Optimal perturbations in transitional Blasius boundary layers: A structured approach

This work employs a structured input-output analysis (SIOA) (J. Fluid Mech. vol. 927, A25) approach to analyze the optimal perturbations in Blasius boundary layer flows. This approach incorporates the non-linear effects into input-output analysis that recovers the important flow features identified using non-linear analysis as demonstrated in canonical wall-bounded shear flows. For the Blasius boundary layer, the structured input-output response recovers the streamwise dependent structures that are most likely to trigger the transition consistent with direct numerical simulations. The associated flow structures most likely to be amplified (optimal perturbations) are identified based on structured uncertainty, which is decomposed into streamwise, wall-normal, and spanwise velocity correlations. The resulting optimal perturbations show that the streamwise velocity perturbations have the highest magnitude and wall-normal velocity perturbations have the smallest magnitude, which is consistent with non-linear optimal perturbations.