Audi, BMW, DaimlerChrysler and Renault

A Revolution in the World of Fluid Dynamics Analysis 

Audi, BMW, DaimlerChrysler, and Renault turn to PolyWorks^® to reduce the time for CFD analysis by up to 83%. 

Major car manufacturers have turned to the polygonal modeling technology of PolyWorks to generate major cost reductions for their Fluid Dynamics analyses. Discover how the unique tools that PolyWorks offers have enabled Audi, BMW, DaimlerChrysler, and Renault to reduce the time required to prepare a CFD-ready polygonal model from seven days to one day.

Fluid Flow analysis is the study of how fluids, like air, liquids, and gases move in and around solid objects, such as airplane wings, automobile bodies, or petroleum pipelines. Most major car manufacturers worldwide face fluid flow problems in their design work, such as air flow over automobile surfaces measuring lift, drag, yaw, and friction. Typically, traditional fluid dynamics analysis is conducted through wind tunnel testing, an expensive and time-consuming operation that requires well-trained technicians. 

The emergence of 3D digitizing technology has revolutionized the way to analyze Fluid Flow by opening the door to “Digital Wind Tunnel Testing”. The millions of data points captured by the non-contact 3D digitizers represent an excellent source of information for digitally simulating fluid flow and replicating typical wind tunnel testing analysis at a fraction of usual cost and time. 

The Challenge

To perform powerful Digital Wind Tunnel Analysis, CFD software such as Exa’s PowerFlow^® necessitates polygonal models that meet strict requirements in terms of accuracy, topology, size, etc. Not too long ago, several steps were required to prepare the model for CFD analysis, which could take up to seven days of work. First, the digitized point cloud had to be transformed into NURBS surfaces using a reverse-engineering software system. The resulting surfaces were loaded into CAD software, and several editing operations, such as feature re-construction and removal of unnecessary details, were conducted. The CAD model then had to be tessellated using another software package to recreate a meshed model. Most of the time, this tessellated polygonal model would necessitate other modifications in order to meet the 100,000-triangle target required for CFD analysis. 

“PolyWorks offers a straightforward approach which has  drastically changed the preparation and optimization of the polygonal models for CFD analysis” said Dr. Hans-Peter Duwe from Duwe-3d in Germany. “PolyWorks offers a wide set of polygon-editing tools that allow us to reconstruct feature curves, remove over-detailed features, and create closed polygonal models that can be used directly inside Exa’s PowerFlow. All operations can be conducted within one software solution, which significantly reduces the time and cost of the Fluid Flow analysis,” he continued.

The Solution

PolyWorks Complete Toolkit for Optimizing the Polygonal Model of an Automobile 

1. Create the polygonal model 

• Car designers create a physical model using clay or another similar composite; the size of the car prototype may vary from full size to 1/2, 1/4, 1/10 reproductions 
• The clay model is entirely digitized using a 3D digitizer 
• The multiple scans are subsequently aligned by photogrammetry (for a full-sized model) or using PolyWorks’ unique best-fit method that quickly aligns scans using the geometric features of the object (for smaller objects) 
• The aligned point cloud is meshed in PolyWorks and a highly accurate polygonal model is created with between 500,000 and 1,000,000 triangles and a tolerance ranging from 10 to 30 microns 

2. Edit the polygonal model for CFD 

A) Reconstructing feature curves 

One of the most important factors that influences the air flow of a model is the quality of its feature curves. Since 3D digitizers cannot capture sharp edges with great accuracy, editing work needs to be performed for the reconstruction. PolyWorks offers a powerful tool that detects and tracks feature curves and best-fits theoretical sharp edges. After the sharp edge curves have been extracted, they can be extended and intersected to create corners. 

B) Produce a compact and watertight polygonal mesh 

Here are the steps to create a compact and watertight model:

• Filling holes produced during the digitizing phase:
  • PolyWorks offers various hole-filling tools to close the surface of a polygonal model. For holes of small and medium complexity, users can rely on an automatic hole-filling method that smoothly interpolates curved sets of triangles within a user-defined 3D bridging distance 
  • For larger and more complex holes, users can create composite Bézier surfaces or NURBS surfaces on top of the polygonal model and insert triangulated surfaces that follow the curvature of the object 

• Deleting unnecessary features of the model:
  • The purpose of this operation is to keep the number of triangles as low as possible. CFD software such as Exa’s PowerFlow is optimized to process polygonal models of up to 100,000 triangles. To reduce the number of triangles, users can remove unnecessary triangles on highly-detailed areas of the model, such as grooves, air traps, etc. 
  •  User can then use composite Bézier surfaces or NURBS surfaces to reconstruct polygons in these areas

C) Inserting CAD surfaces

• Parts from an existing CAD model can be inserted to fill elements, such as the under-body, wheels, windshield, windows, and lights 
• Bézier surfaces and NURBS surfaces can be used to fill the areas for which no CAD is available

3. Reduce the number of triangles and optimize triangle orientation

The adaptive meshing technology of PolyWorks enables the creation of “intelligent” polygonal models, preserving high resolution over edges and fillets while creating larger triangles in flat areas. To comply with Exa PowerFlow’s strict requirements, a polygonal model should: 

• Contain approximately 100,000 triangles 
• Not contain any triangles with poor aspect ratios (height/base) 
• Have triangles whose orientation follows the curvature of the object 

PolyWorks offers advanced techniques to prepare a model that meets these requirements. Users can:

• Set the reduction parameter as a target number of triangles 
• Use a maximum edge length to prevent the creation of large triangles with poor aspect ratios 
• Specify the edge detection angle for preserving feature lines
• Invoke a mesh optimization algorithm that aligns the triangle edges along the curvature flow