On the relation between the self-excited three-dimensionality of laminar separation bubbles and their receptivity to external disturbances

Abstract

© 2021, American Institute of Aeronautics and Astronautics Inc.. All rights reserved.Experimental observations of laminar separation bubbles show that the dynamics of the separated shear layer are dominated by the amplification of external disturbances. For low-to-moderate levels of free-stream turbulence intensity, the laminar-to-turbulent transition process is initiated by the formation of spanwise-aligned vortices associated with inflectional instability. The spanwise coherence of such structures varies strongly depending on the intensity of the flow recirculation and the amplitude of the external disturbances. On the other hand, two-dimensional laminar separation bubbles are intrinsically unstable and tend to become distorted along the spanwise direction even in the absence of external disturbances. This three-dimensional distortion can affect qualitatively and quantitatively the flow receptivity to external disturbances and their subsequent amplification, and thus it needs to be accounted for in the modeling of separation bubbles. This work addresses the receptivity of two-dimensional and three-dimensional separation bubbles to three-dimensional disturbances by analyzing the optimal inputs and outputs via the resolvent operator. A novel matrix-free strategy is used, which provides optimal gains and modes for all frequencies using a single iterative scheme, resulting in total costs an order of magnitude lower than previous methods. Results show that three-dimensional distortion of bubbles may enhance amplification of external disturbances by more than an order of magnitude.