Machine Design

No CAD geometry? No problem for parametric FEA

A parametric FEA program now lets engineers manipulate shapes and positions of features in meshed models without accessing the original CAD model or CAD software


To stretch the rear end of the SUV, Ansys' Beley selected them in a box, named them, and made them pseudo or virtual geometry. Beley first stretched the trunk by typing in 150 mm for the X direction. The software stretches the elements and tells that it has lost only 8% of the element quality.
To increase stiffness, ParaMesh could analyze the pipe weldment first for thicker radial and then for thicker circumferential ribs. The chart shows thickness-versus-stiffness change for both ribs. Thickening the circumferential rib increases stiffness more than adjusting the radial rib.
To add stiffness to the sheet-metal panel, a ParaMesh user could add a bead or raised rib to the mesh of the part. To find a right height, different meshes could be analyzed for deflection, or natural frequency by a modal analysis.

Paul J. Dvorak
Senior Editor

Paramesh from Ansys Inc., Canonsburg, Pa. (, lets analysts extensively change FEA geometry for designs studies. For instance, cross members on a truck frame might be thickened or thinned for crash studies, or adjusted fore or aft to find a stiffer location. The software also works when CAD models are poorly parameterized. Developers say it needs no CAD license, nor much expertise to use.

Most FEA programs link to a CAD system so analysis software cleanly imports geometry by avoiding IGES transfers. If there's a flaw in the design, engineers can quickly adjust the model in the CAD system, and reimport it to the analysis software. But the new capability avoids the necessity of a CAD program.

At a recent exhibition, Jean Daniel Beley, one developer of the software with Ansys, showed how several adjustments could be made to a truck frame that exists only as a 20-year old Nastran FEA mesh. He selected elements in a meshed engine cradle by drawing a box around them and indicated in a menu he wanted to move them 110 mm in a particular direction. "No remeshing is needed because the system modifies existing mesh by slightly reshaping some elements," says Beley. "The system tracks misshapen and poor-aspect-ratio elements." Analysis results can include animations of displacements as the selected elements move from one extreme to another, and plots of element quality. The software works on any type of mesh -- structural, CFD, multiphysics, and magnetic.

The software also indicates how much reshaping the mesh can take. "It generally depends on element size," says Beley. "Finely meshed models can be reshaped more than coarsely meshed ones." If an adjustment is too severe, and an element gets inverted, the analyst just moves it back. The software warns users of element inversion and provides other indicators of mesh quality, such as aspect ratio and color coding.

After the software puts the same amount of stretch in each element, an optimization algorithm smoothes them and minimizes aspect ratios because some elements cannot be stretched as much as others. A few misshapen elements in a stretched SUV body show up along the window edge. Stretching the body to 300 mm turns only a few more elements red. "It still could give useful information," says Beley.

The software does not allow removing elements, for example, to make a hole, because that would change element numbering and the input file. However, users can make an existing hole larger or smaller. "A pin hole, for instance, can be expanded by compressing surrounding elements, or it can be collapsed to almost nothing," says Steve Pilz, a product manager with Ansys. He adds that analysts quickly pick up the technique, and nonanalysts can be doing useful things after a single day of training.

Beley says it's a good idea to move no more than five design features at one time, although there are no established limits. The software works with FEA models from Nastran, Ansys, Patran files, CFD files from Fluent, and those from StarCD.

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