The last part of simulation almost always leads to assessment. Sometimes this is as big a problem as the simulation itself. First, take for example, a structural analysis where the stresses must be classified for a code assessment to be performed. Often a process called stress linearization is conducted to compare the stresses to the code. Stress linearization, while sounding sophisticated, is nothing more than a translator for Finite Element Analysis (FEA) to the code. Unfortunately, almost all the stress linearization routines have errors associated with the problem and they are user-dependent on chosen cross-sections; so, be careful and therefore refer to Rule 2.
FEA exploded in the 1980s and Computational Fluid Dynamics (CFD) grew rapidly in the 1990s. Now there are integrated design packages that will automatically perform CFD and FEA and require little knowledge from the user. Almost all of this analysis is non-linear and highly dependent on boundary conditions, convergence algorithms, model definition and equation parameters. I can give 10 inexperienced engineers a CFD problem and get 10 different answers because theres one thing for certain these packages will yield answers. When assessing the results the reviewer must be aware of this. Having some bench-mark comparisons often is helpful. Data are merely data, unless you are able to correctly interpret those data.
A bright future
Dont be put off by the concerns Ive raised. There are good software tools out there and there are good people using them. The future is exciting. For instance, today we can perform non-linear structural analysis and assess local plasticity, but usually dont. I believe that one day almost all structural analysis will be non-linear and the model will automatically account for local plasticity. In CFD, the tools are gaining speed and efficiency and more data and information are available to tune the solvers to achieve a better solution. Maybe someday the software programs and computers will be smart enough to replace seasoned engineers with common sense and experience (ha ha!) but that day isnt today. Until then, youre better off sticking to the seven rules for successful simulation.
1. Knight, C., Seven rules for successful simulation, Hydrocarbon. Proc., p. 61 (Dec. 2001).
Cliff Knight, P.E., is president of KnightHawk Engineering, Houston. E-mail him at email@example.com.