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Complementary to model testing, Com- putational Fluid Dynamics (CFD) has carved out a firm position in the MARIN portfolio. But when go or no-go decisions are at stake, the question naturally arises: how reliable is a particular CFD application? The advantage of CFD is that it gives insight into flow details, most of which are not directly revealed in a physical ex- periment. However, the CFD practitioner can do a lot of things wrong and even if he is careful, his model is not a perfect representation of the real world. To un- derstand the consequent uncertainty of the results, a research field has evolved known as “Verification and Validation”. MARIN’s CFD team has taken an active part in this research, the greatest efforts been made by Luís Eça within the pro- ductive MARIN – Instituto Superior Téc- nico (IST) cooperation, which has now lasted more than 20 years! Validation CFD is essentially the numerical solu- tion of a mathematical model supposed to govern the behaviour of the flow past an object under proper boundary condi- tion settings. That mathematical model typically contains simplifications and these introduce modelling errors. Valida- tion checks the adequacy of the model and involves the comparison of numeri- cal results with experimental data, taking the uncertainties of both into account. … and Verification Even if the mathematical model would be perfect, the results of CFD still have numerical errors. The process of estimat- ing the numerical error of a computa- tional result of a specific code is called “verification”. Three error sources can be distinguished: round-off, iterative and 22 Maritime Reporter & Engineering News COLUMN EYE ON DESIGN Quality Checking of CFD How reliable is CFD?
