Navier-Stokes simulation of harmonic point disturbances in an airfoil boundary layer

Laminar-turbulent transition mechanisms induced by a harmonic point source disturbance in a hat-plate boundary layer with adverse pressure gradient ar e investigated by fourth-order accurate spatial direct numerical simulation based on the complete three-dimensional Navier-Stokes equations for incomp ressible how The disturbance is introduced into the two-dimensional base fl ow by time-periodic simultaneous blowing and suction within a circular spot at the wall to quietly mimic the momentum input by an active loudspeaker b elow a hole in the surface in respective experiments. Thus a wave train con sisting of pure Tollmien-Schlichting waves of a single frequency and a larg e number of obliqueness angles is stimulated, and its downstream evolution in both physical and spectral space is investigated. A breakdown scenario d ominated entirely by oblique modes is observed that shows a spanwise peak/v alley amplitude splitting with the valley plane at the centerline of the wa ve train. Dominant vorticity structures develop off centerline, and a clear -cut M-shaped structure is formed in final stages related to the wall shear being a footprint of a first pair of Lambda-vortices in the flow.