An experimental study of critical spacing in laminar flow past tandem cylinders.

For uniform steady flow past two cylinders aligned parallel to the flow, an extended bluff body regime (for small cylinder spacing) and a co-shedding regime (for large spacing) can be seen. The "critical spacing" at which the flow structure shifts between these regimes depends on the Reynolds number. For a specific Re, the critical spacing depends on if the cylinder separation is being (quasi-statically) increased or decreased; this hysteresis is also found when fixing the spacing and varying Re. Analyzing the critical spacing is important owing to the quick jump in Strouhal number and surface forces seen during the shift. Accurately quantifying these responses can, for example, guide the development of energy harvesting methods or the prevention of body clashing. Extensive past studies agree on the general trends in critical spacing, but there remains a discrepancy in the details, especially between experiments and computations. Using a flowing soap film "tunnel", Yang & Stremler (2019, JFS 89) found experimental critical spacing values consistent with published computational results for Re=100. We expand on this study by considering multiple cases for 50