Bypass transition

Bypass Transition is a kind of laminar to turbulent transition in a flow over a surface, in which some of pre-transitional events which are generally occurring in natural laminar-turbulent transition, such as generation of two dimensional Tollmien-Schlichting waves, Spanwise vorticity and Three-dimensional vortex breakdown are bypassed and through some secondary instability mode the laminar boundary layer becomes turbulent boundary layer.

The path from receptivity to laminar-turbulent transition as illustrated by Morkovin, 1994.

History

P. S. Klebanoff is the first scientist who experimentally observed bypass transition scenario during his experiments in elevated free-stream turbulence flow. He identified an important aspects of the bypass transition. In his experiment, using hot wires he studied flow over a flat plate which was subjected to 0.3% free-stream turbulence level. At this moderate free-stream turbulence level, he could observe a low-frequency velocity perturbation signal, that is less than 12 Hz which is much smaller than usual Tollmien-Schlichting wave frequency. He also observed thickening and thinning of boundary layer which is not in the case of low free-stream turbulence flow.

Pre-transitional flow structures

In Bypass transition flow, the pre-transitional flow structures are quite different from those of very low turbulent intensity free-stream flow. From various laboratory experiments and computational studies, it has been observed that the low frequency streaky flow structures are present inside the laminar boundary layers. These streaky structures are called as Klebanoff mode or simply K-modes since this was first experimentally observed by the Klebanoff.