COMPUTATIONAL FLUID DYNAMICS VS. MODEL TESTING

A deep-rooted—and healthy—skepticism guides engineers in their quest to balance human safety with trends in equipment and materials. to that end, naval architects have cautiously embraced computational fluid dynamics (CFD) as a means to reduce design-cycle time, especially in the early stages of the design process, while asking whether the results are as accurate as those achieved by using model testing.

Donald Blount, whose company designed the 220-foot (67-meter) record-speed-setting Fincantieri Destriero, had a client so intrigued by the subject that, prior to making a build decision for a 262-foot (80-meter) high-speed motoryacht, funded an exhaustive study to compare the predictability of the two methods.

Employing the testing facilities at SSPA in Göteborg, Sweden, the study examined three hullforms: double chine, round bottom with spray rail, and single chine. Each would be tested at two displacements and as many as three longitudinal center of gravity positions. this program ensured that a wide range of data would be collected to compare the hulls. “the objective of this investigation,” the study states, “was to identify a hullform that achieved suitably low resistance…to satisfy an endurance requirement while remaining dynamically stable…to satisfy a top speed requirement.”

After running the models in each of its various permutations, some of the same test conditions were compared to results from a CFD application—which, the study stated, “demonstrated that it could satisfactorily differentiate the bare hull resistance between several candidate hullforms,” and noting, “the absolute magnitude of the resistance predictions is generally within three percent [of] the experimental data.”

Blount says the results of the two prediction methods indicated some difference in speed when dynamic instability was likely to occur.

“i personally prefer experimental testing, since experimental results are immediate, confirming you have met design expectations or not,” he said. “if expectations are not met, one can readily alter a model to evaluate alternative solutions.

“An analytical model representing hull geometry requires a very experienced hydrodynamicist for input to CFD software to obtain realistic results,” he added. “inexperienced CFD users are dangerous, as they might not recognize the validity or reality of output predictions, which can vary due to poorly distributed elements for hull geometry.” —M.M.