Active control of flow field structure in a transitional flat plate boundary layer

The aim of this study is to develop a method to control two-dimensional Tollmien-Schlichting wave (T-S wave) and oblique wave at the early stage of transition of a flat-plate boundary layer. Small and thin piezo-ceramic (PZT) actuators are used as the control device. In this study, twelve actuators are attached in line in the spanwise direction on the flat-plate surface and wired independently from each other. These actuators are used to generate a wave with the same amplitude and opposite sign of the incoming wave in order to cancel it. From the experimental results, it is found that the cancellation of the velocity fluctuation is successful while the sweep angle of target wave is small. However, as the sweep angle becomes larger, the canceling effect becomes insufficient in some spanwise locations even when the phases and the amplitudes of the controlling signals are fine-tuned. It is also shown that the difficulty in the fine-tuning is due to the deformation of the velocity fluctuation pattern introduced by each actuator.