The short answer – It depends on what you need. The big question is how to decide between the two.
Both methods can take time, depending on resources.
Finite element analysis (FEA) can do everything classical analysis can, but not the other way around. Sometimes, I need to do a classical analysis to back up FEA results. This hasn’t been a problem; it is time-consuming to find equations and calculate numbers.
Are your FEA results accurate?
FEA results can lead to questions about accuracy and reliability. Recipients might wonder, “How do I know this is right?” There’s no absolute way to check analysis results. The best approach is careful detail, good judgment, and thorough documentation.
I have run many check cases to verify FEA results. This doesn’t count the hundreds of test cases available online. Early in my FEA experience, I used many to build my confidence in the software. Comparing results with test data is a great backup for both FEA and classical methods.
Detailed test results sometimes are available
Recently, I ran a sample problem using Fluent, a CFD code from ANSYS. This simple run simulated airflow over a NACA 0012 airfoil. The viscous simulation results closely matched several published wind tunnel tests. The difference was a few percent.
I felt a bit suspicious, as someone can adjust simulations to fit test data. I compared drag figures from my runs to standard wind tunnel tests. This was for drag on a sphere, and the results matched the expected values with a high degree of accuracy.
Other tests showed I could confidently use Fluent and CFX for fluid dynamics and heat transfer simulations.
Even simple errors can be painful
Calculation errors can appear anywhere, no matter the analysis method. The challenge is finding them. It can be as simple as a misplaced decimal point.
Long ago, I spoke with an engineer who worked on the DC-8 design. The calculations involved the internal wing volume for fuel storage.
The estimates puzzled several engineers, as the wing couldn’t hold the required fuel. The issue turned out to be a misplaced decimal point in the pounds per cubic foot of fuel volume. A simple error, yes, but it left designers stuck until someone discovered it.
Comparing to other designs
The Boeing 707 was similar in size, engines, range, and gross weight. It stored all its fuel in the wings. This comparison showed a mistake made by the Douglas engineers.
Over the years, I’ve built confidence in my ability to produce reliable results from both methods. I focus on details, compare results with other investigations, and avoid complacency.
Norman T. Neher, P.E.
Analytical Engineering Services, Inc.
Elko New Market, MN
www.aesmn.org