Note on a discontinuous change in wake pattern for a rectangular cylinder

Base pressure measurements on rectangular cylinders with different side ratios, d/h = 0·4-0·6, where d is the depth and h is the height, are presented. A cylinder with d/h = 0·5 shows characteristic time variations in base pressure, with alternatively high and low values, almost equal to those for d/h = 0·4 and 0·6 (the critical depth), respectively. This is caused by a sudden change in the flow pattern around the cylinder, associated with the vortex formation: for cylinders with depths below the critical, the flow around a cylinder initially changes gradually with increasing d/h, but, d/h reaches a transitional range around d/h = 0·5, there is an abrupt change in the flow pattern.