]

This part of the MEGAFLOW project addresses the hybrid unstructured grid generation. It covers a description of the hybrid unstructured grid generator Centaur from CentaurSoft and its application at DLR and EADS-Airbus using the MEGAFLOW software system with the unstructured finite volume RANS solver TAU. [ABSTRACT FROM AUTHOR]