GEOMETRY-BASED VIRTUAL MODEL VARIANTS FOR SHAPE OPTIMIZATION AND CAD REFEED

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1 GEOMETRY-BASED VIRTUAL MODEL VARIANTS FOR SHAPE OPTIMIZATION AND CAD REFEED *Dr. Werner Pohl, ** Prof. Dr. Klemens Rother *Fast Concept Modelling & Simulation (FCMS) GmbH, Munich, Germany, **University Of Applied Sciences, Munich, Germany ABSTRACT 0 Introduction As of today the exploitation of the potential of numerical optimization methods is still quite limited. In industry mostly so-called sizing optimizations are carried out, with sheet metal thicknesses or material data as optimization parameters. In case more and different parameters like position changes of components within assemblies or general shape changes could be utilized for the generation of an increased multitude of virtual model variants, design spaces for optimization could be larger and more densely populated. Optimization algorithms could produce results of higher quality and higher potential for product performance improvement. The lecture explains, how the FCM software solution (Fast Concept Modeller) as CAA-integrated CATIAworkbench produces and applies such extended capabilities of a comprehensive variant generation of components and assemblies for significantly improved optimization results. 1 About the advantage of geometric variables Geometry models generated with the help of the FCM software are based on wireframes, which have the capability to control a series of simple basic objects. These wireframes create an implicit parameterization (Fig.1), on top of which explicit parameters like position and shape changes can be defined with little effort. A geometry is fundamentally more flexible than a mesh and does not know the limitations of a mesh morpher.

2 Fig.1: geometric basic objects and model generation starting from an implicitly parametric wireframe (example: overland bus MAN-Munich; investigating a bus slice ) FCM-objects within the CAD system (here CATIA) do have a different structure than native CATIA objects, which a designer would utilize for his design. The purpose of such FCM objects is a CAE-ready changeability, which cannot be achieved with native objects. 2 Process improvement by means of geometry-based optimization A geometry object generated within a CAD system or by FCM-in-CAD will - for the purpose of functional safeguarding directly be used by standard software tools like meshers, solvers and optimizers as they are already part of a customer s CAE environment. The automated input for CAE is a FCM CATPART which carries all simulation attributes, connectivities and boundary conditions. Output (result) is the changed geometry which fulfills the previously defined optimization objectives. As a consequence, the traditional manual transformation from a resulting meshed virtual model back to the corresponding geometry is not necessary anymore. The optimizer has changed the geometry during the optimization loop based on the usual design tables. For each loop the corresponding simulation models are automatically derived from that changed geometry und newly subjected to mesher, solver and optimizer for results generation and retrieval. Thus, not only the usual sizing parameters can be used for the optimization, but also parameters for shape and change of topology (position). Latter parameters - by experience -

3 carry a higher potential for e.g. weight optimization under structural and transient dynamic loads. A geometric master model is the source of FEM-shell- or FEM-solid- or mixed-models. FEM shells and FEM solids are generally accepted as accurate. In addition, replacement models may be automatically derived from a master model. They are based on FEM-beams for geometric objects which can be engineering-wise modelled as FE-beams. The advantage of replacement models consisting of a mixture of FEM-beams, -shells and solids is the faster meshing and simulation time. If replacement models are used for the optimization loops, it is possible to perform the optimization on much denser populated meta-spaces for the optimization. The higher the population density of meta-spaces, the higher the chance for better optimization results. Replacement models are approximations. Especially connectivities, e.g. junctions, require special attention. FCM offers fast running approximations like spring-mass-models or previously automatically generated superelements as a solution. For transient dynamic simulations specific CRASH-BEAMS may be used. Replacement models may run faster by up to a factor of 100 for NVH and even up to 1000 for CRASH compared to traditional light-weight shell model representations. This, finally, is the key for a true MDO. Even complex CRASH models formulated as replacement models can be investigated and improved together with other product attributes like NVH, DURABILITY, ACOUSTICS and more. A further performance improvement may be achieved by the coupled adoption of integrated optimizers (e.g. SOL200 in NASTRAN) with the usual stand-alone optimizers. Fig.2 displays the MDO-process. Fig.3 points to the enormous performance gain for CRASH applications solved with CRASH BEAMS.

4 The fact, that all derived simulation models are associative to the geometric master model, allows for a direct validation of the optimized fast replacement model with the accurate shell or/and solid model. Fig.3: Bus-rollover load case with CRASH-BEAM-replacement model (run time 3 min.) allows for 100s of variants within short time; optimization result to be validated with a single run of a conventional explicit transient dynamics solver 3 Process continuity through CAD refeed The classical bottleneck in the virtual product development process is the missing link back from CAE to native CAD. The CAD designer wants to get his CAD model back after a functionality safeguarding check in his familiar format to continue his construction design work. Fig.4 shows the approach thru the use of specific FCM functions, which allow the refeed of an FCM CAE-ready parametric geometry into a CAD-ready construction geometry. Fig.4: process for the refeed of an optimized geometry in FCM format within CATIA back to a CATIA construction geometry format

5 4 Summary By means of a specific CAE-ready parameterization of geometry models embedded into CAD systems (here CATIA) it is possible to lift to date unexplored optimization potentials. This especially applies to the usually high influence of shape and topology parameters on the functional response and weight conditions of mechanical systems in transport industry (body engineering, design of components etc.). The FCM-software is a solution tightly integrated with CATIA capable of generating such models. Integration into other CAD systems is technically possible. A geometric CAE-ready parameterized master model allows for an automated derivation of very fast running CAE replacement models and the validation of the optimization result with accurate simulation models. The process enables comprehensive MDO solutions, which even include the CPU-intensive CRASH product attribute.