Bypass transition to turbulence

Transition to turbulence in wall-bounded flows is one of the most studied problems in fluid dynamics. Notorious for its mathematical complexity, it is also of significant engineering interest. It is well known, for example, that a turbulent

boundary-layer flow results in an higher drag, increased heat transfer between the wall and the fluid and enhanced mixing; it may also be less prone to separation compared to laminar flow. For these reasons, the ability to control, or at

least predict, the onset of laminar-turbulent transition is crucial to the design of aerodynamic components with a significant extent of laminar flow. Despite the relative simplicity of the laminar channel and boundary-layer flows, the various disturbances ultimately leading to transition induce a myriad of complex phenomena. Consequently, compared to other areas in fluid dynamics, the understanding of transition to turbulence has been slow to come. Despite outstanding progress in the area of natural transition, bypass transition in boundary-layers caused by moderate to high amplitude turbulence in the free stream is still not well understood. Recent advances in computing power and the development of efficient numerical algorithms have made many transitional flows amenable to Direct Numerical and Large Eddy Simulation (DNS/LES). The focus of the present studies is on transition caused by coherent disturbances (wakes) generated by obstacles upstream of the boundary layer, as well as on transition caused by to high-amplitude free-stream turbulence.

Animations:

Wake/boundary-layer interaction (Re_D=385)

Wake/boundary-layer interaction (Re_D=3500)

Boundary-layer transition due to free-stream turbulence

References:

V. Ovchinnikov, M. M. Choudhari, and U. Piomelli. Numerical simulations of boundary-layer

bypass transition due to high-amplitude free-stream turbulence. J. Fluid Mech., 613:135–169,

2008. (preprint)

V. Ovchinnikov, M. M. Choudhari, and U. Piomelli. Numerical simulations of boundary-layer

bypass transition induced by a cylinder wake. J. Fluid Mech., 547:413–441, 2006.(preprint)