Vortex shedding behind modified circular cylinders

Circular cylinders modified with an axial slit and/or a concave rear notch were used to study the vortex-shedding phenomenon. Particular attention was paid to the Strouhal number/Reynolds number (StD/ReD) relationship and the determination of drag coefficient CD. For the modified cylinders, it was observed that the StD/ReD relationship was significantly different to that of a circular cylinder, particularly for the cylinder with a slit normal to the flow where StD was always higher than for a circular cylinder over the range of ReD examined (60<ReD<1000). At ReD≈2200, for a cylinder with a slit normal to the flow, and for a cylinder with both a slit normal to the flow and a concave rear surface, estimated values of CD of 1.41 and 2.52, respectively, were large compared to the value of 0.94 for a circular cylinder. Inspection of hot wire traces in the wake at x/D=5 of the cylinder with both the slit and slit with concave rear notch modifications suggested a strong and stable vortex-shedding pattern for these cylinders compared to a circular cylinder at ReD≈2200.