| Disturbances that arise from sources such as wall roughness, acoustic noise and free-stream turbulence are often sufficiently large to cause the natural transition process from laminar to turbulent flow to be bypassed. In such circumstances, mechanisms other than those described by linear stability theory govern the transition process. Some of the effects of free-stream turbulence on transition have been known for many decades, but the phenomenon of bypass transition has remained a mystery. Progress in this field has been hampered by difficulties in obtaining detailed experimental data. The present work contributes to the available data and elucidates the underlying mechanisms of bypass transition.; Direct numerical simulations with free-stream turbulence intensities of 3.5% and 7.0% have been performed on high-performance parallel computers. The simulations utilize the continuous spectrum modes of the Orr-Sommerfeld equation in a newly developed formulation to synthesize an inlet velocity field with free-stream turbulence. This formulation allows the inlet plane to be located downstream of the leading edge which greatly reduces the computational cost.; The concept of shear sheltering is used to explain how the boundary layer is shielded from high frequency freestream turbulence, though low frequency modes penetrate. Once in the boundary layer, an ensuing transfer of energy to low frequency modes is discovered from energy spectra. These modes appear as laminar streaks. To date, the role of these streaks, called Klebanoff modes, has been poorly understood.; It is shown how the bypass process proceeds: algebraic growth causes certain streaks to develop intense streamwise velocity fluctuations, as they lift up in the boundary layer; streaks that reach the outer part of the boundary layer are no longer sheltered by the shear layer; they appear near the boundary layer edge as ‘negative jets’; these are perturbed by free-stream eddies and spawn a local patch of small-scale turbulence. Ultimately this results in a turbulent spot. The formation and development of spots determine the onset and extent of the transition region. |
