Spanwise wake structures of a circular cylinder and two circular cylinders in tandem

The structure of wakes downstream of cylinders is of major importance in the generation of flow-induced sound and vibration and also influences the convective heat transfer process. In particular, it is now thought that the formation of spanwise structures plays an important role. A better understanding of their development is required, and this could lead to ways of reducing both noise and vibration and to an improvement in heat and momentum transfer. Data are presented from tests carried out in a closed- circuit wind tunnel; also presented are data and flow visualizations from tests in a closed-circuit water tunnel. Hot-wire anemometer probes and pressure probes were used to measure velocity and pressure fluctuations at a number of spanwise locations in the wakes. From these measurements the spanwise coherence was calculated. The spanwise coherence increased as the distance between the cylinders in tandem was decreased; when this distance was less than three diameters, spanwise coherence at all spanwise separations was higher than for a single cylinder. Flow visualizations show the complex nature of the flow in the spanwise direction, particularly the existence of streamwise vortices. These appear to be responsible for the sharp reduction in spanwise coherence at large separations between the two cylinders in tandem. The work described is from a preliminary study with the object of developing a better understanding of near-wake flows prior to undertaking a comprehensive three-dimensional flow-field investigation.