The effect of streaks on the breakup of Kelvin-Helmholtz rolls in a geometry-induced laminar separation bubble

This study investigates the influence of boundary layer streaks on the transition process of a geometry-induced laminar separation bubble (LSB). High-fidelity numerical simulations of the T3L test case, proposed by the European Research Community on Flow, Turbulence and Combustion, were performed. The resulting dataset describes a flat plate boundary layer under varying Reynolds numbers and free-stream turbulence levels and was analyzed using Dynamic Mode Decomposition (DMD). The DMD algorithm identifies the most amplified streamwise and spanwise wavelengths within the LSB and enables comparison with those of pre-transitional streaky structures perturbing the boundary layer. Under non-zero free-stream turbulence, elongated streaks develop in the attached boundary layer and shorten the bubble length. The presence of alternating high- and low-speed streaks in the cross-flow direction induces wall-normal oscillations of the separated shear layer, with a spanwise wavelength matching the streak spacing. Nevertheless, the strongest instability detected by the DMD analysis emerges farther downstream, where Kelvin-Helmholtz (K-H) rolls undergo breakdown. Both sinuous and varicose-like instabilities of the K-H structures were observed at the streamwise location corresponding to the highest disturbance growth, involving all three velocity components. The most amplified DMD modes captured a spanwise wavelength matching the streak spacing, characterizing the instability of the K-H rolls. Furthermore, these modes revealed the formation of regions of elevated streamwise vorticity associated with the unstable rolls, which enhances their distortion and subsequent breakdown. Analysis of the vorticity transport equation indicates that laminar streaks within the LSB reinforce the streamwise vorticity field, thereby promoting the instability and breakup of the K-H rolls.