Interference between two circular cylinders; Series of unexpected discontinuities

Abstract

When two cylinders face the flow one behind the other then they are in tandem arrangement. The systematic measurements of the front (gap) and base pressures of the downstream cylinder at various spacings showed a discontinuous ‘jump’ at some critical spacing. The drag force coefficients on the downstream cylinder showed a similar discontinuous change at the same critical spacing. This was attributed to two different flow patterns around the tandem cylinders. Following this, two cylinders arranged side-by-side to the approaching flow were considered. The inherently biased flow in the gap between the cylinders produced two different flow patterns around each cylinder. The biased flow was bistable causing a change over at irregular intervals of time. Thus each cylinder might be subjected to either of the two drag and lift forces. All the available data on measured forces were compiled together with the additional measurements in the range of spacings within which intermittent change-over of drag and lift took place. When two cylinders are neither in tandem nor in side-by-side arrangement then they are staggered. The systematic measurements of drag and lift forces on the downstream cylinder showed two unexpected features. Firstly, the position of the downstream cylinder at which the maximum lift force was exerted on it was plotted in transverse, ( T D ),-longitudinal, ( L D ), plane. Two different curves were obtained which overlapped one above the other within a range of staggered arrangments. This feature of the maximum lift force was considered as a sign that two entirely different flow patterns were the cause. The second feature found was that at some spacings the minimum drag force did not occur in the tandem arrangement but in adjacent staggered arrangements. A plausible explanation was offered for that peculiar C Lmax and C Dmin behaviour. Finally, a discontinuity in the lift force for some staggered arrangements was found and attributed to the bistable flow patterns in the gap.