DNS of harmonic point source disturbances in an airfoil boundary layer flow

Laminar-turbulent transition mechanisms induced by a harmonic point source disturbance in a flat-plate boundary layer with adverse pressure gradient are investigated by spatial Direct Numerical Simulation (DNS) based on the complete three-dimensional Navier-Stokes equations for incompressible flow. A local disturbance is introduced into the two-dimensional (2-D) base flow at the wall by a harmonic point source. Thus Tollmien-Schlichting waves of a single frequency and a large number of obliqueness angles are stimulated and propagate downstream simultaneously, undergoing amplification by primary and subsequent instabilities, and eventually lead to breakdown of the laminar flow. The results of in-flight experiments, performed by other researchers within a joint research project (DFG-Hochschulgruppe), on a wing glove mounted on a motorglider are compared with the DNS results. The linear and non-linear development of the wave train in the boundary layer is investigated in detail by the spectral amplitude evolution and the vorticity/shear-layer dynamics.