Observations of hysteresis in flow around two square cylinders in a tandem arrangement

This paper reports an experimental study on the flow characteristics around two square cylinders in a tandem arrangement. The spacing between the centers of the cylinders ranges from 1.5 to 9 widths of the cylinder and the Reynolds number ranges from 2.0×103 to 1.6×104. Hysteresis with two discontinuous jumps is present for all the Reynolds numbers studied when the spacing is varied in two different ways, one being a progressive increase and the other a progressive decrease. The hysteresis is associated with two different flow patterns, and the discontinuity is attributed to a sudden change of the flow pattern. In the hysteresis regime, only one flow pattern is stable, differing from the bistable phenomenon that involves intermittent switching between two stable flow patterns as reported for higher Reynolds numbers by previous authors. It is found that the hysteresis regime shifts rapidly towards smaller values of spacing as the Reynolds number is increased in the lower range Re=2.0×103–5.3×103, and then remains nearly unchanged between Re=5.3×103–1.6×104. The time history and spectra of velocity as well as the flow visualization corresponding to the change of flow pattern in the hysteresis regime are presented and discussed in detail. The effects caused by hysteresis and Reynolds number on drag forces, mean and fluctuating pressures, and Strouhal numbers of the cylinders are examined.