Interaction of Separation and Transition in Laminar Separation Bubbles in a 3D-Boundary Layer

摘要

The influence of different disturbance combinations and increasing sweep on a family of pressure-induced laminar separation bubbles is studied systematically by means of direct numerical simulation (DNS). Three types of disturbance waves are tested against their potential to stimulate the growth of background disturbances of fundamental or subharmonic frequency. The focus is on 2D-disturbances, which are normally the most amplified disturbances in unswept separation bubbles. For the present 3D-base flows, they are found to lose their dominance for sweep angles larger than 10 to 15 degrees. Instead, oblique waves with a propagation direction between 0 degrees and -6 degrees relative to the potential streamline trigger the strongest growth of background disturbances. Spatial linear stability theory (LST) was utilised to select the most amplified disturbances for each sweep angle. LST turned out to be as reliable as in unswept laminar separation bubbles and its excellent agreement with DNS within the linear domain was not adversely affected by the sweep angle.