Interaction of separation and transition in laminar separation bubbles in a 3D boundary layer

Hetsch, T. and Rist, U. (2004) Interaction of separation and transition in laminar separation bubbles in a 3D boundary layer. Research and Technology Organisation (RTO) Specialists Meeting on Enhancememnt of NATO Military Flight Vehicle Performance by Management of Interacting Boundary Layer Transition and Separation, Prague, Czech Republic. 03 - 07 Oct 2004.

Abstract

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°. Instead, oblique waves with a propagation direction between 0° and -6° relative to the potential streamline trigger the strongest growth of background disturbances. Spatial linear stability theory (LST) was utilised to select the mosta mplified 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.

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