Control of Crossflow Instability Field by Selective Suction System

Abstract

In order to obtain the overall aerodynamic drag reduction for next generation large aircraft development, it is necessary to control the Crossflow instability generated on the main wing. However, mainly because of its complexity, detailed transition process is not well understood. It can be said that at least there are two explanations for the transition process. One explanation is that the primary instability appears as streamwise stationary instability (crossflow vortices), and which drives unsteady secondary instability (inflectional instability) locally along primary instability [1, 2]. Other explanation is that the unsteady wave instability becomes absolutely unstable at certain Reynolds number and eventually brings the flow field into fully turbulent state [3]. Further investigations will be needed until detailed and exact transition process is detected, but we believe that at least in the case of a swept wing flow, transition is being occurred by former process. In our understanding, key process in the crossflow dominant turbulent transition is the generation of the secondary instability. Vortical motion of the crossflow vortices (streamwise vortices) pumps up low momentum boundary layer flow near the wall up to the main flow region where high momentum flow prevails. As a result, unstable flow condition, high shear with point of inflection is created along each streamwise vortex. After the appearance of 7 to 10 cycles of the secondary instability, full turbulent transition is reached. Therefore, it can be said that critical mechanism for turbulent transition is the pumping up of the low momentum flow by the crossflow vortices. Namely, crossflow vortices pump up low momentum boundary layer flow existing very close to the wall surface to much higher momentum flow region near the boundary layer edge, and as a result, unstable flow condition is created locally.