THE NEED TO SPEED UP AERODYNAMIC VEHICLE DEVELOPMENT is of fundamental

CUSTOMIZED SOLUTIONS12 Fluent News·Summer 2005 NL 595 - Copyright © 2005 Fluent Inc.Speeding up AerodynamicVehicle DevelopmentBy Silvestre Artiaga-Hahn, Nicolas Kruse, Frank Werner, Adam Opel AG, Ruesselsheim, GermanyTHE NEED TO SPEED UP AERODYNAMIC VEHICLE DEVELOPMENT is of fun-damental importance to the automotive industry. In vehicle aerodynamics, a number of configurations can be evaluated in a wind tunnel in a matter of days.To compete, numerical simulation must perform with comparable efficiency. The Opel Virtual Aero Lab (OVAL), an automated workflow tool that combines com-mercial and free software, has been created to speed up the numerical simula-tion process [1]. Beside the improvements in terms of standards and efficiency when running numerous variants of a design, the tool provides a better under-standing of the flow field. In short, aerodynamic optimization through simulation can now be realized. The overall impact of this tool on the vehicle development process at Opel has increased significantly since its deployment.With traditional C FD software, the analyst is usually a specialist in both fluid dynamics and numerical simulation. Wind tunnel test engineers have a different set of skills. However, it would be more effective for vehicle development if these two groups could work more closely together. In an ideal situation, CFD would be just one tool for the wind tunnel engineer to use in the process of optimizing cars. At Opel, this goal led to the development of OVAL.The first step in the development of OVAL was an analysis of the effort required for every step of a typical external aerodynamics simulation. The four major steps are CAD clean-up, mesh generation, solution of the Navier-Stokes equations, and postprocessing. The analysis revealed that up to 90% of the tasks are repetitive manual steps for every new simulation. Furthermore, it was determined that much of this work, such as the manual selection of zones during the meshing process, could be done automatically.At Opel, the preparation of watertight surface models is usually done using ANSA™. To keep control over feature representation in the hands of the simula-tion engineer, surface mesh preparation and generation with ANSA is excluded from automation in OVAL. Instead, meshing the virtual wind tunnel volume is done using TGrid, where most functions, like choosing a correct boundary layer resolution, can be automated. The solution is carried out using FLUENT, and scripts (journal files) have been developed over the years to minimize the work required for problem setup. Even with these journal files, some individual settings are still done manually, even though manual setup can be a source of user errors and should be minimized or eliminated if possible. Postprocessing and reporting the results is the last action taken in the simulation process. This can be a time-consuming process, but it is a straightforward one to automate.While TGrid and FLUENT provide user interfaces that allow automation of many actions, static scripts, which can depend on model changes, are not suf-ficient for more complex functions. To avoid such problems, elements of mod-ern programming languages have been introduced as part of the automated tool. Some of these elements, implemented in the form of a graphical user interface, include logical queries, loops, and data input and output. In addition to these functions, MATLAB, a high level programming language, is applied fordata processing, image manipulation, figure generation and HTML report gen-The OVAL start-up windowThe OVAL FLUENT setup windowStarting page of the HTML simulation reportCUSTOMIZED SOLUTIONSNL 595 - Copyright © 2005 Fluent Inc.Fluent News ·Summer 2005 13eration. The scripting language PYTHON [2] is employed for the graphical user Interface (GUI ) and script creation.Communication between the GUI and the internal C functions within TGrid or FLUENT is done with SCHEME [3]. This scripting language is used to create indi-vidual graphical elements (widgets) that can be used to enter information. In TGrid, they are used in the process of volume mesh generation. In addition to the fully automatic mode, an interactive mode for these functions is still available to users, since it is advantageous for error checking. For example, problematic areas, such as those with insufficient surface resolution, might exist that cause errors in the volume mesh generation process. Minimization or elimination of user errors has been a positive outcome of the OVAL tool.In FLUENT, journal files make use of the text user interface (TUI). These text com-mands in combination with SCHEME commands allow an engineer to set up and run FLUENT without user interaction. User-defined functions (UDFs), written in C, provide access to nearly all simulation results, making the automation of the postprocessing stage easy to do.To gain access to FLUENT, the user is required to fill in a standardized form. Three tabs allow the user to select from preprocessing, solving, and postprocessing options. OPEL knowledge is used behind the scenes to ensure that the calculation methods used for the external aerodynamic simulation are consistent with those used in past simulations. Once these forms have been completed, a script is creat-ed using SCHEME and TUI commands that directs FLUENT to run automatically.Some of the automated functions are: the detection of vehicle parts; the calcula-tion of relevant values, such as projection area, wheel center points, and aerody-namic coefficients; and the export of flow relevant data for report generation.The OVAL process was first used for the development of the new Opel ASTRA.Ordinarily, a base calculation requires approximately 20 hours for an experienced user. About half of that time is used for CAD clean-up and surface mesh genera-tion. The other half is needed for volume mesh generation, model setup, and post-processing. OVAL has reduced the required time for this base run to 12 hours. The time savings does not occur within the CAD clean-up and surface meshing process-es, since it is better if the user can decide on his own which model features are of interest and how these should be incorporated into the model. This decision costs time, but fewer user errors are introduced into the CFD model. A variant calcula-tion, on the other hand, requires less model creation, so the time savings are shift-ed more toward volume mesh generation, model setup, and post-processing, and the time savings are larger by comparison.In many cases, an engineer can create a new variant in four hours, run the flow simulation, and have the simulation report the following morning in the form of an automatically generated HTML report. These reports, generated by MATLAB,can be viewed with any browser. They include project development overviews of important aerodynamic coefficients and links to an in-depth analysis report of every simulation run including set-up, key figures, analytical graphs, and a huge standard set of geometry and flow visualization images.Reducing the development timeframe for a new product from 30 to 20 months [4], OVAL is the key enabler for virtual aerodynamic development to reach that target.References1Kleber, A.; Artiaga-Hahn, S.; Werner, F . OVAL – Opel Virtual Aero Lab Eine neueEntwicklungsumgebung für die Fahrzeug-Aerodynamik, 2004, VDI Tagung Wuerzburg.2von Rossum, F .; Drake, L. Jr. The Python Language Reference Manual ; Network Theory Ltd., 2003.3Clinger, W.; Kelsey, R.; Rees, J. Revised Report on the Algorithmic Language Scheme. J. Higher-Order Symb. Comp ., 1993, 11(1), 7-105.4Lepper, K.-H. Integrierte Virtuelle Produktentwicklung vom Entwurf bis zur Serienreife,2004, VDI-Berechnungstagung, Würzburg.A sample of flow visualization that is part of the simulation reportSample project report for variants in one styling themeReduction of elapsed time per model variant; the total time is reduced from 11hours for the old process to only 4 hours workload with OVAL。