Comparison of Techniques for Measuring Flow Residence Time in Flow Separation over a Backward-Facing Step

Flow separation is a common phenomenon in physiological systems. It often leads to increased flow residence time (FRT) has been linked to various physiological adverse events, such as energy loss, blood damage, and thrombus potential. Numerous computational and experimental techniques have been developed to measure FRT. However, direct comparisons between these methods to demonstrate performance differences are lacking. This study aims to provide such a comparison using a canonical configuration for flow separation: flow over a backward-facing step (BFS). We compared popular techniques for measuring FRT, including computational fluid dynamics (CFD), particle image velocimetry (PIV), and dye injection across various BFS configurations. These configurations varied in dimensions (5 mm – 20 mm) and flow conditions to achieve laminar (Re = 654) and transitional-to-turbulent (Re = 3923) flows, which are commonly observed in physiological systems. All techniques showed general agreement with each other. Notably, the dye injection parameters (such as injection speed and position) were fine-tuned as they are critical to obtaining reasonable results. This study directly compared computational and experimental methods for quantifying FRT in a simple, canonical configuration, providing new insights into these techniques. Future studies should include similar direct comparisons of FRT techniques under more physiological conditions.