So my last statement was
rather extreme: "...there is one rule that MUST NOT be
forgotten..." OK, I admit, it does seem rather extreme, but
before you rush to judgment, consider two cases:
Case 1: Several years
ago, I was involved in modeling a pollution control system. The
system was part of a gas turbine power plant and was located after
the gas turbine and was designed to remove NOx from the exhaust
gas stream. The company where I worked was selling one
component of the system and I was using CFD to demonstrate the
effectiveness of our equipment. At the same time, the engineering
firm designing the system had hired an independent consulting firm to
use CFD to model the entire system to help ensure the effectiveness
of the finished design. I supplied 3D models of our equipment to the
consultant who included my equipment in his model and performed the
analysis. When I reviewed the results of the modeling, I discovered
that the consultant had incorrectly used a "symmetry"
boundary condition. The use of this incorrect boundary
condition completely changed the results of the analysis.
OK, the results of this
error weren't too bad; it resulted in more time to fix the boundary
condition and to rerun the analysis and modify the design. The only
cost in this case was money. But what if more than money were at
stake?
Case 2: In Chapter 15 of
his book To Engineer Is Human, Henry Petroski
relates the following incident:
"The two and a half acres of roof covering the Hartford Civic Center collapsed under snow and ice in January 1978, only hours after several thousand fans had filed out following a basketball game..."
Subsequent investigations
revealed that the FEA model used to design the structure did not
accurately represent the combination of the dead weight of the roof
plus the live loads (snow and ice load and other loads) acting
on the roof and its supporting structure. This was the result of an incorrect boundary condition being used to model the loading. In the post accident
analysis, a computer model showed the failure when the correct
boundary conditions were used. Petroski puts in this way: "The
computer provided the answer to the question of how the accident
happened because it was asked the right question
explicitly..." (italics mine). Unfortunately the right
question was only known after the accident. If the failure had
happened a few hours earlier with the arena filled with spectators,
the cost of the failure would possibly have included deaths and
injuries to many people.
So once again, I will repeat my
statement: There is one rule that MUST NOT be forgotten:
GABAGE
IN = GARBAGE OUT
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