Catálogo de publicaciones - libros

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

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.

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.

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.

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.

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.

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 code the 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 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.

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.

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.

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.