Direct numerical simulations with external perturbations have been performed in order to clarify the dynamics of the turbulent coherent structure near a wall, and also the possibility of active control of turbulence. To extract the elementary processes of turbulence, the domain of the numerical calculation is a minimal flow unit with a Reynolds number of 1730. A series of flow injections or suctions with Gaussian distribution are administered on both the lower and upper wall surfaces so that they interact with the coherent structure, especially, the high- and low-speed streaks along the flow direction. The various responses of the streaks to the injection or suction are systematically examined. The observed transient behavior, for example, growth or decay of the coherent structures, affords fundamental knowledge for turbulence control in the future.