Geometric Model Reconstruction - Computational Hemodynamics: Theory, Modelling and Applications - page 56

data, the geometry can be mathematically described with a Non-Uniform Rational
B-Spline (NURBS) for simple integration into a CFD-mesh software. This involves
patching or overlaying a water-tight surface which holds point, line, and face data
that are interconnected and relational. This is important since some CAD models
will not be water-tight. The decision to directly output or apply a water-tight surface
is dependent on the software, since a lot of CAD processing is self-contained and
can be performed within the software itself.
It is important for the reader to appreciate the structure of 3D CAD models and
its relationship to CFD meshing. The exported model should be in a format that
can be read by the CFD meshing software. The format of the model serves as an
intermediary between the segmentation software and the receiving CFD/CAD soft-
ware. Therefore there is a reliance on the file format adhering to standards to allow
smooth data exchange. However this is not always the case and there are varia-
tions in CAD formats among different software vendors. This causes loss of data
and therefore loss of geometry details, leading to what is known as a ' dirty ' CAD
geometry. Such dirty geometries exhibit stray points, non-intersecting lines, cross-
overs, and slivers in the geometry. When this occurs, the user needs to clean up
the geometry which can be performed within the CFD meshing program or exter-
nal CAD-specific software. This is another reason for ensuring that the segmented
model contains a water-tight surface.
The output file can be set as standard 3D CAD formats such as STL, IGES (Ini-
tial Graphics Exchange Specification), or STEP. As an example, the data structure
for the above mentioned CAD formats is given below:
• STL or stereolithography, files are commonly used for rapid proto-typing manu-
facturing. It contains only surface geometry of a three dimensional object only.
The surface is made up of small triangulated faces, called facets. Each facet
is represented by three vertices (corners of the triangle) and its perpendicular
direction. user. If too many triangles are created, the STL file size can become
unmanageable. If too few triangles are created, curved areas are not properly
defined and a cylinder begins to look like a hexagon (see example below).
• IGES (Initial Graphics Exchange Specification) is a platform independent for-
mat used to allow digital translation between CAD software. It contains two
main data entities: geometry (to describe shapes such as curves, surfaces, sol-
ids and relationships) and non-geometry (to describe graphical or computational
data such as colour, mass, time etc). The format is standardised to allow file
interchange between different vendors. Unfortunately, CAD vendors have their
own interpretation of the IGES standard, which renders some the information
useless to others. Additionally, the size of an IGES file can be typically larger
than other formats because of the two data entities it stores. Further information
on IGES format can be found in www.steptools.com.
• STEP , is the acronym for Standard for The Exchange of Product model data
which is a proposed International Standards Organization (ISO) standard for
electronic data exchange. It is also known as the product data exchange using
STEP (PDES) in the United States.