Bypass transition
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A bypass transition is a laminar–turbulent transition in a fluid flow over a surface. It occurs when a laminar boundary layer transitions to a turbulent one through some secondary instability mode, bypassing some of the pre-transitional events which typically occur in a natural laminar–turbulent transition.[a]
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 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.[2]
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. Through 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.[3]
Notes
- ^ Examples of events preceding natural transitions include the generation of two-dimensional Tollmien-Schlichting waves, spanwise vorticity, and three-dimensional vortex breakdown
References
- ^ Morkovin M. V., Reshotko E., Herbert T. 1994. "Transition in open flow systems—a reassessment". Bull. Am. Phys. Soc. 39:1882.
- ^ Kenndal, James M. (1998). "Experiments on Boundary Layer Receptivity to Free stream Turbulence". AIAA. doi:10.2514/6.1998-530.
- ^ Matsubara, M.; Alfredsson, P. H. (2001). "Disturbance growth in boundary layers subjected to free-stream turbulence". Journal of Fluid Mechanics. 430: 149–168. doi:10.1017/s0022112000002810.