Effect of unequal cylinder spacing on vortex streets behind three side-by-side cylinders

The turbulent near-wake of three side-by-side circular cylinders with equal or unequal spacing has been experimentally investigated using various techniques, including the hot wire, laser Doppler anemometer, and flow visualization. The work aims to understand the effect of unequal cylinder spacing on the vortex streets behind the three cylinders. When the cylinder center-to-center spacing is identical, i.e., $T_1{\mathrm{/d=T}}_2\text{/d=1.5,}$ the flow is symmetric about the centerline, with one wide wake behind the central cylinder and one narrow wake on each side of the wide wake. The dominant frequency in the narrow wakes is about 5.4 times that in the wide wake. The observation is consistent with previous reports, thus lending credence to the present measurement. As $T_2\mathrm{/d}$ is slightly increased to 1.6, a remarkable change occurs in the flow. A comparison is made between the cases of equally and unequally spaced cylinders in terms of the pressure around the cylinders, drag, lift, dominant frequencies, and vortex formation mechanisms. The flow topology (vortex patterns) and downstream evolution are also discussed in detail.