

	Mechanics På svenska

Project

Boundary layer transition - Theory and DNS

Research Area:

Fluid mechanics

Project Members:

Berlin, S.

Brandt, L.B.

Henningson, D.S.H.

Hildings, C.

Sponsor:

FOI, VR

Project Description

This project involves research to determine the maximum growth possible of disturbances evolving according to linear theory, as well as to investigate the importance of this growth when non-linearity comes into play. Several shear flow types have been considered. The results show that non-modal growth, i.e. growth not associated with individual eigenmodes but inherently dependent on their superposition, can cause large transient amplification. This growth is mainly associated with streaky structures in the streamwise direction. Non-linear calculations have shown that when the optimal disturbances from linear theory are used as initial conditions, the threshold amplitudes required for transition to turbulence is lower than for general disturbances. Recent calculations have also shown that these results carry over into the spatial development of disturbances in boundary layers. Another part of the project involves direct numerical simulations (DNS) of transition to turbulence where these transient growth mechanisms play a major role. This bypass of the traditional Tollmien-Schlichting instability waves is involved in many shear flow transition scenarios. Previously transition associated with localized disturbances have been investigated, and at present the transition in boundary layers starting with a pair of oblique waves is investigated. These waves generate elongated streakss in the streamwise velocity which rapidly grow due to the non-modal mechanism. The secondary instability of these streaks has also been studied, both using DNS and secondary instability theory. A new direction in the project is to simulate transition in a boundary layer subjected to free-stream turbulence, where the turbulence is initiated close to the leading edge by a random distribution of continuous spectrum modes.

Publications related to the project

Year	Title
2003	On the convectively unstable nature of optimal streaks in boundary layers J. Fluid Mech. 485 221-242 (Published)
2002	Weakly nonlinear analysis of boundary layer receptivity to free-stream disturbances Phys. Fluids 14 1426-1441 (Published)
2002	Transition of streamwise streaks in zero presure gradient boundary layers J. Fluid Mech. 472 229-262 (Published)
2001	On the breakdown of boundary layer streaks J. Fluid Mech. 428 29-60 (Published)
2001	Stability and transition in shear flows Springer (Published)
2001	A numerical and experimental study of a transitional separation bubble Aerospace Science and Technology 5 317-328 (Published)
2000	Direct numerical simulations of growth and breakdown of streamwise streaks in flat-plate boundary layer Proc. 5th Congress Italian Society for Applied and Industrial Mathematics 610-613 (Published)
2000	Secondary instability of streaks in boundary layers Advances in Turbulence VIII. (2000) Proc. 8th European Turbulence Conference, Barcelona, Spain 141-144 (Published)
1999	Optimal Disturbances and Bypass Transition in Boundary Layers Phys. Fluids 11 134-150 (Published)
1999	Numerical and Experimental Investigations of Oblique Boundary Layer Transition. J. Fluid Mech. 393 23-57 (Published)
1999	A nonlinear mechanism for receptivity of free-stream disturbances. Phys. Fluids 11 3747-3760 (Published)
1998	Linear and non-linear development of localized disturbances in zero and adverse pressure gradient boundary-layers. Phys. Fluids 10 1405-1418 (Published)
1998	The Compressible Inviscid Algebraic Instability for Streamwise Independent Disturbances. Phys. Fluids 10 1784-1786 (Published)
1998	On stability of streamwise streaks and transition thresholds in plane channel flows. J. Fluid Mech. 365 269-303 (Published)
1998	Secondary instability of cross-flow vortices in Falkner-Skan-Coke boundary layers. J. Fluid Mech. 368 339-357 (Published)
1996	Transient growth in compressible boundary layer flow. Phys. Fluids 8 826-837 (Published)
1996	Comment on "Transition in shear flows. Nonlinear normality versus non-normal linearity" Phys. Fluids 8 2257-2258 (Published)
1996	Transition Thresholds in boundary layer and channel flows. Advances in Turbulence VI (Ed. Gavrilakis, S., Machiels, L., Monkewitz, P.A.), Kluwer Academic Publishers 381-384 (Published)
1995	Spatial simulations of oblique transition in a boundary layer. Advances in Turbulence V (Ed. Benzi, R.), Kluwer 31-35 (Published)
1995	Bypass transition and linear growth mechanics. Advances in Turbulence V (Ed. Benzi, R.), Kluwer 190-204 (Published)
1995	Spatial simulations of bypass transition in boundary layers. Laminar-Turbulent Transition (Ed. Kobayashi, R.), Springer 263-270 (Published)
1994	Bounds for threshold amplitudes in subcritical shear flows J. Fluid Mech. 270 175-198 (Published)

Internal Reports related to the project

Year	Title	Document Type
1999	An efficient spectral method for simulation of incompressible flow over a flat plate.	Technical report
1998	Oblique waves in boundary layer transition	Doctoral thesis
1998	Control of oblique transition by flow oscillations	Technical report