Effect of inflow on coherent structures in the U.S. Food and Drug Administration's (FDA) idealized benchmark nozzle device using Proper Orthogonal Decomposition

The FDA proposed an idealized nozzle geometry for assessing the performance of Computational Fluid Dynamics (CFD) in blood carrying medical devices. While many CFD simulations and experiments have been performed on this nozzle flow, there is disagreement in the reported locations of jet breakdown, peak viscous stresses and statistics at the FDA-specified transitional and turbulent Reynolds numbers (Re). Hence, the reproducibility of statistics at these Re is questionable, making it difficult to establish a reliable reference case for validation. The crucial fact that the specified Re are too low to define a unique turbulent inflow is often overlooked in literature. We performed direct numerical simulations of the flow through the FDA nozzle at the throat Re of 7500, facilitating a well-defined turbulent inflow, due to self-sustained turbulence in the inlet. A proper orthogonal decomposition (POD) revealed that the throat section housed the most energetic low-wavenumber streak-like structures, potential triggers for the jet breakdown downstream. POD of parabolic inflow at the same Re confirmed that numerical noise led to the jet breakdown, affirming the impact of the inflow condition on the downstream flow and ensuing structures aiding the jet breakdown. Our study provides a reference for the validation of CFD simulations of a turbulent flow through the FDA nozzle and identifies the flow structures arising from the geometry orientation, offering insight into the mechanics of the jet breakdown.