Succeed at Simulation

The “Seven rules for successful simulation” need to be revisited and revised because of advancements in computers. Simulations that once took weeks to compute an answer now take mere hours.

By Cliff Knight, KnightHawk Engineering

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  • Use commercial software that has the theory to back it up. Good software developers of commercial code aren’t afraid to publish a theory manual. I’ve invariably found that the quality of the manual correlates with the performance of the software. Commercial codes that have had a poor theory manual or that lacked one have performed poorly. Ignore any hype that the code is so user-friendly that the engineer doesn’t need to have detailed knowledge of the theory. For you P.E.’s (Professional Engineers) out there, “watch it.” You’re responsible for the design or analysis, not the software vendor. Is the bat liable if a baseball player hits a foul ball or strikes out? Don’t ruin your career because you trusted or used a cheap piece of software that’s relatively unproven or isn’t backed by a detailed theory manual. Just because some software is the latest thing on the market and claims to have all the latest “advances” doesn’t make it the best product to use.

  • Perform hand calculations to check the approach. For most problems you can run hand calculations on a test case to check your approach to a problem and give you confidence in your solution. After completion of the detailed analysis and checking, see what the governing aspects of the problem are and develop test cases that can be calculated by hand to validate your approach. It’s also important to run sensitivity studies on governing parameters to evaluate the accuracy needed for these parameters to get a meaningful solution.
  • 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 there’s 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

    Don’t be put off by the concerns I’ve 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 don’t. 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 isn’t today. Until then, you’re better off sticking to the seven rules for successful simulation.

    Reference


    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 cknight@knighthawk.com.

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