Interactions of lock-on wake behind side-by-side cylinders of unequal diameter at Reynolds number 600

► Lock-on wake characteristics behind side-by-side unequal cylinders are studied. ► Gap and diameter ratios are 0.75 and 2.0. The Reynolds number is 600. ► Wake of large cylinder experiences the primary and one third sub-harmonics lock-on. ► While lock-on, the gap vortices entrain alternately into the wake of each cylinder. ► The lock-on synchronizes with the fundamental or one third of excitation frequency.