Results of FDA’s First Interlaboratory Computational Study of a Nozzle with a Sudden Contraction and Conical Diffuser

The U.S. Food and Drug Administration recently hosted an interlaboratory study to assess the suitability and methodology of computational fluid dynamics (CFD) for demonstrating medical device safety in regulatory submissions. The benchmark study was performed in a generic medical device consisting of a 0.012 m diameter cylindrical nozzle with a sudden contraction and 10° conical diffuser, on either side of a 0.04 m long, 0.004 m diameter throat. Results from 28 simulations from around the world were compared to planar particle image velocimetry (PIV) measurements performed at three laboratories. Five flow rates were chosen that produced laminar, transitional, and turbulent regimes. In general, the CFD results showed modest agreement in global and local flow behaviors. However, all CFD data sets contained wide degrees of velocity variation in comparison to experiment and with each other, much of which could be attributed to the turbulence models used. Some of the velocity discrepancies in the sixteen three dimensional (3D) simulations resulted from substantial flow asymmetries within and downstream of the conical diffuser. Large differences in velocity symmetry were found even when using the same code and many of the same simulation parameters. In contrast, no velocity asymmetries of the same magnitude were observed in any of the planar PIV experiments; relatively minor asymmetries in one experiment were linked to slight asymmetries in the velocity profile at the inlet to the nozzle. CFD predictions of peak wall shear stress at the sudden contraction (normalized to that 0.015 m downstream) varied over two orders of magnitude, with variations of greater than 10 times even when the same software and turbulence model were used. This degree of shear stress variation will necessarily propagate through calculations of blood damage in medical devices. We conclude that CFD simulations used in qualifying medical devices for regulatory purposes need to be conducted following the best practices available, and that targeted experimental validation is essential. The results of this interlaboratory study are freely available in an internet repository (https://fdacfd.nci.nih.gov). We encourage the use of this model in further studies, and support the development of additional benchmarks, better modeling techniques, and consensus standards and guidelines for using CFD in the evaluation of medical devices.