

Article
Streak interactions and breakdown in boundary layer flows
Authors: 
Brandt, L.B., De Lange, H. C. 
Document Type: 
Article 
Pubstate: 
Published 
Journal: 
Phys. Fluids 
Volume: 
20, 024107
116 
Year: 
2008 
AbstractThe objective of this paper is to show that the interaction of streamwise velocity streaks of finite length can lead to turbulent breakdown in the flatplate boundary layer flow. The work is motivated by previous numerical and experimental studies of transitional flows where the highfrequency oscillations leading to turbulence are seen to form in the region of strongest shear induced by streaks in relative motion. Therefore a model for the interaction of steady and unsteady, i.e. slowly moving in the spanwise direction, spanwise periodic streaks is proposed. The interaction of two subsequent streaks is investigated for varying collision parameters. In particular, the relative spanwise position and angle are considered. The results show that the interaction is able to produce both a symmetric and asymmetric breakdown without the need for additional random noise from the main stream. Velocity structures characteristic of both scenarios are analysed. Hairpin and lambdavortices are found in the case of symmetric collision between a lowspeed region and an incoming highspeed streak, when a region of strong wallnormal shear is induced. Alternatively, when the incoming highmomentum fluid is misaligned with the lowspeed streak in front, single quasistreamwise vortices are identified. Despite the different symmetry at the breakdown the detrimental interaction involves for both cases the tail of a lowspeed region and the head of a highspeed streak. Further, the breakdown appears in both scenarios as an instability of threedimensional shear layers formed between the two streaks. The streak interaction scenario is suggested to be of relevance for turbulence production in wallbounded flows.

