On the critical spacing of stationary tandem circular cylinders

For steady flow past two stationary circular cylinders that are aligned in tandem with their axes perpendicular to the flow direction, the wake pattern, vortex shedding frequency, and forces on the cylinders typically show an abrupt change as a function of the dimensionless center-to-center cylinder spacing over a range of critical spacing values. This fundamental problem has been studied for over a century, but knowledge gaps remain and published results are inconsistent. For Re<190, the flow is expected to be laminar and two dimensional, and hence well behaved, yet prior experimental and computational results do not agree regarding the value of the critical spacing. Using flow visualization techniques in a flowing soap film system with Re=100, we have obtained, apparently for the first time, experimental results that are consistent with published computational results. We will discuss the physical characteristics of this flow, particularly the hysteresis that occurs in the value of the critical spacing as a function of the initial conditions.