Gap ratio effect on flow characteristics behind side-by-side cylinders of diameter ratio two

•For side-by-side cylinders of D/d=2, the onset locations and the spatial growing rates of shear layers instability are changed.•The frequency ratio of the narrow and wide wakes depends upon the gap ratio and the diameter ratio; but is independent of the Reynolds number.•The influence on the wide wake is more significant but is less pronounced on the narrow wake. The influences on both wakes are even pronounced for D/d=1 than those for D/d=2.