摘要：

Laminar-turbulent breakdown of a strongly decelerated Falkner-Skan-type boundary layer (Hartree-Parameter beta(sub H)=-0.18) is investigated by direct numerical simulations using the complete Navier-Stokes equations for three-dimensional incompressible flows. The numerical method is based on the so-called spatial model, and allows for simulations of spatially evolving three-dimensional disturbance waves in a two-dimensional growing boundary layer. Transition is initiated by 2-D and a pair of oblique 3-D waves, both with small amplitudes, excited periodically within a narrow disturbance strip at the plate surface. Their streamwise (linear and subsequent nonlinear) evolution resulting in the fundamental breakdown of laminar flow is simulated with high spatial resolution. It is observed that the fundamental breakdown process under adverse pressure gradient is dramatically more complex than the well-known K-breakdown in the Blasius flow: In addition to the (upper) high-shear layer on top of the lambda-vortex, a (lower) characteristic high-shear layer is formed simultaneously in between neighboring lambda-vortices. This shear layer, induced by a secondary vortex system close to the wall, precipitates ultimate breakdown to turbulence. Finally, some results of preliminary studies on the effect of suction on boundary-layer transition are presented that demonstrate the great potential of spatial direct numerical simulations for applied transition research in flows of practical interest.

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