Studies are made of the separated flow behind a normal plate which is aligned to a uniform free stream when the oncoming free stream contains a pulsating component. The discrete-vortex method is applied to simulate this flow situations because this approach is effective to represent the unsteady motions of pulsating flows. The two key parameters, amplitude of the pulsation and the nondimensional frequencies, are dealt in this simulation. The effect of viscosity and the reduction of the strength of circulation in vortices after shedding are incorporated in the present model. In order to simulate the pulsating flow, an appropriate iteration scheme is developed to predict the position and the strength of the nascent vortices. For comparisons of the present method, the vortex tracing method is also calculated. The numerical simulation has fairly reasonable predictions with the experimental results. For a uniform approach flow, the well known Karman vortex is clearly discernible. Inspection of these patterns of a pulsating flow reveals that the formation lengths of the evolution modes tend to be shorter than for the case of a uniform flow. One prominent flow property is the lock-on phenomenon, i.e., the dominant shedding frequency is one half of the pulsating frequency of the approach flow. The present numerical solutions capture the existence of this physical phenomenon, which is in support of the observation of Telionis. By systematically altering the amplitude of pulsation, the ranges of frequencies of the lock-on phenomena are extended.
