Investigations of Suction in a Transitional Flat-Plate Boundary Layer

For the maintenance of a laminar boundary layer flow on transonic wings, it is necessary to integrate a boundary layer suction unit in the nose region. This concept of the Hybrid Laminar Flow control is realized through a suction area adapted to the outer pressure distribution by an array of suction holes. With this, a stabilization of the boundary layer is reached, although cross-flow instabilities are present in the flow. The goal of the experimental examinations was to widen the understanding of the results of flow topology at a single suction hole on hole arrays. Of particular interest was the influence of the hole size, the hole spacing and the suction flow rate on the receptivity of the boundary layer flow. The work was conducted in a refractive index channel, in which laser Doppler velocity measurements in the boundary layer close to the wall are possible. The investigations were carried out under natural transition conditions to clarify the physical mechanisms of the transition acceleration as well as deceleration in dependence of the suction rate. The results show that the transition process follows completely a strong stochastic behavior.