The turbulent recirculating flow behind a backward-facing step was excited by an acoustic perturbation issued from a thin slit at the separation point. The two key parameters, i.e., the pulsating velocity, $U_f$, and the pulsating frequency, $f_f$, are dealt with in the present experiment. Reynolds number based on the step height was varied 25,000-35,000. A particular frequency gives a minimum reattachment and the most effective frequency is dependent on the nondimensional frequency, $St_\theta$, which is based on the momentum thickness near the pulsating point. In order to investigate the effect of pulsation, the structure of turbulent flow are scrutinized, which includes the time mean velocity and fluctuations of the velocity. The speading rate of mean flow and the growth of shear layer for the pulsating flow are larger than those for the natural flow. The lock-on effect of shedding frequency near the reattachment region is also demonstrated. A comparison between the pulsating and natural flow suggests that the large-scale vortical structure is associated with the pulsating frequency and flow instabilities.
