Dynamics of Boundary Layer Transition: Measurement and Visualization C. B. Lee State Key Laboratory for Turbulence Research and Complex System, Peking University, Beijing Shiyi Chen State Key Laboratory for Turbulence Research and Complex System, Peking University, Beijing and the Johns Hopkins University

OVERVIEW Oblique Wave Interaction

Existence of the universal mechanism for Receptivity （ Lee 2000 ， 2001 ） The new mechanism for the streamwise vortex formation (Stretching or others)? （ Lee 2001 ） Generation of the chain of ring vortices (self-induction or other possibility ) (Lee 2001; Kachanov 2002; Bake 2002) QUESTIONS

In turbulence production, which structure dominates the turbulence Bursting (Lee 2000; 2004) Low frequency fluctuations for the long streak(Gong, Lee & Lee 2001 ， Asai2002)

Experimental facility Low turbulence level water channel T ≦ 0.1% Free stream velocity 22 cm/s Reynolds number 0.2 ×10 6 1/m T-S wave frequency is 2Hz

Experimental Set-up

T-S Wave Features

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∧ -vortextex

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Long streak generation People believed that the low speed streak is the source of the turbulence production （ Kline et al. 1967; Asai 2002, 2004; Lee 2000 ) Low speed streak instability ( Asai 2002)

Slug Phenomena (Tritton 1988)

Video and Slug ModelideoSlug Model

A common flow structure The possible mechanism for the generation of the SCS is simple enough. Therefore, it is possible a common flow structure in all the shear flows. The mechanism is the two oblique wave interaction. Oblique wave can be generated by any kind of disturbances

Generation of the secondary closed vortex Video1 Video2

Outline for the vortex generation

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Streamwise Vortex(or rolls) and the chain of ring vortices Generation Stretching Interaction between the ∧ -vortex and the secondary closed vortex

Kachanov’s Hypotheses Self-induction of two parallel vortex can generate the ring-like vortices one by one under Crow’s Long wave instability theory(1968) Bake (2002) followed this conclusion

Determination of the number of the vortices in a chain (= 4) Strong effect on the velocity fluctuations in whole boundary layer Different mechanism for their formation (Experiments in Fluids 30, 354; Chinese Phys Letts 17, 583)

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Breakdown of the Chain of Ring Vortices

Details of CRV breakdown Part filament of the vortex oscillates back and forth Increase of the diameter of the vortex ring Breakdown happens at first in the near wall region Then the breakdown appears in the outer layer

(continuous) All the four ring vortices are in the same manner of breakdown

Breakdown of the SCS

The breakdown of the SCS as well as the low speed streak is generated by the Chain of Ring Vortices

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Conclusions Long streak or low speed streak: family of the SCS without any other members Chain of ring vortices : son’s son of the SCS Streamwise vortex : son’s son of the SCS

Thank You ！