Oblique waves in boundary layer transition

Sammanfattning: Traditional research on laminar-turbulent transition has focused on scenarios that are caused by the exponential growth of eigensolutions to the linearized disturbance equations, e.g. two-dimensional Tollmien-Schlichting waves. Recent research has reveled the existence of other non-modal growth mechanisms, for example associated with the transient growth of streamwise streaks. Oblique waves may trigger transition in which the new mechanisms is an important ingredient. We have investigated the role of oblique waves in boundary layer transition, using an efficient spectral code for direct numerical simulations.In the initial stage of this transition scenario oblique waves have been found to interact nonlinearly and forte streamwise vortices, which in turn forte growing stream wise streaks. If the streak amplitude reaches a threshold value, transition from laminar to turbulent flow Will take place.In the late transition stage, large velocity fluctuations are found at flow positions associated with steep spanwise gradients between the streaks. At those positions we have also found h-vortices, structures that are also characteristic for traditional sec ondary instability transition. The h-vortices are shown to be due to the interaction of oblique waves and streaks that seem to play a more important role in the late stage of transition than previously appreciated.The numerical results are compared in detail with experimental results on oblique transition and good agreement is found.A new nonlinear receptivity mechanism is found that can trigger boundary layer transition from oblique waves in the free-stream. The mechanism continuously interact with the boundary layer and the resulting transition scenario is characterized by the growth of streamwise streaks. The same structures that are observed in experiments on transition caused by free-stream turbulente. A linear receptivity mechanism that interact with the boundary layer downstream of the leading edge is also identified. It is related to linear receptivity mechanisms previously studied at the leading edge. The nonlinear and linear mechanisms are of comparable strength for moderate free-stream disturbance levels.Two strategies for control of oblique transition are investigated, both based on spanwise flow oscillations. The longest transition delay was found when the flow oscil lations were generated by a body forte. When the control was applied to a transition scenario initiated by a random disturbance it was more successful and transition was prevented.