Three-dimensional steady disturbance modes in the Blasius boundary layer – a DNS study

Direct numerical simulation is used to investigate the nature and behaviour of steady spanwise flow modulations excited internally in the Blasius boundary layer, primarily for a Reynolds number based on the displacement thickness ( Æ ) greater than 1,600. Both types of disturbances known – -/streak-/Klebanoff modes or longitudinal vortex modes – exhibit a periodic spanwise variation, but the -modes are characterized by a dominant streamwise disturbance velocity component and a negligible swirling motion, whereas for the vortex modes the latter is strong. We investigate how the respective modes can be identified and in which situation they are present, using direct excitation at the wall locally as well as non-locally, and nonlinear generation in the oblique and fundamental breakdown scenario in transition to turbulence. Understanding their occurrence contributes to understanding non-eigenmodal disturbance growth and secondary flow instability eventually causing final laminar breakdown.