The role of stationary cross-flow vortices in boundary-layer transition on swept wings

The spatial evolution of stationary cross-flow-vortex packets in a laminar boundary layer on a swept wing is computed by direct numerical simulation with the incompressible Navier–Stokes equations. Steady suction and blowing at the wing surface is used to generate disturbances that are periodic and equally spaced in the spanwise direction. The initiated disturbances become unstable and lead to distinct stages of instability evolution. Initially, each vortex packet undergoes a region of chordwise and spanwise linear independent growth; then the individual packets coalesce downstream and lead to spanwise wave adjustments and linear superposition of adjacent packets; finally, the vortex packets reach sufficiently large amplitudes in later stages of disturbance development to nonlinearly interact, which results in the rapid growth of the disturbances. In this later stage, the low-velocity fluid near the wing surface is lifted out into the boundary layer and rolled over the high-speed fluid in the direction of the positive spanwise velocity component. Highly inflectional chordwise and spanwise velocity profiles are recorded in this region. Previous studies have demonstrated that these inflectional profiles spawn the rapid growth of secondary instabilities, which lead to the catastrophic breakdown to turbulence.