Vortical structures behind three side-by-side cylinders

The flow behind three side-by-side circular cylinders is experimentally investigated using various techniques, including particle image velocimetry, hot-wire anemometry, laser Doppler anemometry and laser-illuminated flow visualization. The work aims to study the vortical structures and the interactions in the turbulent wakes of three side-by-side cylinders. The cylinder centre-to-centre spacing T/d is chosen to be 1.5, at which both wide and narrow wakes are formed. It is observed that the base pressures of the upper and lower cylinders are identical but smaller than that of the central cylinder. Consequently, the gap flows between cylinders are deflected towards the upper and lower cylinders, respectively, forming one wide wake behind the central cylinder and one narrow wake on each side of the wide wake. The normalized dominant frequency is 0.373 in the narrow wakes and 0.069 in the wide wake. Photographs from flow visualization and the downstream evolution of the hot-wire signal spectra suggested that, while the vortical structures in the narrow wakes may be generated from the shear layer separation from the upper or lower cylinder, those in the wide wake may originate from the shear layer instability. The flow topology (vortex patterns) at different downstream stations is also discussed in detail.