An experimental study was made of flow structures behind a local suction and blowing in a flat plate turbulent boundary layer. Local forcing was given to boundary layer flow by means of sinusoidally oscillating jet issuing from a thin spanwise slit at wall. The Reynolds number based on the momentum thickness is Reθ = 1700. Effect of local forcing on the flow structure was scrutinized by altering the forcing frequency ($0.0027 ≤ f^+ ≤ 0.0162$) and $A_0$=0.2. Small local forcing decreased the skin friction coefficient and increase the displacement thickness. A phase average technique was employed to capture the coherent structures due to the local periodic forcing. The turbulent flow characteristics were decomposed into a periodic part and a random part. It is shown that an organized spanwise vortical structure is generated by the local forcing. Relatively larger vortical structures were made by the lower frequency of local forcing, where the turbulent energy is concentrated at the core of the vortical structure.
