Active control of boundary-layer transition using a thin actuator

An experimental study to generate T-S (Tollmien-Schlichting) waves and oblique waves in a flat plate boundary layer is carried out. This is considered to be the first step towards controlling boundary-layer transition. In order to generate T-S waves and oblique waves, an array of piezoelectrically driven actuator pieces attached to a flat plate surface was used. The actuator pieces were sufficiently thin to keep the boundary layer in the laminar state. In order to generate various oblique waves, ten actuator pieces were aligned in the spanwise direction, each of which could be controlled separately. It is shown that both two-dimensional T-S wave and oblique waves of various angles could be generated using this device. It is also shown that when a flow pattern similar to the Craik-type three-dimensionalization mode was excited, the onset of turbulence took place early and locally at a certain spanwise location.