Stephen Ferguson
Senior Consultant
Engineer
CD-adapco
Melville, N.Y.
Edited by Leslie Gordon
Although better
translators and CADembedded
CAE has
smoothed out the route
from model to solution,
there is still a significant
group of problems in
which geometries are
difficult to mesh. Estimates
from the automotive
industry suggest
that as much as 80% of a
typical CAE simulation
goes to generating the
computational model,
with most of that time
spent in surface preparation
and repair.
A good example of
dirty CAD is a collection of parts,
such as automotive underhood
components, that must be combined
into a single surface before
building a computational mesh
suitable for analysis. Individual
components might not fit together
perfectly, with volumes that overlap
or surfaces that interfere with
each other. Another example is a
fully detailed CAD part that requires
defeaturing. Yet another
example is a part assembly configured
for manufacture. It contains
gaps, likely to prevent meshing,
where welds, screws, rivets, and the
like are to be placed.
Shrink-wrapped CAD
To help solve the problem of
dirty CAD, “shrink-wrapping”
tools — such as our Surface Wrapper
which works inside our analysis software — reduce the time
it takes to get simulation results,
while producing more accurate
results. That’s because shrinkwrapped
models do not need geometric
simplification for meshing,
so that, if required, they can represent
the full complexity of component
geometry. However, when
model complexity is greater than
required for simulation (for example,
rivets on an aircraft wing), the
surface wrapper can also be used to
remove extraneous features.
The wrapper works for FEA,
CFD, crash, or any kind of analysis
where a high-quality triangulated
surface is necessary for a simulation-
ready mesh. From a user’s
point of view, surface wrapping is
automatic. The user first imports
a CAD geometry, sets a representative
“base-size” that determines the level of feature resolution in
the final surface, and presses the
“surface-wrap” button. The surface
wrapper always generates a closed
manifold or watertight surface.
Behind the scenes, the surface
wrapper works by shrink wrapping
a high-quality triangulated surface
mesh onto the geometry, closing
holes in it, and joining disconnected
and overlapping surfaces. The wrapper
quickly calculates the wetted
surface of the geometry (the total of
all the object’s surfaces that interact
with the surrounding material, for
example, air), discarding surfaces
outside the calculation-domain to
eliminate unnecessary detail.
Users can specify the level of
resolution surface-by-surface,
or use volume regions to specify larger areas of refinement. All size
specifications are relative to the
base size so wrapped surface can be
fine-tuned by just altering a single
parameter. Importantly, the surface
wrapper respects the fidelity
of the original CAD. Unlike other
technologies, it accounts for sharp
edges and corners of the original
model, as well as for any other “feature
curves” the user prescribes.
The time it takes to surface wrap
depends on geometry complexity,
surface refinement, and computing
power available. A structure
such as an offshore oil platform or
automotive underhood can usually
be wrapped in an hour. In most
cases, surface wrapping takes just a
few minutes using a desktop PC.
Joining a mannequin
to a racing bicycle
The surface wrapper recently
gave Felt Racing, a manufacturer of
high-end racing bicycles in Irvine, Calif., an edge in developing a new
UCI-legal, aerodynamic carbonfiber
bicycle. The company used
CFD simulation to determine the
most aerodynamically efficient
designs, testing only the best in a
wind tunnel. Although the company
had CAD data for the frame
and most of the components, it
used a third-party mannequin to
represent the rider. The rider had
to be joined to the frame before analysis. The company used the
surface wrapper instead of joining
the models manually, a process
that would have entailed many
hours of intensive manual surface
stitching. The company also used
the wrapper on the bicycle’s geartrain
components.
Make Contact
CD-adapco, (631) 549-2300, www.cd-adapco.com