Laminarizing effect of nonlinearly saturated crossflow vortices sustained by a sinusoidal roughness

Turbulent transition of three-dimensional boundary layer can be delayed by an artificial leading edge roughness that excites a less unstable (or subcritical) crossflow mode. This mode wins the competition against the most linearly-unstable mode and dominantly generates crossflow vortices in its nonlinear saturation phase. The growth of the most unstable mode is then suppressed, because the mean boundary layer flow is nonlinearly modified due to crossflow vortices. However, crossflow vortices tend to decay shortly after the saturation phase and hence the laminarizing effect naturally occurs only in a limited region. In this study, we propose a sinusoidal roughness which can sustain crossflow vortices for a long distance. When the ridge lines of the sinusoidal roughness are aligned to a specific streamline within the boundary layer, crossflow vortices do not decay but continue to be saturated nonlinearly over a wide range of the chord position. The boundary layer is strongly and widely stabilized by them and the crossflow-induced transition can be completely suppressed or significantly delayed.