Bypass transition

A bypass transition is a laminar–turbulent transition in a flow over a surface, in which some of the pre-transitional events which generally occur in a natural laminar-turbulent transition (such as the generation of two-dimensional Tollmien-Schlichting waves, spanwise vorticity and three-dimensional vortex breakdown) are bypassed, causing the laminar boundary layer to become a turbulent boundary layer through some secondary instability mode.

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. Klebanoff identified an important aspect of the bypass transition. In his experiment using hot wires, he studied flow over a flat plate which was subjected to a 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 the 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 in the laminar boundary layers. These streaky structures are called Klebanoff modes or simply K-modes since this was first experimentally observed by Klebanoff.