Effects of periodic local forcing on a turbulent boundary layer

An experimental study is performed to analyze flow structures behind a local suction/blowing in a flat-plate turbulent boundary layer. The local forcing is given to the boundary layer flow by means of a sinusoidally oscillating jet issuing from a thin span wise slot at the wall. The Reynolds number based on the momentum thickness is about Reθ=1700. The effects of local forcing are scrutinized by altering the forcing frequency (0.011≤f+≤0.044). The forcing amplitude is fixed at A0=0.4. It is found that a small local forcing reduces the skin friction, and this reduction increases with the forcing frequency. A phase-averaging technique is employed to capture the coherent structures. Velocity signals are decomposed into a periodic part and a fluctuating part. An organized span wise vortical structure is generated by the local forcing. The larger reduction of skin friction for the higher forcing frequencies is attributed to the diminished adverse effect of the secondary vortex. An investigation of the random fluctuation components reveals that turbulent energy is concentrated near the center of vortical structures